Halo Gas and Galaxy Disk Kinematics Derived from Observations and CDM Simulations of Mgii Absorption Selected Galaxies at Intermediate Redshift
We obtained ESI/Keck rotation curves of 10 Mgii absorption selected galaxies () for which we have WFPC–2/HST images and high resolution HIRES/Keck and UVES/VLT quasar spectra of the Mgii absorption profiles. We perform a kinematic comparison of these galaxies and their associated halo Mgii absorption. For all 10 galaxies, the majority of the absorption velocities lie in the range of the observed galaxy rotation velocities. In 7/10 cases, the absorption velocities reside fully to one side of the galaxy systemic velocity and usually align with one arm of the rotation curve. In all cases, a constant rotating thick–disk model poorly reproduces the full spread of observed Mgii absorption velocities when reasonably realistic parameters are employed. In 2/10 cases, the galaxy kinematics, star formation surface densities, and absorption kinematics have a resemblance to those of high redshift galaxies showing strong outflows. We find that Mgii absorption velocity spread and optical depth distribution may be dependent on galaxy inclination. To further aid in the spatial–kinematic relationships of the data, we apply quasar absorption line techniques to a galaxy ( km/s) embedded in CDM simulations. In the simulations, Mgii absorption selects metal enriched “halo” gas out to kpc from the galaxy, tidal streams, filaments, and small satellite galaxies. Within the limitations inherent in the simulations, the majority of the simulated Mgii absorption arises in the filaments and tidal streams and is infalling towards the galaxy with velocities between km s. The Mgii absorption velocity offset distribution (relative to the simulated galaxy) spans km s with the lowest frequency of detecting Mgii at the galaxy systematic velocity.
Subject headings:galaxies: halos — galaxies: kinematics and dynamics — galaxies: intergalactic medium — quasars: absorption lines
In a cosmological context, galaxy formation occurs via accretion of gas from the cosmic web and from galaxy–galaxy mergers. The galaxy kinematics reflect these processes. As galaxies evolve, a complex interplay develops between the star formation, which can generate winds, and an array of kinematic structures such as tidal streams, galactic fountains, and filamentary infall that comprise an extended gaseous halo. This overall picture is suggested by observations and CDM cosmological simulations of galaxy formation. However, we lack a thorough understanding, both observationally and theoretically, of how these processes precisely affect the dynamics of galaxies and their extended halos. High quality, detailed observations are required to further develop this working scenario and produce a comprehensive model of galaxy evolution in the cosmological context.
Observations of local galaxies provide detailed views of gas disks and the inner kpc of their halos. Oosterloo et al. (2007) obtained deep Hi observations of NGC 891 and found lagging, differentially rotating halo gas kinematics with velocities decreasing with distance above the galaxy plane. Lagging halos are observed in several other local galaxies (e.g., Sancisi et al., 2001; Swaters et al., 1997; Rand, 2000; Heald et al., 2007). There are also cases where halo gas has been detected with velocities opposite to galaxy rotation (Fraternali et al., 2001; Oosterloo et al., 2007).
Evidence for the accretion of cold gas, which may lower angular momentum and play a role in the development of lagging halos, has also been rapidly accumulating. Several galaxies are observed to have galactic fountains, and be surrounded by Hi cloud complexes, minor merger tidal tails, and IGM filaments (e.g., Heald et al., 2007; Sancisi et al., 2008).
Quasar absorption lines provide powerful probes of halo gas kinematics to large galactocentric distances. In three galaxies, Côté et al. (2005) found that low column density Ly absorption does not follow galaxy rotation to kpc. They suggested that the gas arises from the cosmic web. Bowen et al. (2002) found that Ly absorption strength correlates with galaxy over density. At large distances, the halo gas kinematics may no longer be coupled to the galaxy kinematics, but reflect the motions of cosmic web.
Quasar absorption line studies of the Mgii doublet produced by gaseous halos of foreground galaxies (e.g., Bergeron & Boissé, 1991; Le Brun et al., 1993; Steidel, Dickinson, & Persson, 1994; Churchill, Steidel, & Vogt, 1996; Churchill, Kacprzak, & Steidel, 2005; Zibetti et al., 2007; Kacprzak et al., 2007) probe the low ionization metal enriched gas–galaxy dynamics and IGM interface. With Mgii absorption lines, we can study the kinematic conditions of galactic halos over a wide range of redshifts out to projected galactocentric radii of several hundred kpc. Mgii absorption arises in low ionization, metal enriched gas with neutral hydrogen column densities of cm, and thus selects a large dynamic range of gas structures in the environments associated with galaxies (Rigby, Charlton, & Churchill, 2002; Churchill et al., 2000).
The idea that Mgii absorbers could arise from spherical infall, disk–like rotation, or both, is also a topic of much interest. In a small sample of high resolution Mgii absorption profiles, Lanzetta & Bowen (1992) inferred that rotation kinematics dominated at smaller impact parameters, whereas infall kinematics dominated with increasing impact parameter. Armed with a larger high resolution sample, Charlton & Churchill (1998) applied statistical tests to a variety of kinematic models and concluded that pure disk rotation and pure halo infall models are ruled out. However, models with contributions from both disk rotation and spherical infall statistically reproduced absorption profiles consistent with observed kinematics.
Among the more extreme structures, are galactic winds generated from star forming galaxies. At , Tremonti et al. (2007) detected Mgii blueshifted 500–2000 km s relative to post-starburst host galaxies and Weiner et al. (2009) found km s blueshifts in star forming galaxies. In other ionic species, similar outflows have been observed at (Pettini et al., 2001; Shapley et al., 2003; Steidel et al., 2003; Simcoe et al., 2006; Cabanac et al., 2008). None of these surveys have studied the dynamics of the galaxies themselves, which are clearly needed in order to obtain a full picture of the galaxy–halo dynamics.
A direct comparison of the galaxy disk kinematics and absorbing Mgii halo gas kinematics has been performed for six highly inclined galaxies (Steidel et al., 2002; Ellison et al., 2003). Ellison et al. (2003) found that the systemic velocity of a galaxy coincided with the center of the absorption system, which spanned more than 100 km s about the systemic velocity. Bond et al. (2001) used expanding shell models to explain that this peculiar absorption profile is likely caused by expanding supernovae–driven superbubbles. Steidel et al. (2002) found that, in four of the five cases, the velocities of all of the absorption components lie to one side of the galaxy systemic redshift. The fifth case had a narrow, weak absorption centered at galaxy systemic velocity. Since the halo gas velocities align in the same sense as the galaxy rotation, the velocity offsets of the absorbing gas relative to the galaxy systemic velocity strongly suggest “disk–like” rotation of the halo gas. Using simple disk halo models, Steidel et al. (2002) concluded that an extension of the disk rotation with a lagging halo component (based upon properties of local galaxies’ halo gas kinematics) was able to explain some of the gas kinematics. However, the models were not able to account for the full velocity spreads of the gas.
From a theoretical stand point, semi–analytical models and isolated galaxy simulations (e.g., Mo & Miralda-Escude, 1996; Burkert & Lin, 2000; Lin & Murray, 2000; Maller & Bullock, 2004; Chen & Tinker, 2008; Kaufmann et al., 2008; Tinker & Chen, 2008) have been invoked to study isolated galaxy halos. In these models, Mgii absorption arises from condensed, infalling, pressure confined gas clouds within the cooling radius of a hot halo. These models are quite successful at reproducing the general statistical properties of the absorber population. However, they lack the important dynamic influences of the cosmic structure and local environments.
CDM simulations have been able to synthesize the formation and evolution galaxies within large scale structures. Recently, Ceverino & Klypin (2009), were able to naturally create, without ad–hoc recipes, extended galactic scale outflows and metal enriched multi–phased galactic gas halos. This was accomplished by studying the detailed physics of the formation and evolution of the multi–phase ISM in parsec resolution simulations. These same prescriptions were then successfully applied in their large scale cosmological simulations.
Since these cosmological simulations include all the potential structures that can influence halo gas dynamics and include the local environment, they provide a promising technique for understanding the role of gas in galaxy evolution. The quasar absorption line method can be applied to simulations to examine structures selected by species such as Mgii, in the vicinity of galaxies. The goal is to compare directly observed absorbing halo gas kinematics and host galaxy kinematics to those extracted from the simulations. In order to arrive at a deeper understanding, the observations should target redshifts where detailed high quality kinematics can be obtained for a sample of galaxies with a wide range of orientations with respect to the quasar line of sight.
We have obtained ESI/Keck rotation curves of 10 intermediate redshift () galaxies for which we have high resolution HIRES/Keck or UVES/VLT quasar absorption profiles of Mgii, as well as WFPC–2/HST images. In this paper we perform a kinematic comparison of 10 galaxies and their associated halo Mgii absorption. We define halo gas to be metal enriched structures that give rise to Mgii absorption such as extraplanar gas, outflows, tidal streams, filaments, and satellite galaxies. We compare our observations with a simple rotating thick disk halo model (similar to the one employed by Steidel et al., 2002) and with the cosmological simulations of Ceverino & Klypin (2009).
The paper is organized as follows: In § 2, we present our sample, and explain the data reduction and analysis. In § 3, we present the results of our galaxy–Mgii absorption kinematic observations, and in § 4, we compare the observed absorption velocities with a simple disk kinematic halo model. In § 5, we discuss the details of the cosmological simulations. We study a simulated galaxy and its halo structures in detail. We analyze the integrated total hydrogen and Hi column density maps and the absorbing gas velocity distributions. We use these results to infer possible structures and kinematics drivers of that produce the observed Mgii absorption profiles. We also compute the star formation rate and star formation surface density of the simulated galaxy and compare them to previous observational results, and with two of the galaxies in our sample. We end with our conclusions in § 7. Throughout we adopt a , , cosmology.
2. Data and Analysis
2.1. Sample Selection
The selection of the sample presented in the study is based upon three steps. (1) We compiled a list of Mgii absorbers in high resolution ( km s, with Å) HIRES/Keck (Vogt et al., 1994) or UVES/VLT (Dekker et al., 2000) quasar spectra. We make no cut to the sample based upon equivalent width. (2) We then compiled all subsequent deep ground based imaging and spectroscopic redshift surveys of the quasar fields and selected the galaxies that have confirmed redshifts aligned with Mgii absorption (Bergeron & Boissé, 1991; Bergeron, Cristiani, & Shaver, 1992; Steidel, Dickinson, & Persson, 1994; Lowenthal et al., 1995; Guillemin & Bergeron, 1997; Chen et al., 1998; Lane et al., 1998, This paper). (3) Finally, we selected galaxies for which WFPC–2/HST images were available and from which we can extract detailed galaxy morphological parameters (see Kacprzak et al., 2007). The final sample of ten galaxies have an impact parameter range of kpc.
