The nature of nuclear emission in LINERs
Key Words.:Galaxies:active, Galaxies:nuclei, Galaxies:evolution,X-ray:galaxies
To get insight in the nature of the ionized gas in the nuclear region of LINERs we have performed a study of HST H imaging of 32 LINERs. The main conclusion from this analysis is that for the large majority of LINERs (84%) an unresolved nuclear source has been identified as well as extended emission with equivalent sizes ranging from few tens till about hundredths of parsecs. Their morphologies appear not to be homogeneous being basically grouped into three classes: nuclear outflow candidates (42%), core-halo morphologies (25%) and nuclear spiral disks (14%). Clumpy structures reminiscent of young stellar clusters are not a common property on LINERs. The remaining 5 galaxies are too dusty to allow a clear view of the ionized gas distribution.
A size-luminosity relation has been found between the equivalent radius of the H emission and the (2-10 keV) X-ray luminosities. Both ionised gas morphologies and the size-luminosity relation are indistinguishable from those of low luminosity Seyferts, suggesting the same origin for the NLR of LINERs and Seyferts. Also a relation between soft X-rays and ionized gas has been suggested for the first time in LINERs. From multiwavelength data, only 4 out of the 32 LINERs have no evidences on an AGN nature of their nuclear sources from multiwavelength data, but extremely obscured AGNs cannot be discarded out given the Compton thick signatures of their X-ray emission. For the confirmed AGN LINERs, their H imaging favour core-halo and outflow morphologies (65% of the cases). Finally, their calculated Eddington ratios show that our LINER sources radiate at sub-Eddington regime, with core-halo systems having on average larger Eddington ratios than outflow candidates.
It has been suggested that Low Ionization nuclear emission-line regions (LINERs, Heckman 1980) are placed at the low luminosity end among the active galactic nuclei (AGN) family (Ho 2008). Although LINERs are found in a large population of nearby galaxies (30%; Ho et al 1997), a debate still does exist on the nature of their energy source.
Ho (2002, 2008) summarizes the main lines of evidence supporting the AGN nature of LINERs: host galaxies properties similar to Seyferts, most of the more massive black holes residing in LINERs, incidence of broad line regions (hereinafter BLR), compact nuclei at both radio and X-ray frequencies. He atributes the large progress made during the last two decades to multifrequency analyses and HST high spatial resolution studies.
The extensive work made by Nagar et al. (2000, 2002, 2005) have shown that radio cores are found in 44% of LINERs, a percentage similar to that observed in Seyferts (47%). Also when radio data at different frequencies exist their spectra tend to be flat as it is expected when non-thermal processes take place.
At X-ray frequencies large progress has also been made thanks to the large X-ray facilities Chandra and XMM-Newton. X-ray observations can be considered of paramount importance, constituting one of the best tools to identify AGN. From the different studies carried out in the last decade (Ho et al. 2001, Eracleous et al. 2002, Dudik et al. 2005, González-Martín et al. 2006 and 2009a), it has been proved that an AGN is present in at least 60% of the LINERs. Moreover when multifrequency information is taken into account (basically the incidence of broad lines and the properties at radio frequencies) the percentage of AGNs rises up to 90% (González-Martín et al. 2006 and 2009a).
On their hand, HST observations have provided a large advance in the physics of LINERS. The pioneering UV imaging surveys by Maoz et al. (1995) and Barth et al. (1998) concluded that 25% of the observed LINERs had an UV compact source in their nuclei. But of course one of the most outstanding results during the last decade has been the discovery that sources with detected radiocores show variability at UV frequencies on month scales (Maoz 2007). Four of their 13 sources (namely M81, NGC 3998, NGC 4203 and NGC 4579) have been confirmed to be variable also at X-ray frequencies (Pian et al. 2010). Recently González-Martín et al. (2010) have also detected X-ray variability for the LINER NGC 4102.
HST optical works (Pogge et al. 2000; Simões Lopes et al. 2007; González-Delgado et al. 2008; González-Martín et al. 2009a) have confirmed that almost all the observed LINERs show a nuclear source on top of an irregular distribution of circumnuclear dust. Dust obscuration can explain the existence of dark-UV LINERs. The importance of an obscuring environment, maybe linked to the acretion physiscs, has been recognized in our X-ray aproach to the nature of LINERs (González- Martín et al. 2009b). We found that a large percentage of them (50%) show clear signs to be Compton thick. This fraction is even larger than that reported for Seyferts (30%) (González-Martín et al. 2009b, Panessa et al. 2006) and so the location and nature of their obscuring matter needs to be further investigated. Until new high resolution X-ray images become available, only indirect information can be obtained on the nature of LINERs by looking for correlations between X-ray properties taken at lower resolution and optical/NIR properties taken at much larger spatial resolution. In this vein, it is worthwhile to search for the properties of the ionized gas and its relation to the X-ray results.