Our goal is to study the relationship between the spatial and kinematics relations between a galaxy and the Mgii absorbing gas in its vicinity. In our sample, two of the Mgii absorbers appear to be associated with galaxies of similar luminosity that exhibit signs of interaction in the HST images. These galaxies are G1 and G2 in the field of Q0450–132 and G1 and G2 in the field of Q1127–145 (see Figures 3 and 5, respectively). The Q0450–132 galaxies show tidal asymmetries and have projected separated of 12 kpc. There are no additional candidate galaxies in the image within 100 kpc (projected) of the quasar. The Q1127–145 galaxies also exhibit tidal asymmetries. Their projected separation is roughly 50 kpc. In the Q1127–145 field there is a third galaxy, G4, with much smaller luminosity, with a redshift that places it within 70 km s of G2 and 25 km s of G1.
These particular systems, which are characterized by two roughly comparable luminosity galaxies with signs of interacting, pose an interesting challenge. They indicate that some Mgii absorption is arising in the complex environment of a major–major galaxy interaction. It is probably a fair statement to assert that interacting galaxies of roughly equal luminosity (and that is an important point) will have local environments very different than those of galaxies that clearly have no companion of comparable luminosity. It becomes an intractable problem to discern what portion of the absorption may be arising with gas associated with one or the other galaxy in such a pair. In cases where a single galaxy candidate can be assigned as the luminous host of the Mgii, it is possible to unambiguously study the spatial and kinematic relationships. Of course, it is always possible a very low luminosity counterpart is below the detection of the images; but such a companion would indicate a minor–major interaction and not a major–major interaction like the Q0450-132 and Q1127–145 pairs. Minor–major interactions would be more akin to the Magellanic galaxies in the 50 kpc vicinity of the Milky Way; they can be considered part of the Milky Way halo. Such a distinction would equally apply to the single galaxies in this sample if there is an unseen minor companion.
In order to keep our sample as uniform as possible for the study of the spatial and kinematic connections between a galaxy and its associated absorbing gas, we limit our analysis to the single galaxy candidates when ambiguities would arise, such as comparison with galaxy inclination, position angle, and impact parameter. There is a very different nature to the major–major interacting pairs toward Q0450-132 and Q1127–145 in that they may have a common gas envelope and therefore provide a slightly different probe of absorption galaxy properties. In cases where ambiguities do not arise in the analysis, we include all the galaxies in our sample.
2.2. Quasar Spectroscopy
Details of the HIRES/Keck and UVES/VLT quasar observations are presented in Table 1. The HIRES spectra (except for Q) were reduced using IRAF111IRAF is written and supported by the IRAF programming group at the National Optical Astronomy Observatories (NOAO) in Tucson, Arizona. NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation.. The spectrum of Q was reduced using the MAKEE222http://spider.ipac.caltech.edu/staff/tab/makee package. The UVES spectra were reduced using the standard ESO pipeline and a custom code called the UVES Post–Pipeline Echelle Reduction (uves popler, Murphy, 2006). The quasar spectra are both vacuum and heliocentric velocity corrected. Analysis of the Mgii absorption profiles was performed using graphic–based interactive software of our own design (see Churchill et al., 1999, 2000; Churchill & Vogt, 2001) for local continuum fitting, objective feature identification, and measuring absorption properties. The redshift for each Mgii system is computed from the optical depth weighted mean of the absorption profile (see Churchill & Vogt, 2001). The typical absorption redshift uncertainty is km s. The Mgii rest–frame equivalent widths are adopted from Kacprzak et al. (2007). Velocity widths of absorption systems are measured between the pixels where the equivalent width per resolution element recovers to the detection threshold (Churchill et al., 1999).
|QSO Field||Instrument||Date (UT)||(sec.)|
|Q||1.90||HIRES||1994 Jul. 05||2700|
|Q||2.06||HIRES||1999 Feb. 08||3600|
|Q||2.25||HIRES||1995 Jan. 24||5400|
|Q||0.53||HIRES||1995 Jan. 22||5400|
|Q b bfootnotemark:||2.70||HIRES||1998 Feb. 26||5400|
2.3. HST Imaging and Galaxy Properties
All WFPC–2/HST images were reduced using the WFPC–2 Associations Science Products Pipeline (WASPP333Developed by the Canadian Astronomy Data Centre (CADC) and the Space Telescope–European Coordinating Facility (ST–ECF): http://archive.stsci.edu/hst/wfpc2/pipeline.html). The WFPC–2 astrometry is calibrated to the USNO2 Catalog (Monet et al., 1998). WASPP data quality verifications include photometric and astrometric accuracy and correctly set zero–points. Details of the WFPC–2/HST observations are presented in Table 2. Galaxy apparent and absolute magnitudes are adopted from Kacprzak et al. (2007, 2008), respectively. The and magnitudes are based upon the Vega system. As described in Kacprzak et al. (2007), we used GIM2D (Simard et al., 2002) to model the galaxy morphologies, and measured the quasar–galaxy impact parameters, galaxy sky orientations, inclination angles (), and position angles () of their major axes with respect to the quasar line of sight. We fit each galaxy surface brightness profile with a Sersic bulge component (for ) and an exponential disk component. Additional modeled galaxy morphological parameters will be presented elsewhere (Kacprzak et al. 2010, in preparation).
|QSO Field||z||Date (UT)||(sec.)||PA|
|Q G1||0.851407||2001 Oct. 16||7200|
|Q G1||0.417337||2006 Dec. 24||6500||134|
|Q G1||0.493937||2006 Dec. 24||5300|
|Q G2||0.493937||2006 Dec. 24||5300|
|Q G1||0.483338||2006 Dec. 24||4800||276|
|Q G1||0.786726||2006 Dec. 24||5300||130|
|Q G2||a afootnotemark:||2006 Dec. 24||5300||130|
|Q G1||0.312710||2006 Dec. 24||3900||129|
|Q G2||0.312710||2006 Dec. 24||4200||87.5|
|Q G3||0.328266||2006 Dec. 24||600||87.5|
|Q G1||1.017040||2001 Oct. 16||1800||75|
|QSO Field||ID||Reference a afootnotemark:||(kpc)||Å||This Study|
|G3 c cfootnotemark:||0.755||8||0.751923|
2.4. Galaxy Spectroscopy
The ESI/Keck (Sheinis et al., 2002) galaxy spectra were obtained over two nights; two were obtained in October 2001 and eight were obtained in December 2006. Details of the ESI/Keck observations are presented in Table 3. For each galaxy, the slit position angle was chosen to lie along the galaxy major axis (except for Q G2). The slit length is . Thus in some cases, we were able to simultaneously position two galaxies on a slit. Exposure times range between s per galaxy. The wavelength coverage of ESI is 4000 to 10,000 Å, which allows us to obtain multiple emission lines (such as [Oii] doublet, , [Oiii] doublet, , [Nii] doublet, etc.) with a velocity resolution of km s pixel ( km/s).
In October 2001, the data were obtained with a slit and binning. The mean seeing was () with partial cloud coverage. In December 2006, the data were obtained using a slit with binning. Binning by two in the spatial directions results in pixel sizes of over the orders of interest. The mean seeing was () with clear skies.
The data were reduced using the standard echelle package in IRAF. We used internal quartz illumination flat fields to eliminate pixel to pixel variations. In the science frames, sky subtraction was performed by fitting a polynomial function to each spatial column. A quasar or bright star spectrum in the same field was obtained and used as a trace in order to facilitate the extraction of the galaxy spectrum. A spatially integrated spectrum was extracted in order to obtain an accurate galaxy redshift from the centroids of multiple emission lines. The lines listed in the legends of Figures 2–6 were used to determine the galaxy redshifts. The rest–frame vacuum wavelength used for each emission line was obtained from the National Institute of Standards and Technology (NIST) database. Each spectrum was wavelength calibrated using CuArXe arc line lamps. Spectra were calibrated in IRAF using standard stars taken during the night of the observation. The flux is accurate to 10% and we have made no corrections for slit loss, or Galactic reddening.
In Figure 1, we show an example of a 2D spectrum of the spatially resolved [Oii] doublet from the galaxy in the Q field. We used a Gaussian fitting algorithm (see Churchill et al., 2000), which computes best fit Gaussian amplitudes, widths, and centers (redshift), to the galaxy emission and absorption lines. The galaxy redshift was computed from the mean redshift of all the detected lines. Emission lines and absorption lines used to calculate the galaxy redshift must have been detected at the level. The adopted redshift uncertainty for each galaxy was computed from the standard deviation in the redshifts computed from each emission line. The galaxy redshifts are listed in Table 4; their accuracy ranges from 2–45 km s. The galaxy velocity offsets from the optical depth weighted mean Mgii absorption are also listed in Table 4 and range from to km s.
The rotation curve extraction was performed following the methods of Vogt et al. (1996) and Steidel et al. (2002). We extract individual one–dimensional spectra by summing three–pixel wide apertures (corresponding to approximately one resolution element of ) at one pixel spatial increments along the slit. An error spectrum is also extracted for each of these apertures. To obtain accurate wavelength calibrations, we extracted spectra of CuArXe arc line lamps at the same spatial pixels as the extracted galaxy spectra. Fitted arc lamp exposures (CuArXe) provided a dispersion solution accurate to Å, or about km s at the wavelengths of interest. Galaxy spectra are both vacuum and heliocentric velocity corrected for comparison with the absorption–line kinematics. Each galaxy emission line (or absorption line in some cases) was fit with a single Gaussian (except the [Oii] doublet was fit with a double Gaussian) in order to extract the wavelength centroid for each emission line.
An example of a three–pixel wide spectral extraction from the 2D spectrum is shown in Figure 1 where the [Oii] and lines are detected at the and level, respectively (the significance level is the ratio of the measured equivalent width to the uncertainty in the equivalent width based upon error propagation using the error spectrum extracted for the same three–pixel aperture). The dashed line in Figure 1 provides the spatial cut for which the spectrum in Figure 1 is illustrated. The velocity offsets for each emission line in each extraction were computed with respect to the redshift zero point determined for the galaxy (Table 4). The rotation curves for the 10 galaxies obtained with ESI/Keck are presented in Figures 2–6.
3. Discussion of Individual Fields
Here we discuss the halo gas and galaxy kinematics of ten galaxies in seven different quasar fields. In Table 5, we list all the galaxies in each field that have spectroscopically confirmed redshifts. The table columns are (1) the quasar field, (2) the galaxy ID, (3) the galaxy redshift, (4) the reference(s) for the galaxy identification, (5) the quasar–galaxy impact parameter, , and uncertainty, (6) the Mgii absorption redshift, and (7) the rest–frame Mgii equivalent width, , and uncertainty. Three new galaxies (Q G1, Q G2, and Q G3) have been spectroscopically identified in this work.
Most of the quasar fields listed in Table 5 have been spectroscopically surveyed for all galaxies with , which translates to for , out to (e.g., Steidel, Dickinson, & Persson, 1994; Guillemin & Bergeron, 1997; Chen et al., 1998). These works have been instrumental in developing our current picture of the galaxy–absorber connection at intermediate redshifts. It remains possible that additional galaxies below this luminosity lurk in front of the quasar and may be associated with the absorbing gas. It is also possible that a less than 100% completeness in the confirmation of galaxy redshifts may result in an additional galaxy or galaxies also associated with the absorption. The concerns and caveats associated with incorrect idenitifications and the conclusion drawn for those works therefore also apply to this study. Galaxies fainter than , or low mass galaxies hidden by the quasar PSF, such as satellites or minor companions, can be considered part of the halo of the large host galaxy.
In the following subsections, we discuss only the ten galaxies selected for this study (see column 8 of Table 5). Detailed images of the galaxies are presented in Kacprzak et al. (2007). A summary of the impact parameters, galaxy maximum rotation velocities, and GIM2D model inclinations and position angles ( angle between the galaxy major axis and the quasar line of sight) is listed in Table 6. Galaxy redshifts will only be quoted to four significant figures from here on for simplicity. We will later discuss kinematic halo models in § 4.
3.1. Q0002+051 G1
A WFPC–2/HST image of the Q field is shown is Figure 2. The galaxy, G1, was targeted for this study. It was first assumed by Steidel, Dickinson, & Persson (1994) to be the absorbing galaxy associated with the Mgii absorption at given its colors and its proximity to the quasar. We report the first redshift confirmation of this galaxy. The galaxy redshift was identified by an [Oiii] and a weak [Oii] emission line (the [Oii] line was used only for the determination of the galaxy redshift and provided no kinematic information). In Figure 2, we note that the ESI/Keck slit was placed across both G1 and G2 (where ’’ and ’’ on the slit indicate the positive and negative arcseconds relative to the center of the galaxy, respectively). We did not detect any identifiable emission lines from the galaxy G2.
The Mgii absorption was first reported by Bechtold et al. (1984) and then confirmed by Steidel & Sargent (1992). The HIRES/Keck spectrum was originally presented by Churchill, Steidel, & Vogt (1996) and has Å.
The G1 galaxy has a compact morphology. It has an absolute band magnitude of which translates to a luminosity . The galaxy has inclination angle and . G1 is at a projected distance of kpc from the quasar line of sight. In Figure 2, the spatial radial velocity, as derived from [Oiii], is shown. It appears G1 is not predominantly rotating. The projected velocity shear is km s. Displayed in the lower panels are the Mgii and Mgi absorption profiles, shown on the same velocity scale. The mean optical depth absorption redshift is offset by km s from the galaxy systemic velocity. This Mgii absorption system is the most kinematically complex absorption profile in our sample. Four absorption sub–systems are spread out over a total velocity of km s. The blue–shifted extreme wing of G1’s velocity shear is partially consistent with the velocities of the dominant saturated component of the Mgii. However, it is inconsistent with the Mgii gas at large positive velocities. The Mgi gas, which may trace more neutral and higher column density gas, has velocities is also inconsistent with the velocities of G1.
3.2. Q0229+131 G1
A WFPC–2/HST image of the Q field is shown in Figure 2. The galaxy, G1, was initially spectroscopically identified by Bergeron & Boissé (1991). The first detection of Mgii absorption at the galaxy redshift was reported by Sargent, Boksenberg, & Steidel (1988). A HIRES/Keck spectrum of this absorber was first presented by Churchill, Kacprzak, & Steidel (2005) and has Å.
This moderately inclined, , bulge dominated galaxy has two large grand design spiral arms similar to a local Sb galaxy. One of the arms contains a bright Hii region. The galaxy position angle is . G1 has an absolute band magnitude of which translates to . G1 is at a projected distance of kpc from the quasar line of sight.
In Figure 2, the rotation curve, derived from [Oii], , and [Oiii] emission lines, is shown. The asymmetric rotation curve has a maximum observed velocity of km s. The asymmetry may be a result of the asymmetric spiral arms. Below the rotation curve, the Mgii absorption profiles are shown on the same velocity scale. The mean absorption redshift is offset by km s from the galaxy systemic velocity. The Mgi profile is not shown here since it is completely bended with a four component Siiv complex from a Civ absorber (Sargent, Boksenberg, & Steidel, 1988). This Mgii absorption system has a velocity spread of roughly 112 km s with a single cloud residing roughly km s from the main component. The main absorption component aligns with the redshifted wing of the galaxy rotation curve arising from the spiral arm nearest the quasar. The galaxy rotation velocities are not consistent with the outlying cloud.
3.3. Q0450-132 G1,g2
A WFPC–2/HST image of the Q field is shown in Figure 3. Two galaxies were placed along the same slit. The redshifts of this double pair were initially obtained by Steidel, Dickinson, & Persson (1994). This absorption system was discovered during the survey of Sargent, Boksenberg, & Steidel (1988) and the HIRES/Keck spectrum was first presented by Churchill (1997). The Mgii absorption has an equivalent width of Å.
The galaxy closer to the quasar, G1, has a redshift of and is at an impact parameter of kpc. The galaxy has an inclination of with a . G1 has an absolute magnitude of which translates to a galaxy. The galaxy further from the quasar, G2, has a redshift of and has an impact parameter of kpc. The galaxy has an inclination of with a . G2 has an absolute magnitude of which translates to . The galaxies are spatially separated by only kpc projected, and by a line of sight velocity of km s. Both galaxies appear to have single-sided tidal tails and show strong morphological evidence of a previous interaction or harassment.
The rotation curves of both galaxies, as derived from [Oii], , , [Oiii], , and [Nii], are presented in Figure 3. The maximum observed rotation velocities for G1 and G2 are km s and km s, respectively. Below the rotation curve, the Mgii and Mgi absorption profiles are shown on the same velocity scale. The mean absorption redshift is offset by km s from the galaxy systemic velocity of G1. The Mgii absorption is a single kinematic region with a velocity width of km s. The absorption aligns, in velocity, with the blueshifted wing of the rotation curve of G1. The Mgi aligns more closely to the systemic velocity of G1. The galaxy G2 does not have much rotation since the slit position angle is close to its minor axis.
3.4. Q0454-220 G1
A WFPC–2/HST image of the Q field is shown in Figure 3. The galaxy, G1, was targeted for this study. G1 was spectroscopically identified by Chen et al. (1998). The Mgii absorption was first reported by Bergeron & Kunth (1984) and then confirmed by Tytler et al. (1987). The HIRES/Keck spectrum was originally presented by Churchill & Vogt (2001). The Mgii equivalent width is Å.
G1 is a spiral galaxy that has a perturbed morphology with one extended spiral arm. It has a compact bulge and several bright Hii regions. The galaxy has an inclination of and a position angle of . It has an absolute band magnitude of which translates to . G1 is at a projected distance of kpc from the quasar line of sight.
The rotation curve, as derived from [Oii], , [Oiii], , and [Nii], is presented in Figure 3. The rotation curve flattens out at a maximum observed velocity of km s. Below the rotation curve, the Mgii and the Mgi absorption profiles are shown on the same velocity scale. The mean absorption redshift is offset by km s from the galaxy systemic velocity. This Mgii absorption is a single kinematic system having a velocity spread of roughly 93 km s. The blue shifted component of the rotation curve has velocities that are consistent with the Mgii and Mgi absorption profiles.
3.5. Q0836+113 G1
A WFPC–2/HST image of the Q field is shown in Figure 4. The galaxy, G1, was initially spectroscopically identified by Lowenthal et al. (1995). The first Mgii absorption detection at the galaxy redshift was reported by Turnshek et al. (1989). The HIRES/Keck absorption profiles of this absorber is first presented in this work (Figure 4) and has Å.
The galaxy G1 appears to be an edge–on spiral with an asymmetric brightness profile. The morphology is similar to that of the two galaxies in Q, which have single–sided tidal tails. This almost edge–on galaxy has an inclination of and . G1 has an absolute band magnitude of which translates to . The galaxy is at a projected distance of kpc from the quasar.
The spatial radial velocity of G1, as derived from [Oii], is presented in Figure 4. The data suggest that G1 exhibits more of a global shear than rotation. The maximum observed shear velocity is km s. Below the velocity curve, the Mgii and the Mgi absorption profiles are shown on the same velocity scale. The mean absorption redshift is offset by km s from the galaxy systemic velocity. The Mgii absorption is a single kinematic component having a velocity spread of roughly 282 km s. Most of the component is composed of highly saturated clouds. From the Mgi profile, one can resolve the individual clouds that are saturated in Mgii. The galaxy velocities are consistent with the velocities of the absorbing gas. However, there is a large amount of Mgii gas that has greater velocities than those of the galaxy. The bulk of the more neutral/high column density gas, as indicated by Mgi, is at the galaxy systemic velocity.
3.6. Q1127-145 G3
A WFPC–2/HST image of the Q field is shown in Figure 4. We present a newly identified galaxy, G3, at . The galaxy was identified by and [Nii] emission lines. The absorption was also recently discovered, and is presented here in Figure 4. This weak system has an equivalent width of Å.
The face–on galaxy has an inclination of with a (the is highly uncertain since ). The galaxy has a large bar with a sizable bright bulge; similar to a local SBb galaxy. It has absolute magnitude of which translates to . The galaxy is at a projected distance of kpc from the quasar.