Previous works have concluded that the H morphology of LINERs mainly consists on a point source embeded in an extended structure sometimes clumpy, filamentary and in some particular cases with clear indications of nuclear obscuration, but mostly indistinguishable from what is observed in low luminosity Seyfert galaxies (Pogge et al. 2000, Chiaberge et al. 2005 and Dai & Wang 2008). Based on STIS spectroscopic observations of 13 LINERs, Walsh et al. (2008) clearly demonstrate that at scales of tens of parsecs their energy source is consistent with photoionization by the central nuclear source, but with a NLR kinematics dominated by outflows. Following Barth’s (2002) considerations, by analogy with Seyfert unification models, it is natural to wonder whether the various types of low luminosity AGN (LLAGN), which LINERs could belong to, are different manifestations of the same underlying phenomenon, with observed differences being only orientation or obscuration. The main goal of this paper is twofold: (1) to evaluate if the ionized gas in the central regions of LINERs shows characteristics indicative of ionized emission from the AGN (a NLR), and also (2) to investigate their relation to the Seyfert population.
In this paper, we present an update of the properties of the Narrow Line Region for a large sample of 32 LINERs. Archival HST narrow imaging data have been used (WFPC2 and ACS). In Section 2 the sample and the HST image processing are described. In Section 3 we present the results and discussion. Section 4 summarizes our main conclusions.
2 Sample and data reduction
|(2000)||(2000)||number||PI name||time (sec)|
We have searched for archival HST data for the 82 LINERs in our
sample (González-Martín et al. 2009a) in the Hubble Legacy
Archive (HLA hereinafter) web page
HST absolute astrometry does not guarantee the centering of two images at the level of its spatial resolution. For that reason, when needed, the narrow- and wide-band images have been aligned according to the center of the galaxy (maximum peak in brightness) and with the stars present in the field.
In order to maximize the chances to get a reliable estimation of the continuum level at the wavelength corresponding to the emission line, we use a common procedure to get continuum subtracted images, based in a relative calibration as follows. The fluxes in the narrow () and broad band () filters are:
where , , and are the intensities measured in the line itself, the continuum under the line, and the continuum in the wide filter, respectively.
To assume that line emission is less extended than the continuum is equivalent to say that, far enough from the center, for flux calibrated images one should have . Previous to any calculation, the background of the two images, narrow and wide-band, has been set to zero.
Then, we have to calculate the
factor required to put the two images at the same level. To do so,
we obtain the surface brightness profiles (with ellipse
The emission line images coming from equation 3 are not flux calibrated, but allow to recover emission-line morphologies and sizes, what is our main purspose, as it will be explained below.
Fig. 2 shows the resulting continuum subtracted images for all the galaxies listed in Tab. 1. Both H and [O III] images are shown when available (for NGC 4579 and NGC 3379). Overlaid to the grey scale images a number of contours are shown, the faintest corresponding to 3 times the dispersion of the background, .
Fig. 2 also shows the sharp divided continuum images (SD hereinafter). Briefly, the sharp-dividing method consists on dividing the original image by another one that results from applying a filter to the original image, with a box size several times that of the PSF FWHM. In our case, a median box of 30 pixels has been used. SD images provide an enhancement of the small structures and thus will be in many cases a good tracer of the dust structures (see Marquez et al. 2003 and references therein), as it is the case, for instance, for NGC 4374 (see Fig. 2).
Appendix A gives a description on the morphology for each object together with additional relevant information.
For estimating the sizes of the emission-line regions, we consider a
3 level above the background and measure the area of the region inside the
corresponding contour. The size is parametrized as the equivalent
radius of such an area, i.e.: . The
of the background level for each image has been measured
in several regions around the galaxy, so that the final is the
median of these values, and its accuracy is provided by the dispersion of
the various around the median. The results are presented
in Table 2. No radius has been determined neither for
NGC 3379 due to low S/N of the images, nor for
NGC 3627 and NGC 5866
due to the large amount of dust wich difficults their determination. Col. 1 shows
the galaxy name, Col. 2 the filter used for continuum substraction, Col. 3 the
distance as taken from González-Martín et al. (2009a),
Cols. 4 and 5 show the X-ray soft and hard luminosities taken from
Gonzalez-Martín et al. (2009a)
Nevertheless, since the data are inhomogeneous, a S/N threshold does not have a well defined physical meaning, which complicates the interpretation of equivalent radius. Therefore, we have used the flux calibration corresponding to the images taken with narrow band filters, (done in the standard way, using the information available on the image headers), for the resulting emission-line image, once the continuum is rescaled and subtracted, . Due to the uncertainties in the flux calibration for ramp filters, the images obtained with such filters have not been used. For the flux calibrated images, we have also calculated R (Col. 8 in Table 2) as the isophotal equivalent radius at the isophotal level of 2.9 10 erg scm arcsec. This rather arbitrary surface brightness was chosen to optimize the measure for all the available data. This radius allows a measure of a physical characteristic size of the regions independently on the individual S/N ratios of the images. This has been done for the 22 objects with flux calibrated images.