The rotation curve of G3 is presented in Figure 4. The maximum observed rotational velocity is km s. Given that significant rotation is observed, the galaxy is most likely not completely face–on as the GIM2D model inclination suggests. Below the rotation curve, the Mgii and the Mgi absorption profiles are shown on the same velocity scale. The absorption redshift is offset by km s from the galaxy systemic velocity. The Mgii absorption contains two separate single cloud components. Both clouds have a velocity spread of km s and are separated by km s. No significant Mgi (, Å) is detected. The projected galaxy rotation velocities are consistent with both cloud velocities; each cloud aligns with each side of the rotation curve.
3.7. Q1127-145 G1,g2
The history of the Q absorption system is quite complex. The Mgii absorption was initially detected by Bergeron & Boissé (1991) and was determined to be a DLA (see Rao & Turnshek, 2000) since HST UV data show a damped Ly with cm (Lane et al., 1998). The equivalent width of the system is Å.
The true identity of the absorbing galaxy has been a topic of debate in the literature. A WFPC–2/HST image of the Q field is shown in Figure 5. Bergeron & Boissé (1991) spectroscopically identified G1 and G2 to be at the redshift of the absorption. G1 exhibits strong emission lines and G2 has no detectable emission lines. G1 was assumed to be the absorbing galaxy, since it is closer to the quasar and has significant star formation (Bergeron & Boissé, 1991). Lane et al. (1998) later spectroscopically identified G4 via an [Oiii] doublet with , which was also consistent with the absorption redshift and was assumed to be the absorber only due to its proximity to the quasar. Lane et al. (1998) state that it is also possible that the three galaxies may have undergone a strong interaction where the absorption could arise from tidal debris. In Figure 5, it is apparent from the tidal disturbances, that G1 has undergone interactions in the past. G2 also exhibits some tidal material to the north of the galaxy (which is less apparent in the figure).
Rao et al. (2003) and Nestor et al. (2002) suggest, from ground–based multi-band imaging, that the low surface brightness emission detected around the quasar (see Figure 5) could arise from a foreground low surface brightness galaxy at the absorption redshift. However, it is possible that the low surface brightness signal is coming from the quasar host galaxy at . The background quasar is radio–loud and has strong X–ray emission. An X-ray jet extends north east kpc from the quasar, projected (Siemiginowska et al., 2002, 2007). There is a diffuse halo, both detected in radio and X–ray, around the host quasar. It is not yet clear whether the X-ray halo is real or a result of blurring from the instrument PSF (Siemiginowska, A. 2007, private commutation). Thus, it is possible that the low surface brightness detected by Rao et al. (2003) may be from the background quasar. (Chun et al., 2006) found a possible underlying galaxy from the quasar. Again, this may be a foreground galaxy, or structure from the quasar host galaxy which is commonly observed (e.g., Bahcall et al., 1997).
Galaxies G1 and G2 were targeted for this study. These galaxies are spatially separated by only kpc projected, and by a line of sight velocity of km s. G1 is an almost edge–on () spiral that displays asymmetries on both sides of the galaxy. The galaxy has . It has absolute magnitude of , which translates to . The galaxy is at a projected distance of kpc from the quasar line of sight. G2 is an interesting galaxy; it has a major dust lane and a large bulge. Given that we detected no emission lines, this galaxy could either be classified as Sa or as an early–type S0 galaxy. G2 also has a tidal disturbance along the major axis of the galaxy towards the north. The galaxy has an inclination of and . It has absolute magnitude of which translates to . The galaxy is at a projected distance of kpc from the quasar.
The rotation curve of G1, obtained from the [Oii], , and the [Nii] doublet, is presented in Figure 5. The maximum observed rotational velocity is km s. The rotation curve of G2, obtained from the Nai absorption doublet, is presented in Figure 5. The maximum observed rotational velocity of G2 is km s. Below both rotation curves, the Mgii and the Mgi absorption profiles are shown on the velocity scale defined by G1. The mean absorption redshift is offset by km s from the systemic velocity of G1. The Mgii absorption can be broken up into two kinematic components. The first large saturated component has a velocity spread of roughly 235 km s. The second component, blue–ward of the main component, contains a few weak clouds and has a velocity spread of roughly 68 km s. In the Mgi profile, one can resolve the individual clouds that are saturated in Mgii. Almost all of the Mgi gas is aligned with the saturated Mgii component. Only a very weak Mgi cloud is detected in a second kinematic component. The main component of the Mgii gas aligns with the blue–ward wing of the G1 rotation curve, which is on the side toward the quasar. The Mgii absorption velocities start at the systemic velocity of G1 and is centered on the maximum galaxy rotation velocity. Since the rotation curve of G2 is derived from Nai absorption lines, it is likely that the maximum rotation speed of the galaxy extends to larger velocities then detected. The velocities of G2 are consistent a portion of the large saturated component of the Mgii and the small blue–ward clouds.
3.8. Q2206-199 G1
A WFPC–2/HST image of the Q field is shown is Figure 6. The galaxy, G1, was targeted for this study. The galaxy was spectroscopically identified by Bergeron, Cristiani, & Shaver (1992). The Mgii absorption was first reported by Sargent, Boksenberg, & Steidel (1988). The HIRES/Keck spectrum was originally presented by Prochaska & Wolfe (1997). The Mgii equivalent width is Å.
The spiral galaxy G1 is quite unusual in brightness and morphology. The galaxy has an absolute magnitude of which translates to . Galaxies with super- luminosities are quite rare and represent only a few percent of the galaxy population. The galaxy spiral structure is tightly wound with a large leading arm. There appears to be numerous Hii regions. The bulge is compact and offset from the isophotal center. Note that at the galaxy redshift, the rest–frame mean wavelength of the F702W filter is around Å, roughly rest-frame U–band. The galaxy inclination is with a . The galaxy is at a projected distance of kpc from the quasar.
The rotation curve, obtained from the [Oii] doublet, is presented in Figure 6. The maximum observed rotational velocity is km s. Given the low observed rotation velocities, the galaxy is likely more face–on than the GIM2D model inclination suggests. The galaxy morphology is asymmetric and unusual which makes it difficult to determine the inclination. Below the rotation curve, the Mgii and the Mgi absorption profiles are shown on the same velocity scale. The mean absorption redshift is offset by km s from the galaxy systemic velocity. The Mgii absorption can be broken up into two kinematic components. The first one has velocity spread of roughly 150 km s and is mostly saturated. The second component is a single cloud with a velocity spread of roughly 28 km s that is offset 198 km s red–ward of the main component velocity center. From the Mgi profile, one can resolve the individual clouds that are saturated in Mgii. Most of the Mgi gas is shifted km s from the systemic velocity of the galaxy. The red–ward arm the the rotation curve aligns with only a small portion of velocity of the main Mgii component. However, there is a large amount of Mgii gas that has greater velocities than those of the galaxy. The bulk of the more neutral/high column density gas, as indicated by Mgi, has velocities greater than that of the observed galaxy rotational velocities.
3.9. Summary I: Observational Kinematic Comparisons
In our direct comparison of galaxy disk and halo gas kinematics, traced by Mgii and Mgi absorption, we find the following: (1) in all ten cases, the observed galaxy rotation velocities show substantial overlap with the bulk of the absorption velocities. (2) in seven of ten cases, the Mgii and Mgi absorption resides to one side of the galaxy systemic velocity. In the remaining cases, (Q G1, Q G1, and Q G3) absorption resides on both sides of the galaxy systemic velocity. Our findings are similar to those of Steidel et al. (2002), even though their sample targeted highly inclined and edge–on disk galaxies with . Here we have attempted to select galaxies with a range of inclination and position angle with respect to the quasar line of sight (see Table 6).
To see if there are differences in the Mgii absorption profiles as a function of inclination, we have separated the galaxies into two inclination bins with (five galaxies) and (six galaxies). The galaxies from the Steidel et al. (2002) sample are included here. For this comparison, multiple galaxies that can be associated with a single absorption system (such as G1 and G2 in Q and ) were removed since we cannot confidently know whether one or both galaxies host the Mgii absorption.
In Figure 7, we show the co–added Mgii absorption profiles. The data are plotted as an absolute velocity difference from the galaxy systemic velocity. As seen in Figure 7, the combined spectra of the galaxies with shows that absorption resides between km s with a peak in the optical depth at km s. In Figure 7, the combined spectra show that galaxies with are associated with Mgii absorption with smoothly varying optical depth spread over km s. A Kolmogorov-Smirnov (K-S) test reveals the probability of the two optical depth distributions being drawn from the same sample is . This rules out the null hypothesis of similar Mgii optical depth distributions as a function of inclination at the 99.91% confidence level (). The data are suggestive that Mgii absorption velocity spread may be a function of galaxy orientation; galaxies with higher inclination have Mgii absorption with larger velocity spreads and more evenly distributed optical depths (on average at any given velocity). However, since the number of galaxies per inclination bin is small, we need to acquire a larger sample to see if the trend holds.
A trend with inclination might be expected if the absorbing gas kinematics is well represented by a monolithic rotating halo. Mgii absorption velocity spreads in four out of the five galaxies in the Steidel et al. (2002) sample were shown to be consistent with a monolithic rotating halo model that allowed for lagging rotation with increasing height above the disk. Given the apparent kinematic trend with inclination, we investigate whether the lagging halo model (Steidel et al., 2002) can successfully predict the Mgii absorption velocity spreads of our ten galaxies.
4. Galaxy Kinematics and Halo–Disk Models
We apply the simple halo model of Steidel et al. (2002) to our systems in order to determine whether an extended disk–like rotating halo is able to reproduce all or most of the observed Mgii absorption velocity spread. The model is a co–rotating disk with velocity decreasing as a function of scale height.
The line of sight velocity, , predicted by this disk halo model is a function of the measurable quantities , , (the angle between the galaxy major axis and the quasar line of sight), and , which is the maximum projected galaxy rotation velocity,
where the free parameter, , is the lagging gas velocity scale height and where is the projected line of sight position above the disk plane. The parameter represents the position at the projected mid–plane of the disk. The range of values is constrained by the model disk–halo thickness, , such that . The distance along the line of sight relative to the point were it intersects the projection of the disk mid–plane is then . There are no assumptions about the spatial density distribution of Mgii absorbing gas, except that is the effective thickness of the gas layer capable of giving rise to absorption.
In order to maximize the rotational velocity predicted by the model, we assume kpc, which effectively removes the lagging halo velocity component (such that the exponential in Equation 4 is roughly equal to unity).
In Figure 8, we show the Mgii absorption profiles for each galaxy, where the shaded regions indicate detected absorption. Below each absorption profile is the disk halo model velocities as a function of derived for each galaxy (solid line) using Equation 4 and parameters in Table 6. Recall that, at kpc, the model line of sight intersects the projected mid–plane of the galaxy. The dashed curves represent the disk halo model velocities derived from the combination of the minimum and maximum uncertainties in the and . In some cases (see Figure 8) the values of the and are well determined such that the dashed curves lie on the solid curves. The model also predicts the line of sight position, , of the halo gas at each velocity, .
The disk halo model is successful at predicting the observed Mgii absorption velocity distribution when the solid (or dashed) curves span the same velocity spread as that of the Mgii absorption gas. The model curves must occupy the full shaded region to be 100% successful. If this is not the case, one can conclude that disk–like halo rotation is not the only dynamic mechanism responsible for the Mgii kinematics. In the following subsections we discuss the disk model of the individual galaxies.
4.1. Q0002+051 G1
The galaxy G1 exhibits a low level velocity shear. Given the velocity spread of the gas ( km s), it is impossible for the bulk of the absorption gas to be consistent with the observed velocities of G1. In Figure 8, we see that the galaxy disk halo model is counter rotating with respect to the dominate saturated Mgii component. There is no overlap between the predicted halo model velocities with those of the Mgii and Mgi absorption. Even if the galaxy had a highly significant velocity shear, the bulk of the Mgii clouds would not be consistent in velocity space. Given the number of high velocity components, it is unclear that this absorption profile represents a gravitationally bound gaseous galactic halo.
4.2. Q0229+131 G1
The galaxy G1 has an asymmetric rotation curve with the largest rotation velocity observed in the direction of the quasar line of sight. In Figure 8, we see that the disk halo model velocities are consistent with the bulk of the Mgii absorption. Thus, extended disk–like halo rotation could be invoked to explain most of the observed halo gas velocities. However, there remains a small single Mgii halo cloud km s from systemic that that cannot be explained by halo rotation alone. This suggests other dynamic processes give rise to some of the Mgii absorption.
4.3. Q0450-132 G1, G2
Galaxies G1 and G2 are potentially interacting galaxies, as evident from their morphologies and strong emission lines. It is possible that these interactions are an effective mechanism in producing extended Mgii absorption in the halo (Bowen et al., 1995; Kacprzak et al., 2007; Rubin et al., 2009). In Figure 8, we plot the disk halo models for both galaxies. G1, the galaxy closest to the quasar, has model halo gas kinematics that are counter–rotating with respect to the Mgii absorption. G2, on the other hand, has modeled halo velocities that are consistent with those of the Mgii absorption. The halo model of G2 is also consistent with the bulk of the Mgi. Given that G1 was observed along the major axis and G2 was not, if we assume that G2 had comparable rotation speeds as G1, the halo model velocities would overlap with most of the absorption velocities. We will discuss the difficulties of disentangling these multiple galaxy systems in § 4.9.
4.4. Q0454-220 G1
Galaxy G1 has a symmetric rotation curve that completely flattens out at the maximum velocity of km s. In Figure 8, we see that the disk halo model has velocities consistent with the bulk of the Mgii absorption velocities. They are also consistent with the Mgi absorption velocities. However, there is an inconsistency of km s between the halo model and Mgii absorption velocities. Thus, the halo model is unable to reproduce the total observed spread of Mgii absorption velocities.
4.5. Q0836+113 G1
The galaxy G1 exhibits minimal rotation; the velocities are more indicative of a global shear. The galaxy systemic velocity is centered roughly in the middle of the Mgii and the bulk of the Mgi absorption profiles. In Figure 8, we see that the halo model also shows little rotation, roughly km s. If the model was a true representation of the halo, then more than 50% of the absorbing gas has velocities inconsistent with disk rotation that are larger than the model velocities. Even if G1 has a more significant velocity shear, the model would still not be able to explain the gas blue–ward of the galaxy systemic velocity. Here, the models fails to predict the bulk of the absorption velocities.
4.6. Q1127-145 G3
The galaxy G3 appears face–on, however it exhibits a maximum rotation of km s. The model inclination of is likely incorrect given the observed rotation velocities. In Figure 8, the disk halo model for G3 exhibits little line of sight velocity ( km s) given its orientation with respect to the quasar line of sight. Given the model parameters listed in Table 6, we varied the galaxy inclination, such that , in an attempt to reproduce the observed absorption velocities. Even with , the halo model fails to reproduce the observed Mgii absorption velocities. For the disk halo scenario, the halo gas is expected to reside to one side of the rotation curve. The nature of this Mgii absorption profile is interesting; two weak clouds separated by km s. It is likely that these clouds could arise in either a patchy diffuse halo or the line of sight is intercepting small scale structure near the galaxy halo. In any case, the model is unable to reproduce the observed Mgii absorption velocities.
4.7. Q1127-145 G1,g2
Galaxies G1 and G2 are potentially interacting galaxies, as evident from their morphologies. These galaxies appear to be in a small group including G4. In Figure 8, we plot the disk halo models for G1 and G2. The model for G1 has velocities that are consistent with up to km sspread of the saturated component of the Mgii and all of the Mgi absorption. The disk halo model of G2 is counter–rotating with respect to G1 as viewed from the quasar line of sight. The model velocities are consistent with the remaining absorption of the saturated component which was not covered by G1. The model velocities of both G1 and G2 overlap km s. The second weaker kinematic component, km s blue–ward of systemic velocity of G1, cannot be explained given the predicted halo velocities. It is possible that the saturated component of the Mgii absorption could arise from either G1, G2, or both. The weaker component may arise from tidal debris stirred up by the apparent interactions. It is also possible that some of the Mgii absorption is associated with G4. It is likely that the saturated component is associated with a nearby galaxy since it is a DLA and many other low ions have also been detected. We will discuss the difficulties of disentangling these multiple galaxy systems in § 4.9. In any case, the model is mostly successful, except that it does not reproduce all of the Mgii absorption velocities.
4.8. Q2206-199 G1
The G1 galaxy is the largest in our sample, and the second furthest away from the quasar line of sight. In Figure 8, the disk halo model shown has very little line of sight velocity ( km s). It is clear that the model of this moderately inclined galaxy does not reproduce the observed absorption velocities. In fact, the bulk of the Mgii and Mgi has velocities km s greater than the galaxies maximum observed rotation velocity. This is peculiar, since the dominant saturated component is commonly expected to be associated with the galaxy disk, yet the kinematics here suggest otherwise. This galaxy–absorber pair is a another example demonstrating that disk–like halo rotation cannot be the only mechanism driving the kinematics of halo gas.
4.9. Mgii Absorption from Galaxy Pairs/Groups
Since Mgii absorbers were first associated with galaxy halos, it has been common practice to associate one galaxy with an absorption system at a given redshift (e.g., Bergeron & Boissé, 1991; Steidel, Dickinson, & Persson, 1994; Churchill, Steidel, & Vogt, 1996; Guillemin & Bergeron, 1997). It is now becoming evident that Mgii absorption also arises in small groups of galaxies (Bowen et al., 1995; Churchill & Charlton, 1999) and even clusters (Lopez et al., 2008). In this paper, we present two such examples: Q G1, G2 and Q G1, G2, G4. The pair of galaxies in Q are close to each other in both projected distance and velocity. Both galaxies have morphological evidence (one sided tidal tails) that is classically associated with interacting/merging galaxies. The Q field contains three galaxies at similar redshift (we have recently spectroscopically identified two additional galaxies with similar redshifts within kpc from the quasar [Kacprzak et al., in prep]). The two galaxies studied here, G1 and G2, have evidence of morphological perturbations and extended tidal material.
The Q and Q galaxy pairs, have observed rotation velocities that overlap with those of the Mgii absorption. Given these two fields, it is clear that it can be difficult at times to associate one particular galaxy with an absorption system. One alternative interpretation is that the material responsible for the absorption is tidal debris originating from both galaxies due to past mergers and harassments (similar to the Magellanic stream). Given that the galaxies are in close proximity (projected), it is also possible that these galaxies share a common gas structure that gives rise to the absorption.
4.10. Summary II: Disk Halo Model
In an effort to reproduce the Mgii absorption velocities, we used a disk halo model to compute the expected absorption velocities. In only one case, Q G1, we were able to reproduce the full spread of the Mgii absorption velocities in a disk halo model. In four other cases, Q–, Q– (G2), and Q– (G1 and G2), the velocity region with the strongest absorption is consistent with the model. However, the halo model of the galaxy G2 toward Q– does not account for roughly 35% of the absorption. In the cases of both Q and Q, the models cannot account for the unsaturated small cloud structures at higher velocities relative to systemic. For each case, the models do reproduce some of the absorption velocities, however, the disk rotating halo model is insufficient to account for the full observed Mgii absorption velocity range.
We emphasize that the disk halo model applied to the data is an extreme version of the spatial–kinematic relationship in that (1) all the gas is assumed to rotate at the maximum observed velocity of the galaxy, and (2) the scale height of the models ( kpc) is highly unrealistic. These extreme conditions were required in order to obtain the greatest degree of agreement between the model and the data. If these conditions are relaxed, the level of agreement we found is diminished substantially. None the less, even under these extreme and unrealistic model parameters, the disk halo model provides insight into the degree at which rotation kinematics can account for limited regions of the absorption velocity spread.
What we learn from the disk halo model is that it is reasonable to suggest that additional dynamical processes (such as infall, outflow, supernovae winds, mergers, etc.) and/or additional satellites or unidentified galaxies giving rise to some of the Mgii absorption contribute to the observed velocity spreads. The possibility of unidentified galaxies is difficult to quantify, for the assigning of a given galaxy to a given absorption system is by its very nature not 100% certain. It would, however, require the galaxies we have assigned to not be a significant contributor to the absorption and/or the unidentified galaxy to have very extended absorbing gas. The former statement is not strongly supported by a body of previous studies (e.g., Bergeron & Boissé, 1991; Le Brun et al., 1993; Steidel, Dickinson, & Persson, 1994; Guillemin & Bergeron, 1997; Steidel et al., 1997; Churchill, Kacprzak, & Steidel, 2005; Tripp & Bowen, 2005; Zibetti et al., 2007; Chen & Tinker, 2008; Tinker & Chen, 2008) The later statement is based upon the detected galaxies in the quasar fields. The statement would not apply to putative galaxies below our detection limit that are in close projection to the quasar.