Distances have been taken from table 1 in González-Martín et al. (2009a)
Note that these luminosities have been corrected from intrinsic absorption; L (2-10 keV) has been also corrected for Compton-thickness. L and L hold for the logarithm of the (2-10) kev (from Gonzalez-Martín et al. 2009b) and (0.3-2) keV (from Gonzalez-Martín et al. 2009a)
R is the equivalent radius corresponding to a level 3 times higher that the dispersion of the background. R corresponds to the isophotal level at 2.9 10 erg scm arcsec.
The resulting radius is smaller than 2 pixels.
3 Results and discussion
3.1 H emission as a tracer of the morphology of the NLR in LINERs
The first result from our analysis is that for most LINERs the H emission is composed of a nuclear source and extended emission, revealing a complex structure, with a large range of different morphologies. The exceptions are NGC 2639, NGC 3379, NGC 3627, NGC 4036 and NGC 5005, for wich an unresolved nuclear source has not been identified. We have grouped our sample galaxies into 4 types of objects according to the morphology of the extended H emission in the central 1-2 arcseconds. The objects belonging to each sub-cathegory are shown in Table 3.
Core-halo: When a clear unresolved nuclear source, surrounded by diffuse emission, has been identified. Nine out of the 32 objects belong to this class. In most cases the putative nucleus is sitting in a linear elongated structure. In five cases (IC 1459, NGC 315, NGC 2639, NGC 3623, and NGC 5055; see individual comments in Appendix A) the extended emission appears to be sitting in the disk of the galaxy and the elongation of the emission follows the major axis of the galaxy (taken from the NED database
9). In three of them (NGC 2787, NGC 3998, and NGC 4111), the nuclear disk axis seems to be perpendicular to the galaxy major axis. NGC 2681 does not show any elongation (see Fig. 2).
Outflows: Eleven galaxies show morphological evidences to have nuclear outflows (Veilleux et al 2005). Some of them present debris/filamentary extension (NGC 4486, NGC 4676A and B, NGC 4696, NGC 5005, and NGC 5846), biconical structures (NGC 4036 and NGC 5005) and also buble-like structures (NGC 3245 and NGC 4438) coming out from the nucleus. The high spatial and spectral resolution spectroscopic data (HST-STIS) for NGC 3245, NGC 4036 and NGC 4579, reported by Walsh et al. (2008), indeed evidence outflow kinematics strengthening our suggestion. For the remaining objects, such a kinematical confirmation has to await until similar spectroscopic data are available.
Disky: Seven galaxies present face-on structures that can be associated to H emission along the spiral arms (NGC 2681, NGC 2841 and NGC 4736), diffuse emission along the disk (NGC 3379, NGC 4552 and NGC 4636), nuclear plus star formation rings (NGC 4314). NGC 4594 has also been included in this class because, although it is not seen face on, it appears that its H emission is concentrated in the nuclear region and their spiral arms.
Dusty: Those where clear dust lanes obscure the underlying H structure. This prevents us from getting information on the morphology of these inner regions. Five objects have been classified as such (NGC3226, NGC3607, NGC3627, NGC4374 and NGC5866). Different structures can be identified depending on the dust distribution along the galaxy, but mostly nuclear sources surrounded by an inhomogeneous dusty disk are found.
|IC 1459||NGC 3245||NGC 3226||NGC 2681|
|NGC 315||NGC 4036||NGC 3607||NGC 2841|
|NGC 2639||NGC 4438||NGC 3627||NGC 3379|
|NGC 2787||NGC 4486||NGC 4374||NGC 4314|
|NGC 3623||NGC 4579||NGC 5866||NGC 4552|
|NGC 3998||NGC 4636||NGC 4594|
|NGC 4111||NGC 4676A||NGC 4736|
|NGC 4278||NGC 4676B|
|NGC 5055||NGC 4696|
Our main concern here is to understand whether the detected H nuclear regions correspond to the expected NLR for AGN. Pogge et al. (2000) made an extensive HST investigation on the NLR of 14 LINERs, and concluded that at HST resolution the NLRs are resolved showing complex morphologies, different from galaxy to galaxy, that come from a combination of knots, filaments and diffuse gas. Dai & Wang (2008) concluded similarly with an extension of Pogge’s sample up to 19 LINERs.