5. CDM Cosmological Galaxy Simulations
To further understand the halo gas dynamics and the mechanisms driving the observed gas velocity spread, we investigate high resolution cosmological simulation of galaxy formation, which include the dynamical processes of infall, outflow, supernovae winds, mergers, etc. Simulations provide the only theoretical means to fully incorporate these dynamical processes in a cosmological setting. We use the method of quasar absorption lines through the simulations to “observe” the Mgii absorption kinematics. Here we analyze a single simulated galaxy in detail to study the possible structures and mechanisms that give rise to the observed Mgii halo gas kinematics. By comparing halo gas kinematics in the simulations to the spatial and dynamic processes of the simulated galactic environments, we can gain further insights into the observed Mgii absorption properties.
5.1. Description of The Simulations
The CDM cosmological simulations are performed using the Eulerian Gasdynamics plus N–body Adaptive Refinement Tree (ART) code (Kravtsov, 1999, 2003). Physical processes implemented in the code include star formation and stellar feedback, metal enrichment from type II and Ia supernovae, self–consistent advection of metals, and metallicity–dependent cooling and photoionization heating due to a cosmological ultraviolet background (Haardt & Madau, 1996). The code does not include radiative transfer, magnetic fields or Kelvin-Helmholtz instabilities. The cosmological model has , , and .
These simulations have high star formation efficiency at the resolution scale. This assumption is motivated by studies of star formation in simulations of isolated disks with a similar resolution (Tasker & Bryan, 2006). The gas consumption time-scale is yr for the star-forming cells, however, only a small fraction (1%) of the gas in the disk is forming stars in a typical time step of 40 Myr at z1. As a result, the disk-averaged star formation efficiency is low: the gas consumption time-scale is yr, consistent with observations (Kennicutt, 1998).
The computational region is a 10 Mpc co–moving box. We apply a zooming technique (Klypin et al., 2001) to select a Lagrangian volume of three viral radii centered in a MW–size halo at redshift . The volume is then re–simulated from with higher resolution and hydrodynamics. The high-resolution region has a radius of about 1.5 co-moving Mpc, and has about dark matter particles with M per particle. The volume is resolved with about hydrodynamic cells with different levels of resolution.
The combined effect of stellar winds and supernova explosions at the resolved scale (Ceverino & Klypin, 2009) prevent the over–cooling problem of galaxy formation at high redshifts (White & Frenk, 1991) and reduce the angular momentum problem found in early simulations (Navarro & Steinmetz, 2000). This results in models with flat rotation curves consistent with observations (Ceverino & Klypin, 2009), and is achieved without typical ad–hoc assumptions about the physics at sub-resolution scales. These high resolution simulations allow us to resolve the regime in which stellar feedback overcomes the radiative cooling. By resolving this regime, simulations naturally produce galactic scale outflows in star-forming galaxies (Ceverino & Klypin 2010, in preparation) and galaxy formation proceeds in a more realistic, although violent way, through a combination of cold flows accretion, mergers, and galaxy outflows.
5.2. Simulated Spectra
To study the Mgii absorption arising in the gas halos within the simulations, we employ the following methods. For a given gas cell probed by a line of sight through the simulation box, the total hydrogen density, temperature, and metallicity is used to obtain the Mgii ionization fraction assuming photoionization conditions. Post simulation, we use Cloudy (V96b4, Ferland, 2001) with the Haardt & Madau (1996) UV background spectrum at the appropriate redshift. The line of sight the redshift, Mgii column density, and Doppler parameter (assuming thermal broadening) are computed for each cell.
Absorption spectra with the instrumental and noise characteristics of the HIRES spectrograph are generated assuming each cell gives rise to a Voigt profile at its line of sight redshift. We give each spectrum a signal–to–noise ratio of 50 per pixel, which corresponds to a limiting equivalent width detection of 0.005 Å for unresolved lines. The spectrum for each sightline is then objectively analyzed for detectable absorption above the equivalent width threshold of 0.02 Å, which corresponds to cm for km s. The mean optical depths (mean redshifts), rest–frame equivalent widths and velocity widths, and other quantities are then measured (see Churchill & Vogt, 2001). The velocity zero point of the simulated absorption lines is set to the line of sight velocity of the simulated galaxy (center of mass of the stars).
To examine the 3D spatial and kinematic properties of gas giving rise to Mgii absorption, we identify Mgii “absorbing gas cells” along each sightline as those which contribute to detected absorption in the simulated spectra; they are defined as cells that align within the range of line of sight velocities of the absorption. We account for multiple kinematics subsystems (Churchill & Vogt, 2001), regions of absorption separated by continuum.
5.3. Discussion of Simulated Galaxy Observations
We focus on a single typical galaxy at . The galaxy star formation rate is M yr. The galaxy has a maximum rotation velocity of 180 km s when observed edge on. Based upon the the Tully–Fisher relation, we derive a luminosity of . The average luminosity for our sample is excluding the galaxy. Thus, this simulated galaxy is well representative of our observational sample.
The simulated galaxy is probed with with a square grid of sightlines at intervals of 7.5 kpc that span kpc to kpc on the “sky” and Mpc along the line of sight444For this and all subsequent discussions, spatial quantities are quoted as proper lengths. There are 900 total sightlines. The highest resolution of the adaptive mesh at is pc. The gas contributing to detectable Mgii absorption is found in a range of cell resolutions from pc with the majority of the gas arising in cells of resolution 905 pc. We examine simulated quasar lines of sight for this galaxy with three different inclinations, (face–on), , and (edge–on).
In Figure 9, we show the integrated total hydrogen column density, N(H), over a kpc cube for the edge-on view of the galaxy. There is no absorption detected along the line of sight outside of this cube. The galaxy is clearly not in isolation. In the image, filaments and tidal stream material can be seen. Several low mass satellites galaxies are also in the process of interacting with the main galaxy. We have superimposed squares over the sightlines where Mgii absorption was detected in simulated quasar spectra. Increasing square sizes indicate the absorption strength in four bins: , , , and . Out of the 900 lines of sight, Mgii absorption was detected in 87 for the edge–on case. Note that the absorption is not distributed ubiquitously on the sky around the galaxy, but traces the various structures around the galaxy. The covering fraction of the Mgii gas is low (%).
In Figure 10, we show the integrated neutral hydrogen column density, N(Hi), over the same cube. The N(Hi) range from roughly to cm. The structures that are associated with absorption stand out more in Hi. We see as expected (Churchill et al., 1999) that Mgii absorption is detected in regions with N(Hi) cm. Regions in which N(Hi) cm that fall between the line of sight grid sampling are also expected to produced Mgii absorption. Note that as one goes to lower N(Hi), the gas covering fraction increases.
In Figures 11–12, we present N(H) and N(Hi), respectively, for the face–on view of the galaxy. Out of the 900 lines of sight, Mgii absorption was detected in 96. The covering fraction remains roughly 10%. Again, the absorption primarily arises in streams and filaments.
We do not show the 45 degree view of the galaxy here. Out of the 900 lines of sight, Mgii absorption was detected in 124, and the covering fraction is roughly %.
Overall, there are many complicated structures that reside within what we classically call a halo. Note that we find pockets of DLA Hi column densities [N(Hi) cm] out to kpc. Although, the covering fractions of these dense regions are low, their presence suggests that it is possible to observe DLA absorption at high impact parameters. These large impact parameter DLA systems arise from the inner regions of satellite galaxies. These satellites have a luminosity range . Assuming standard K-corrections, their apparent magnitude in the HST F702W filter range between . These DLA producing satellites are well below the typical detectability of WFPC–2/HST and ACS/HST. Also note that the satellite in the upper right corner of Figure 12 has an Hi morphology similar to galaxies seen on their first pass through a cluster, where gas is being stripped (e.g., Chung et al., 2007).
Focusing on the two small satellite galaxies and tidal stream in a kpc region of the lower left quadrant of Figure 10, we present an example of simulated Mgii absorption spectra in Figure 13. The velocity zero point of the simulated spectra is set to the galaxy systemic velocity. The absorption profiles are quite similar to those detected in Mgii surveys. The simulated profiles have velocity spreads of km s, some comprising multiple kinematic components and complex structures. The Mgii absorption spans across the galaxy systemic velocity or resides blueward of the galaxy systemic. Absorption arising near the galaxy systemic velocity or entirely to one side is consistent with the majority of our observational data.
In Figure 14, we show the Mgii absorption spectra over the kpc central region of the edge-on galaxy shown in Figure 10. Individual absorption profile velocities, in the inner regions of the galaxy, show a variety of complex kinematics and optical depths. In our observational sample of galaxies, we have no galaxy–absorber systems that have impact parameters less than kpc. In the simulations, the inner kpc contains some km s outflowing gas. The outflows are not strong, but their signatures are reflected by the complex kinematics of the absorption profiles in the inner regions. The reason why there is a line of sight near the center of the galaxy that does not produce Mgii absorption is because it is dominated by K gas. The profiles reduce in kinematic complexity rapidly as impact parameter increases.
To the degree that the simulations reflect the reality of the gaseous environments around galaxies, we find that Mgii absorption arises in many types of structures (metal enriched filaments, minor satellites, tidal streams, and within the region of the galaxy itself). As inferred from our simulated absorption line survey, and guided by analysis of the 3D simulations (examples provided by Figures 9–14), the simulations are not suggestive of Mgii absorption arising from spherical or disk–like halos with uniform gas covering fraction. We revisit the spatial and kinematic distribution of the absorbing gas in § 5.5.
5.4. Disk Halo Models of The Simulated Galaxy
Given the structures shown in Figures 9–12, it would seem to be unrealistic to treat the halo as a monolithic thick disk. However, we have modeled all the simulated Mgii absorption profiles for each of the three galaxy inclinations with the disk halo model (§ 4).
We find that for the edge–on view, out of 87 lines of sight with detected Mgii absorption, 45% have kinematics consistent with the model (the full range of velocities can be explained). For , out of 124 lines of sight, 26% have kinematics consistent with the model. For the face–on view, out of 96 lines of sight, only one is consistent with the model. This is not surprising, since the projected maximum rotation velocity is small compared to the absorption spreads.