Among our 32 sample galaxies, 17 LINERs are studied in this paper for the first time. Pogge et al. (2000) already analized 7 of the LINERs in our sample (namely NGC 3998, NGC 4036, NGC 4374, NGC 4486, NGC 4579, NGC 4594 and NGC 5005) and Dai & Wang (2008) studied another 4 (namely NGC 404, NGC 2768, NGC 3718 and NGC 4192 ) not included in the sample because of our X-ray selection.
All together, including the new 17 LINERs from our work plus the 19 ones from Dai & Wang (2007) (we have 15 objects in common with them), they conform a rather homogeneous set of data for 36 LINERs, which seems to be the larger sample homogeneously analyzed so far. It is worth noticing that the 4 objects from Dai & Wang’s paper not included in our sample can be fit into the outflow-like group. Thus from the total sample of 36 LINERs, 42 % would be outflow candidates, 25% core-halo systems, 19% disk-like systems and 14% dusty LINERs. These results stress the intereresting possibility of shock heating as an extra contribution to the ionization in addition to nuclear photoionization. This scenario needs to be explored at length with high S/N spectroscopy for the outflow candidates to investigate if at least for these LINERs the long standing problem of ionizing-photon deficit can be solved (see Eracleous et al. 2010b for a full discussion).
The question then to be answered is whether the origin of the outflow can be circumnuclear star formation or it is a nuclear outflow predicted by the unified AGN models (Elvis 2000). From the STIS spectroscopic analysis by González Delgado et al. (2004) it is found that recent star forming processes (with ages lower than 10 years) are almost absent in LINERs, being the dominant stellar population that of old stars with, in some particular cases, some contribution from intermediate age (10 years) stars. The H identified structures appear to be consistent with such a picture. Indeed, at the HST resolution of few tens of parsecs, a knotty appearance should be expected when young star clusters are present, which is not observed in most of the images. Their inspection appears to indicate that such knotty structures are only present in the Mice system. In disk-like systems, star formation can be distinguished in their disks (e.g. see the star formation ring on NGC 4314 at 200 parsecs from the nucleus, Fig. 2). The structure of core-halo galaxies is more likely originated from the gas ionized by the nucleus. For dusty galaxies, although a faint nuclear source is visible in most of them, the dust distribution prevents us from drawing any conclusion on the extended ionized gas.
3.2 Quantification of the H emission: equivalent radius
To further investigate the origin of the extended H emission,
and considering its irregularity, we
have calculated a characteristic radius for estimating the
size of the ionized region: the equivalent radius,
, and R provided in
We searched for a distance dependence that could bias our result, so we plotted equivalent radii in arcseconds versus distance and did not found any correlation between the two quantities. In Fig. 3 the distribution of in parsecs is shown as the empty histogram and the corresponding R distribution as the black filled area. A range of values between 43 and 528 pc with a median value of 200 pc has been obtained for and between 16 and 469 pc, with a median value of 116 pc for R. Comparing both estimations, it is found that with the exception of NGC 4111, NGC 4314 and NGC 5005 for which it is found the largest values of R and much smaller values from the estimation , for the remaining cases R is equal or lower than . Thus we can conclude that the currently more extended used size estimation at 3 detection limits tend to overestimate the true physical size of the nebula. Finally it is worth to note that no significative difference is found on the size among the different morphologies.
This range of values is similar to that reported by Dai & Wang (2008). For the 14 galaxies with measured radii in common in both works, our estimations for NGC 2787, NGC 4111, and NGC 4594 are smaller; large discrepancies are found for three objects (for NGC 4314 and NGC 4736 Dai & Wang measured very small values and for NGC 4374 a rather large value was measured compared to ours); for the remaining ten objects our estimations are larger than theirs. We stress that the method used by Dai & Wang (2008) relies on the estimation of the annulus at which the 3- level above the continuum is reached (see also Bennert et al. 2002). The general irregularity of the isophotes makes this method rather uncertain, what has motivated us to use , that we consider a more realistic estimation of the size of the emitting regions.
Our sizes cover the lower end of the distribution of values for
the major axis obtained, with [OIII]-HST imaging (Schmitt et al. 2003)
for the NLR of Seyfert galaxies. For the 10 Seyfert galaxies with HST
data from Schmitt et al.’s sample included in the X-ray catalog
3.3 Luminosity - size relation
The luminosity - size relation can also be used to get insight onto the nature of the ionized emission. This has been raised as an important relation for AGN since Peterson et al. (2002) found that it can be defined for the BLR of Seyferts. Greene et al (2010) have revisited such a dependence and found that it is consistent with R based both in Balmer lines and hard (2-10 keV) luminosities. This is the expected dependence when the BLR density is independent on luminosity. Their data also suggest a steeper relation for the narrow line luminosities, R L.
Bennert et al. (2002) and Schmitt et al. (2003) searched for such a relation for the NLR of Seyferts and Dai & Wang (2008) extended the work to LINERs. They concluded that LINERs follow the same relation than Seyferts and QSOs. In this work we present this relation, but for the first time using the X-ray luminosity instead of that of H, which is a more robust tracer of the power of the AGN (Maiolino et al. 2002). H is expected to be more contaminated by other processes as recent star formation events.
The X-ray luminosity can be used as a measure of the bolometric intrinsic luminosity of an AGN (Gonzalez-Martín et al 2006, 2009a and b). Therefore, it is worthwhile to investigate whether it is related to the size of the NLR. In Fig. 4 the hard (2-10 keV) X-ray luminosity versus the two determinations of the equivalent radius is presented. The different H morphologies described in Section 3.2 are plotted with different symbols. The following three galaxies have been excluded from the plot: NGC 3379, NGC 3627 and NGC 5866. NGC 3379 was excluded because the low count rates of its narrow line image impede the determination of a reliable equivalent radius; NGC 3627 and NGC 5866 were excluded because large amounts of dust hampers the detection of their NLR. The two galaxies conforming the Mice system (NGC 4676A and NGC4676B) show a large knotty extension of star formation regions together with typical structures of outflowing material, leading to a rather large value of equivalent radius exceding the hypothetical NLR. Therefore, despite their inclusion in the plots, they won’t be used for any correlation below.
A first attemp to look for a correlation between X-ray luminosities and is based on a least square linear fit, that results in the values reported in Tab. 4, and not plotted in Fig. 4 for clarity. The correlations are quite bad, with all the galaxies classified as disky (but NGC 4594) showing larger sizes than those expected from their luminosities for the remaining galaxies. This is not unexpected, since emission in disky galaxies also comes from the contribution of ionized regions in their disk. Therefore we tried again a linear fit, but this time excluding disky galaxies. The result is the full line in Fig. 4. The resulting coefficients imply better correlations in this case (see Tab. 4). Finally, we fitted just the core-halo systems (dashed line in Fig. 4), resulting in the best correlation (see Tab. 4).
Therefore, two main results appear from Tab. 4: (1) The luminosity-size relation is better when using X-ray luminosities at harder energies and (2) a better correlation is found when disky systems are excluded from the fitting, the best fit resulting when only core-halo systems are taken into account.
This later result can be due to the fact that core-halo systems appear to be less dusty and therefore provide a clear insight on the NLR. Since the hard X-ray luminosities cannot be produced by stars, this relation support the nature of the emission regions as the result of the ionization by the AGN. In that respect it is very suggestive of the similarity of LINERs with higher power AGN, that the slope for core-halo systems ( 0.38) has the same value than that reported for Seyfert galaxies by Schmitt et al. (2003) by using the [O III] luminosity as a proxy of the AGN power.
The resulting correlations for the subset of calibrated data are shown in Fig. 4 (bottom) and in Tab. 4. It is very interesting to notice that a significant correlation remains only for the core-halo systems. Both dusty and outflow galaxies appear to have lower equivalent radii for their X-ray luminosities. For the dusty systems it is obvious that the presence of large amounts of dust obscures the inner regions and thus lowers the measured size of the H emission. The explanation for the outflow candidates is not so straightforward. It appears that they cover a narrow range on X-ray luminosities. This result may suggest a different origin for the emission mechanism in these systems but needs to be further investigated.
|energies||slope||Correl. coeff.||slope||Correl. coeff.|
|(0.5-2 keV)||0.324||0.144 0.064||0.530|
3.4 Soft X-rays vs NLR morphologies
For a collection of 8 Seyferts, Bianchi et al. (2006) have reported a spatial correlation between the soft X-ray emission and the NLR as reflected by the [OIII] emission, taking this result as an important evidence on the photoionized nature of soft X-rays. Given the morphological similarity between the NLR of Seyferts and LINERs (Schmitt et al 2003, Pogge et al 2000), it will be interesting to explore if such a relation does exist also in LINERs.
In Fig. 6 the soft X-ray isocontours are overploted (in black) over the H images. Only the 28 galaxies with available Chandra imaging have been included. The remaining 4 galaxies have only XMM-Newton X-ray data, as indicated in Table 5 with an asterisk in Col. (6). Althought a very detailed comparison cannot be made due to the different resolutions at both wavelenghts (around 1” and 0.1” for Chandra and HST data, respectively) it is remarkable that soft X-rays and H data show a rough coincidence in their shapes, the soft X-ray contours following the structures identified with HST. This is not the case for the hard X-rays (red contours in Fig. 6). Few galaxies depart from the general behaviour, NGC 3226, NGC4486, NGC 4676A and B, NGC 5846 and NGC 5866. NGC 3226 show a compact structure both at soft and hard X-rays, whereas the H distribution seems to suggest an outflow coming out from that compact nucleus. For NGC 4486, both soft and hard X-rays follow the radio jet also visible in the continuum images, being the H outflow perpendicular to it. NGC 4676A and B and NGC 5846 shows in H a structure non-coincident with either hard or soft X-rays, suggesting that the emitting gas has a different origin. No conclusion can be obtained for NGC 5866: its H emission appear to be very obscured by large amounts of dust and soft X-rays show a spatial distribution which appears to be out of the plane of the galaxy.