The conclusion to be drawn here is, that even if the halo gas does not rotate as a monolithic disk in the simulations, Mgii absorption detected along some lines of sight can still appear to be consistent with a disk halo model.
5.5. Halo Gas Spatial and Velocity Distribution
To understand what kinematic mechanisms are responsible for the Mgii absorption velocity spreads measured in the simulated spectra, we examine the velocities and spatial distributions Mgii absorbing gas.
In Figure 15, we present the 3D spatial distribution of N(Mgii) cm gas contributing to the Mgii absorption along the lines of sight. The edge–on galaxy is located at the origin and the lines of sight are parallel to the x–axis. Gas with N(Mgii) cm is not shown for clarity (which corresponds to an equivalent width limit of 0.012 Å for km s). From this view, the tidal streams and filaments can be visually discerned. There are also isolated clouds that produce Mgii absorption. Regions that contribute to absorption have physical sizes of pc (simulation resolution) to kpc. All the absorption along the lines of sight occurs within kpc from the center of the galaxy.
The gas is color coded as a function of line of sight velocity relative to the galaxy systemic velocity. Gas that is colored red (blue) is moving away from (towards) the observer. The gas has line of sight velocities ranging from km s. The line of sight velocity dispersion in the inner regions near the galaxy appears inconsistent with disk rotation. The somewhat randomized velocities reflect the winds. Upon carefully examining the gas velocities along a particular line of sight, velocity gradients can be observed. For example, the region seen in Figure 15 at kpc, has velocity gradients of km s.
In Figure 16, we present the same three dimension spatial distribution shown in Figure 15, except that the gas is color coded as a function of radial velocity relative to the galaxy. Gas colored red (blue) has a radial velocity component that is outflowing from (infalling towards) the galaxy. This combined spatial and kinematic representation provides an holistic view of the halo dynamics. The gas has radial velocities ranging from km s. If one focuses on the filament structure ( kpc, kpc) in the plane of the galaxy, one can see the strong velocity gradient. The gas along the filament far from the galaxy increases in infall velocity from to km s as it approaches the galaxy center. The same dynamics can be seen along the tidal stream ( kpc, kpc) originating from the two small satellite galaxies.
In Figure 17, we show the probability distribution of the total gas velocity of the Mgii absorbing gas cells. What we call the probability distribution is the area normalized frequency distribution that we detected in the simulations. The total velocity is the magnitude of the gas velocity vector relative to the galaxy. Since our observational data have only impact parameters greater than kpc, we exclude all absorbing gas within kpc of the simulated galaxy. The velocities range from km s with a peak at km s and secondary maxima at km s. In Figure 17, we show the probability distribution of radial velocities of the Mgii absorbing cells. The velocities range from km s with a maximum at km s. It appears that, beyond kpc, most of the gas is infalling towards the galaxy and very little is outflowing. In Figure 17, we show the ratio of the radial to the total velocity. The bulk of the gas is dominated by radial infalling velocities.
Drawing from the number of lines of sight we have through the simulations, we produced the probability distribution of absorption velocity offsets from the galaxy systemic velocity ( km s) using the simulated absorption profiles. The quantity is obtained by calculating the optical depth weighted mean of the profiles (the velocity at which there is equal optical depth to both sides along the profiles). In Figures 18, and , we show the velocity offset probability distributions for the edge–on, , and face–on orientations, respectively. For the edge–on case, the absorption velocity offset ranges from km s with a strong peak around km s; it is highly probable to detect absorption to one side of the galaxy systemic velocity. For the case, the velocity spread increases to about km s and develops multiple peaks at km s and at km s. The probability of detecting gas at the galaxy systemic velocity is significantly smaller than over the range km s. For the face-on case, the velocity dispersion is still around km s. Again, it is unlikely that absorption will be detected at the galaxy systemic velocity.
Since most of the Mgii gas arises between km s, with very little at the galaxy systemic velocity, and since most projected galaxy rotation curves have maximum velocities of km s, it may not be a surprise to observe Mgii absorption aligned with the observed galaxy rotation curve. These results are consistent with the findings of Bouché et al. (2007), who detected galaxy emission within km s of the optical depth weighted mean Mgii absorption redshift.
5.6. Shortcoming of the Simulations
The technique of quasar absorption lines through cosmological simulations is one of several promising approaches to understanding the dynamics of galaxy halos. At the present time, simulations of galaxy formation in the cosmological context still need to achieve greater accuracy for modeling stellar feedback. For this study, we employed a feedback recipe that successfully results in extended metal enriched gas around galaxies. These simulations result in an equivalent width distribution with an under abundance of larger equivalent widths and a relative over abundance of smaller equivalent widths. They also under predict the observed Mgii mean absorption covering fraction range of (Tripp & Bowen, 2005; Chen & Tinker, 2008; Kacprzak et al., 2008; Barton & Cooke, 2009). The covering fractions for all absorption above 0.02 Å are as follows: (1) (edge–on): total 10%, weak 6%, strong 3%; (2) (face–on): total 10%, weak 6%, strong 3% (3) : total 14%, weak 10%, strong 5%.
These mismatches with observations could either be a result of the method in which Mgii column densities are determined in the simulations or observational biases. In the simulations, the determination of the Mgii ionization fraction may be underestimated due to the fact that we do not account for shielding of UV photons in the ionization corrections. It is also possible that the resolution of the simulations may influence the derived Mgii column densities and that higher resolution may in fact lead to higher column densities. We aim to analyze such issues in future work. A possible observational biases that galaxies are selected, identified, and assigned to already known absorption systems. This may elevate the inferred covering fraction (e.g., Tripp & Bowen, 2005). We do emphasize, however, that in the simulations we do detect Mgii absorption out to kpc, as seen in current observations (Churchill, Kacprzak, & Steidel, 2005; Zibetti et al., 2007; Kacprzak et al., 2008).
As an additional caveat, we also remark that the experiment to examine the spatial and kinematic relationship between the galaxy and the Mgii absorbing structures in the simulations is very different than the observational experiment in one regard. We examine a grid of sight lines through a single simulated galaxy, which is in a unique environment and undergoes a unique evolution in the IGM. The observational data, on the other hand, sample a single line of sight through various galaxies in various environments and with various evolutionary histories and with random orientation through the galaxy and environment.
We re–emphasize that the simulations, as we have applied them here, provide a first view of the types if physical structures and their spatial and kinematic relationship to a galaxy in the cosmological environment that give rise to Mgii absorption. Though the details of the Mgii absorbing properties are not yet tuned to observations (and we mention that our experiment is only for a single galaxy using an incomplete sampling of its halo– a full comparison with survey statistics is not entirely applicable), the dynamical structures themselves in the simulations are robust. That we find some parts of these structures give rise to Mgii absorption is not outside expectation given the densities and temperatures of the gas. The upshot is that, given these discrepancies and concerns, the simulations are not expected to provide a fully quantitative comparison with the data. However, the simulations do provide a fully self–consistent galaxy model from which we acquired valuable insights for interpreting Mgii absorption line observations.
5.7. Summary III: Galaxy Simulations and Halo Gas Distributions
In our study of galaxy at three inclination angles with respect to the simulated quasar lines of sight, we have detected Mgii absorption in a variety of structures. Mgii absorption was detected in inflowing metal enriched filaments, tidal streams, small satellites, and gas around the host galaxy. The absorption resides in a “halo” of about 100 kpc in size. The types of structures that form in the simulations (see Figures 9–12) are a challenge to models in which the absorption is assigned to thick disks (e.g., Charlton & Churchill, 1996) and symmetrically distributed halos (e.g., Mo & Miralda-Escude, 1996). The structures also may provide further guidance for expanding upon radial density dependent halo occupation models (e.g. Tinker & Chen, 2008). The spatial distribution of filaments, tidal streams, and satellite galaxies are asymmetric, patchy (low volume filling factor), complex, and part of a cosmological setting.
DLA Hi column densities are seen out to kpc, and arise in low mass satellite galaxies. These galaxies are below the detection limits of even deep HST images. Although, the covering fractions of these dense regions are low, this might explain why some bright galaxies at DLA redshifts are found at large impact parameters (e.g., the 3C 336 DLA at kpc; Steidel et al., 1997).
In the simulations, the kinematics are closely coupled to the gas structures. Metal enriched tidal streams and filaments are dominated by infall to the central galaxy, and these structures are selected by Mgii absorption (see Figures 15 and 16). In fact, we find that gas giving rise to the Mgii absorption is dominated by inflow (see Figure 17). Inflow velocity gradients are apparent, such that the infall increases as the gas approaches the galaxy. This inflow is not spatially symmetric.
Similar to the observations, the velocity and spatial distributions from the simulations conspire to give rise to Mgii absorption to one side of the galaxy systemic velocity even though the absorbing gas is not rotating with the star forming component (see Figures 13 and 14). The velocity offset probability distribution (relative to the simulated galaxy) spans km s with lowest probability of detecting Mgii at the galaxy systematic velocity (see Figure 18). Thus, the fact that we observe Mgii absorption velocities consistent with the galaxy rotation curves may be a natural consequence of the spatial and kinematic distributions of gas in complex environments surrounding galaxies.
The current feedback recipes used in the simulations successfully produces in extended metal enriched halo gas around galaxies. However, for this single galaxy we derive covering fractions that are lower than the current observed mean values of (Tripp & Bowen, 2005; Chen & Tinker, 2008; Kacprzak et al., 2008; Barton & Cooke, 2009). In this particular galaxy halo, we also find an under abundance of larger equivalent widths and a relative over abundance of smaller equivalent widths. We have yet to determine if some of these mismatches are due to observational biases in current studies or issues with the feedback/baryon physics implemented in the simulations. However, the structures in the simulations still provide valuable insight for interpreting Mgii absorption line observations.
6. Large Scale Galactic Winds: Observations and Simulations
In our analysis of the simulated galaxy, we do not find systematic rotation near the galaxy, as was suggested by Lanzetta & Bowen (1992), but that the gas kinematics reflects processes such as winds and chimneys. These are characteristic of stellar feedback processed that successfully circumvent the over cooling and angular momentum problems that have plagued simulations (Ceverino & Klypin, 2009).
Observationally, Weiner et al. (2009) has inferred outflows in Mgii absorption associated with star–forming galaxies ( M yr). The outflow velocities are proportional to . The star formation rate in our simulated galaxy is M yr. Calibrating to their observations, our galaxy is expected to have winds in the range km s. Our simulated Mgii profiles indicated some outflowing gas with velocities of km s in the inner kpc (independent of galaxy disk inclination). The absorption profiles produced by the outflows are saturated and span both sides of the systemic velocity of the simulated galaxy (see Figure 14). Thus, the saturated Mgii absorption profile that spans both sides of the galaxy systemic velocity might be direct signature of outflows.
Two galaxies in our sample, Q G1 and Q G1, have saturated profiles that span both sides of the galaxy systemic velocity. These saturated, symmetric profiles are also observed at high redshift () and span the galaxy systemic velocity. The absorption is attributed to large scale galactic outflows which can be detected at least out to impact parameters of kpc (Steidel et al. 2010, in preparation). The outflows are determined to be more or less symmetric and radial, giving rise to symmetric absorption profiles. These high redshift galaxies do not exhibit substantial kinematic structure and are instead consistent with dispersion-dominated velocity fields around km s (Law et al., 2007), similar to the shear observed for Q G1 and Q G1. Given the similarities between these two galaxies and those at high redshift observed to have outflows, one possibility is that large scale outflows are responsible for the absorption velocities associated with both galaxies. To explore the outflow scenario for Q G1 and Q G1, we examine the galaxy star formation rates.
For Q G1, we measure a [Oii] line flux of erg s cm. We use the relation from Kewley et al. (2004) to obtain a star formation rate of 6.5 Myr. For Q G1 we determined the star formation from the UV flux at 1700 Å which was derived from the –band magnitude of the galaxy (Steidel, Dickinson, & Persson, 1994). The robustness of this method has been demonstrated by Erb et al. (2006). From the measured UV flux of erg s cm, we derive a SFR of 6.5 Myr. The SFRs are not corrected for the internal extinction of the galaxies and are thus lower limits.
Using the results of Weiner et al. (2009), both galaxies are expected to have outflows with velocities in the rage km s. Both galaxies are at impact parameters of kpc, which is near the boundary of where we lose the signature of outflows for our simulated galaxy. Although Q G1 and Q G1 have Mgii absorption velocity widths of 290 and 470 km s, respectively, which are comparable to the outflow velocity range predicted from the results of Weiner et al. (2009).
Heckman (2002, 2003) discusses that the star formation per unit area is a more relevant indicator of galaxy outflows. These outflows are ubiquitous in galaxies where the global star-formation rate per unit area exceeds M yr kpc, where the area is defined by the half light radius of the galaxy. This criteria applies to local starbursts and even high redshift Lyman Break galaxies. The ISM entrained in the winds have outflow speeds of to km s. For Q G1 we obtain a M yr kpc. For Q G1 we obtain a M yr kpc. Thus, both of the galaxies are expected to have outflow signatures.
We estimate that the simulated galaxy has a M yr kpc, which is slightly less that the criterion of Heckman. This is consistent with our outflow velocities derived from the Mgii absorption profiles since we do not see evidence of strong large scale outflows. This particular simulated galaxy may not be well representative of the Q G1 and Q G1 galaxies.
In summary, galaxies Q G1 and Q G1 are kinematically similar to high redshift absorption selected galaxies. The SFRs and s for both galaxies exceed the limits were strong outflows are expected. Given the large impact parameters that outflows are detected at high redshift, it is quite possible the observed Mgii absorption kinematics for galaxies Q G1 and Q G1 are signatures of outflowing gas.
We have examined and compared the detailed galaxy and Mgii absorbing kinematics for a sample of 10 intermediate redshift galaxies. The galaxies have a wide range of inclinations and orientations with respect to the background quasar. The galaxy–quasar impact parameters range from kpc. The galaxy rotation curves were obtained from ESI/Keck spectra and the Mgii absorption profiles were obtained from HIRES/Keck and UVES/VLT quasar spectra. In an effort to compare the relative kinematics, we used a thick disk halo model to compute the expected absorption velocities through a monolithic gaseous halo.
To obtain theoretical insights into the gas dynamics and spatial distribution of halos, we used the technique of quasar absorption lines to analyze Mgii absorption around a galaxy in a high resolution cosmological simulation of galaxy formation. The galaxy was probed with a square grid of sightlines at intervals of 7.5 kpc that span kpc to kpc for a total of 900 sightlines. We examined this galaxy at three different inclinations, face–on, , and edge–on.
Our mains results can be summarized as follows:
For all ten galaxies, the velocity of the strongest Mgii absorption component lies in the range of the observed galaxy rotation curve. In seven of ten cases, the Mgii and Mgi absorption velocities reside fully to one side of the galaxy systemic velocity. The strongest absorption usually aligns with one arm of the rotation curve. In the three remaining cases, the absorption velocities span both sides of the galaxy systemic velocity. Two of those three (Q G1 and Q G1) have strong saturated absorption on both sides of the galaxy systemic velocity. The third (Q G3), has two very weak clouds, and therefore probes low column density gas.
For galaxies Q G1 and Q G1, we have determined that large scale galactic outflows might be giving rise to the observed Mgii absorption kinematics. Both galaxies have a M yr and M yr kpc and M yr kpc, respectively. These SFRs and s are typically found for galaxies exhibiting outflow velocities of several hundred km s. The Mgii absorption velocities associated with the two galaxies span both sides of their systemic velocity. Such profiles have been interpreted, in both our simulations and at high redshift, as signatures of outflows.
We find that the observed Mgii absorption velocity spread and optical depth distribution may be a function of galaxy inclination. Galaxies with higher inclination exhibit a Mgii absorption velocity spread of km s with a somewhat even distribution of optical depths, whereas, galaxies with lower inclinations exhibit a narrower velocity spread of km s with a clear optical depth peak at km s. A K-S test shows that the Mgii optical depth distributions for the high and low inclination bins are not consistent at the level. These results suggest that the absorbing gas is either disk–like or the spatial distribution and kinematics of the structures producing the absorption (i.e., filaments, tidal streams, satellites etc.) are closely coupled to the disk orientation.
We employed simple rotating disk halo models to examine whether disk–like rotation is consistent with the observed galaxy–gas kinematics. For model parameters that allow for a 1 Mpc gas scale height and maximum rotation velocity (rigid rotation) the the bulk of the observed absorption kinematics can be explained by co–rotation with the galaxy. In all cases, the rotating disk halo models we present are unable reproduce the full spread of observed Mgii absorption velocities. This model is a highly unrealistic representation of galaxy gas. When the parameters are relaxed to better reflect reasonable gas scale heights and a slowing of the rotation speed with height above the disk plane, the relative proportion of the gas velocity spread that can be made consistent with galaxy co–rotation diminishes such that some absorbers cannot have but a tiny fraction explained by co–rotation. In this simple scenario, even if some of the absorbing gas arises in a thick disk, what we learn from the exercise is that some additional type of dynamical process (such as infall, outflow, supernovae winds, etc.) must be invoked to explain the range of absorption velocities hat cannot be made consistent with the simple rotating disk halo model.
In two quasar fields, we find pairs of galaxies that align in velocity within km s of a single, saturated Mgii absorption system. For one case, the observed velocity range of the strong saturated component can be explained by a rotating disk model only if both galaxies contribute to the absorption. This challenges the idea that an individual Mgii absorber can be assigned to a single galaxy, and understood as an isolated halo.
In the simulations, Mgii absorption selects gas structures such as metal enriched tidal streams, filaments, small satellite galaxies, and the region within kpc of the galaxy. Together, these structures extend roughly kpc around the galaxy, suggesting that galaxy “halos” are a complex composite of the these various structures.
For this simulated galaxy the Mgii covering fraction is %, which is below the current observational estimated means of 20–80% (Tripp & Bowen, 2005; Kacprzak et al., 2008; Barton & Cooke, 2009). This may reflect a need for additional tuning of the feedback/baryon physics in the simulations, or indicates current observational biases.
In the simulations, DLA Hi column densities arise in low mass satellite galaxies at impact parameters as large as kpc. These galaxies are below the detection limits of deep HST images. Although, the covering fractions of these dense regions are low, this might explain why some bright galaxies at DLA redshifts are found at large impact parameters.
In the simulations, the majority of the Mgii absorbing gas is infalling in filaments and tidal streams towards the galaxy with velocities between km s. The velocity offset probability distribution (relative to the simulated galaxy) spans km s with lowest probability of detecting Mgii at the galaxy systematic velocity. Thus, observed Mgii absorption velocities can fall within the range of the galaxy rotation curve velocities, even though the gas arises in a variety of kinematics structures.
The gas structures selected by Mgii in the simulations (see Figures 9–12) cannot be described as simple thick disks or spherical halos. If the simulations reflect reality, it would appear that Mgii absorption arises in large kpc halos that are built from the local cosmological environment of a moderate mass galaxy. Complicating the picture is the fact that we find groups and pairs of galaxies that align in velocity within km s of a single Mgii absorption system. This challenges the idea that an individual Mgii absorber can be assigned to a single galaxy or understood as an isolated halo. Though considered subcomponents of halos, smaller scale structures like the Magellanic-type galaxies and tidal streams, may contribute significantly to the detections of Mgii absorption (York et al., 1986; Kacprzak et al., 2007). These considerations lead us to suggest that galaxies and Mgii absorbers should be studied and modeled in a environmental context if they are to be fully understood.
In the simulations, the kinematics are closely coupled to the gas structures (i.e., filaments, tidal streams, small satellite galaxies, and the inner 20 kpc of the central galaxy). As observed in our data, the simulated Mgii absorption velocities fall within the range of the galaxy rotation velocities, and rarely at the galaxy systematic velocity. Thus, the simulations suggest that observing Mgii absorption velocities consistent with the galaxy rotation curves can naturally occur even if the absorption arises in many different structures in the complex environment of the galaxy. It is these structures that comprise halos.
A natural extension of the work presented here would be to perform a similar study (simulations and observations) that incorporates the kinematics of higher ionization Civ and Ovi doublet absorption. These ions probe lower density and/or higher temperature structures and provide a more comprehensive view of the gaseous environment around galaxies. Future observations with the Cosmic Origins Spectrograph are perfectly suited for the galaxy sample presented in this paper. It is also important to expand the number of galaxy environments studied in the simulations.
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