Unfortunatelly there does not exist yet a sample of good RGS XMM-Newton data for LINERs to allow the modelling of the soft X-ray emission. However the data reported by Starling et al. (2005) on the LINER galaxy NGC 7213 and those collected for 53 LINERs by González-Martín et al. (2010, in preparation) seem to suggest that their soft emission comes from photoionized gas, in good agreement with the conclusions obtained with the systematic work on Seyfert 2 galaxies by Bianchi and colaborators (Bianchi et al. 2006, Bianchi et al. 2010).
3.5 Multiwavelength properties
Different authors (Ho et al. 1999; Maoz 2007; Eracleous et al. 2009, 2010a;
González-Martín et al. 2009a)
have recognized the importance of the multiwavelenght information to get a
clear picture of the enegy source in LINERs. Table 5 shows relevant
information for the LINERs in this sample. The information in Cols. from
(2) to (10) has been extracted from Tab. 12 in
González-Martín et al. (2009a), with
Col. (1) providing their number code for each galaxy. In Col. (6) the word CT
has been added when a LINER shows
Compton-thick (CT hereinafter) characteristics as defined in
González-Martín et al. (2009b). Col. (7) has been updated with the corresponding
references for three objects. Col. (9) gives the final
classification considering the multiwavelength information from
Cols. (3) to (8). Col. (10) provides the HST morphological class
as defined in this paper. In Col. (11) we present
the Eddington ratio, R, calculated using the formulation given in Eracleous et al. (2010a):
R = 7.7 x 10 L M
where L is the bolometric luminosity in units of 10 erg s and M is the black hole mass in units of 10 M☉. The SED obtained by Eracleous et al (2010) for LINERs leads to a bolometric luminosity 50 times L. Both L and M values have been taken from Gonzalez-Martín et al (2009b).
|9||NGC 2639||Non-AGN* CT||Y||Y||Y||core-halo||7.9x10|
|11||NGC 2681||Y||AGN CT||Y||N||Y||core-halo||3.2x10|
|20||NGC 3245||AGN CT||Y||N||Y||outflow||1.0x10|
|21||NGC 3379||Non-AGN CT||N||N||disky||2.0x10|
|24||NGC 3607||Non-AGN CT||Y||Y||dusty||5.0x10|
|27||NGC 3627||Non-AGN* CT||Y||Y||dusty||7.9x10|
|33||NGC 4036||AGN CT||Y||N||Y||outflow||3.2x10|
|39||NGC 4314||N||Non-AGN CT||N||N||disky||1.0x10|
|39||NGC 4278||N||Non-AGN CT||N||N||disky||1.2x10|
|41||NGC 4374||AGN CT||Y||Y||dusty||1.6x10|
|43||NGC 4438||N||AGN CT||Y||N||Y||outflow||6.3x10|
|53||NGC 4636||Non-AGN CT||Y||Y||Y||outflow||2.1x10|
|59||NGC 5005||Y||AGN* CT?||Y||N||Y||outflow||1.0x10|
|60||NGC 5055||Y||AGN CT||N||Y||core-halo||1.0x10|
|70||NGC 5846||Non-AGN CT||Y||Y||Y||outflow||2.0x10|
|71||NGC 5866||Non-AGN CT||Y||Y||dusty||5.0x10|
From Wu & Cao (2005)
No BH mass is available for this source
From Dunn et al. (2010)
As we did in Sect. 3.2., the two components of the Mice system will not be included in the discussion; their rather complex nature resulting from the merger-like interaction between NGC 4676A and B, is unique among our sample galaxies and may contaminate our results. Our discussion will therefore be dealing with the remaining 30 galaxies.
From the 4 LINERs with a final classification as non-AGNs according to Col. (9), NGC 3623 has an uncertain X-ray classification since it is based in XMM-Newton data. The other 3 (namely NGC 3379, NGC 4314 and NGC 4278) show an H classification as disk-like systems. Their X-ray data show evidences to be Compton-thick (Gonzalez-Martín et al 2009b), what suggests that they could well host extremely obscured AGN activity.
For 3 out of the 26 confirmed AGN LINERs (namely NGC 3607, NGC 3627 and NGC 5866), their classification is only based on the detection of a broad H line, being classified at X-rays as non-AGNs. They all have an HST classification as dusty objects and appear to be Compton thick at X-ray frequencies, so in these three cases a hint of a relationship between the obscuring materials could be claimed.
In addition to these, 10 more AGN LINERs show evidences of CT nature. In 5 of them (namely NGC 2639, NGC 4374, NGC 4552, N4636 and NGC 5846) BLRs have been detected. Only a dusty environment is seen with the HST data for NGC 4374. For the other 4 galaxies the obscuring material seems to be most probably sitting in the innermost regions. Seven out of the 13 CT LINERs show their BLR, among which 4 of them have dusty H morphologies. Therefore, for the remaining 3 out of the 7 CT LINERs there is no obscuring material at HST resolution that could be invoked as the origin of its CT nature. A similar result has been found for Seyferts 1 (Malizia et al. 2009, Panessa et al. 2008), questioning the dichotomy type 1/type 2 AGNs in the current unification models (Urry and Padovani 2000). Finally for the remaining CT narrow-line LINERs (NGC 2681, NGC 3245, NGC 4036, NGC 4438, NGC 5005 and NGC 5055) the obscuration cannot be atributed to important dust lanes obscuring the nuclei. Summarizing the results on CT LINERs, a large incidence is found in the dusty systems, since 4 out of the 5 dusty systems are CT; the remaining are distributed among the different types.
Althought the number statistics are rather low, it is very interesting to notice that among secured X-ray AGN-classed LINERs, based on Chandra observations, ( 19 out of 24 galaxies, see Col. (6) in Tab. 5) outflow and core-halo morphologies prevail (6 outflow systems, 7 core-halo, 4 disk-like and 2 dusty) amounting to 68%. Taking the 26 galaxies AGN-classed based on multifrequency data, 8 have been classified as core-halo, 9 as outflow, 4 as disky and 5 as dusty. Therefore outflows and core-halo represent 65% of the AGNs.
Considering the Eddington ratios, the large range obtained (from 10 to 10) overlaps with the values found for Seyfert galaxies (Panessa et al. 2006, Gonzalez-Martín et al 2009a), suggesting that LINERs are not always the low accretion cousins of Seyferts. We have found a slight trend for the Eddington ratios to decrease when moving from core-halo to outflow and disky systems (see Fig. 5). Dusty galaxies are not considered in the general trend since in the absence of dust they should fit in one of the other 3 classes. Different authors have claimed that strong radio jets are responsible for the bulk of the radio emission observed in LINERs (Nagar et al. 2005, Filho et al. 2002) and that the radio loudness parameter (see Maoz 2007) can be related to the Eddington ratios in the sense that Eddington ratios are larger for lower radio loudness ratios. Maoz (2007) speculate that, in order to explain high Eddington ratios in low luminosity AGNs, mechanisms preventing gas to reach the inner parts of the accretion disk would be at work; they suggest radio loudness at low luminosities as such a solution, with the gas joining a jet or an outflow. Our data seem to support such a hypothesis since 1) radio loud systems are found in core-halo and outflow systems and, even more important, 2) all the outflow systems appear to be radio-loud.
4 Summary and conclusions
We have presented HST-H imaging of 32 LINERs, selected from the X-ray sample studied in our previous works (González- Martín et al. 2009a and b). A full description of the extraction and reduction process is given and the resulting emission-line images are also presented together with the sharp divided continuum images for each galaxy. The description of the most relevant properties for each individual galaxy is also given.
The main conclusion from this analysis is that, for the large majority of LINERs, an unresolved nuclear source has been identified, together with extended emission with equivalent sizes ranging from few tens of parsecs till about 500 pc. Adding up additional 4 LINERs from the literature to our sample, we conclude that their emission-line morphologies appear not to be homogeneous, being basically grouped into three classes: nuclear outflow candidates (42%), core-halo morphologies (25%) and nuclear spiral disks (14%), being the remaining 5 galaxies too dusty to allow a clear view of the ionized distribution. Except maybe for the only case of a merger-like interaction (the two galaxies in the Mice system), no signatures of clumpy structures reminiscent of star clusters have been identified, in agreement with results from stellar population analisis (González-Delgado et al. 2004 and Sarzi et al. 2006).
A size-luminosity relation has been found between the equivalent radius of the H emission and the hard X-ray luminosity. This correlation ressembles that reported for the NLR of Seyferts galaxies based in the [OIII] luminosity (Schmitt et al. 2003). This relation is another piece of evidence confirming the AGN-NLR nature of the ionized gas in LINERs (Pogge et al. 2000, Walsh et al. 2008).
Indications of a relationship between soft X-rays and H emission in LINERs are also reported for the first time. This spatial correlation looks similar to the one reported by Bianchi et al (2006) for Seyferts, evidencing the photoionized nature of the soft X-rays.
For the only 4 LINERs with no evidences for AGN nature of their nuclear emission, a CT AGN cannot be discarded out given the properties of their X-ray emission. For the confirmed AGN-LINERs, their H morphologies favour core-halo and outflow systems (65% of the cases). Finally, Eddington ratios have been calculated showing that LINER nuclei radiate in the sub-Eddington regime, in agreement with previous data (Maoz 2007, Ho 2008, Eracleous et al. 2010a). However core-halo systems tend to have larger Eddington ratios than outflow candidates on average. These result may be consistent with the suggestion by Maoz (2007) of radio-loud outflow related systems showing smaller Eddington ratios.
Acknowledgements.J.M. and I.M. acknowledge financial support from the Spanish grant AYA2007-62190 and Junta de Andalucía TIC114 and the Excellence Project P08-TIC-03531. O.G-M acknowledges finantial support by the EU FP7-REGPOT 206469 and ToK 39965 grants. A.R. acknowledge finantial support from CONACyT grant number +081535. We acknowledge the valuable feedback from an anonymous referee. Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive (HLA hereinafter), which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Appendix A Comments on individual objects
At HST resolution Lauer et al. (2005) classified this
galaxy as an starting dusty nuclear ring and Verdoes Kleijn et al.
(2000), based in H+[NII] WFPC2 images, identify an ionized gas disk
with PA 37 and inclination 60 following what is found
by Goudfrooij et al. (1990) at larger scales (100 arcsec).
Here we report a residual H
emission with a central source and a rather biconical
distribution extended 500 pcs at PA 37 (Fig. 2).
X-ray energies (0.3-2 keV), it extends along the same direction than that in H
HST data (Fig. 6).
A compact unresolved source of
ionized gas on top of a dusty
disk, together with an extension
of the disk of 200 pc at PA 49 is detected (see Fig. 2; see also Verdoes-Keijn et al. 1999). The high spatial resolution
provided by Chandra imaging allowed the detection of the X-ray jets
(Donato et al., 2004; Worrall et al., 2003, 2007; Gonzalez-Martin et al.
2009). Soft X-rays are extended along the axes of the jet and the host galaxy (Fig. 6).
At HST resolution, its H shows an elongated asymetric extended
structure at PA -29, but not nuclear compact source is
identifiable. It also shows extended filaments being more prominent
towards the NW. The SE region is maybe obscured by dust. The SD images
and the broad band data show a rather
dusty morphology (see Fig. 2 and Simões Lopez et al. 2007).
The H emission image show a central source with extended emission along
the central spiral structure with major axis at PA 40 and a radial
extension of 4 arcsecs ( 440 pcs). Spiral dust lanes are clearly detected
in the SD image (Fig. 2). Broad agreement is found in the elongation of soft X-ray and H emissions (Fig. 6).
On the HST SD images, a near-nuclear dust-lane
is clearly resolved into a spectacular set of concentric, elliptical dust
rings, covering a radial range of 5â10″(see also Shields et al. 2007, Simões-Lopes et al.
2008, Gonzalez Delgado et al. 2008). In H,
a nuclear component has been
detected, in good agreement with Dai & Wang (2009). An elongation
at PA 49 can also be identified, which is perpendicular to the major axis of the galaxy (Fig. 2). Soft X-rays roughly follow the H emission (Fig. 6).
At HST resolutions
it shows a rather face-on ring-like structure and a clearly well identified
unresolved nuclear source. A small NLR can be identified at PA of 90. Dust morphology becomes
apparent from the SD image (Fig. 2).
Soft X-rays extend along two main axes, one following the hard X-ray emission (PA about 10) and the other one close to that of the H emission (Fig. 6).
This galaxy shows a bright nucleus with some evidences of
dusty environment clearly seen in the SD image (see also Gonzalez Delgado et al. 2008) .
In it shows an extended morphology quite similar
to that observed in continuum (Fig. 2). At X-ray frequencies
it shows a compact structure both at soft and hard energies. The H however
seems to suggest an outflow-like morphology coming out from that compact nucleus
shows a kidney-like structure slightly brighter
to the North, with a nuclear unresolved source
(see also (Gonzalez Delgado et al. 2008, Walsh et al. 2008). Kinematical data from
Walsh et al. (2008) support our outflow classification.
The western dust structure is clearly appreciated in the SD image. (Fig. 2). One of the two axes shown by the soft X-ray contours follows the H emission (Fig. 6).
An extended structure
emerging from the nucleus can be apreciated
althought the S/N ratio on the H image is low. A tiny dust-lane crosses the nuclear regions at PA -50 in SD (Fig. 2). At HST resolution Lauer et al. (2005), based in the F555W filter,
classified this galaxy without a clear nuclei but with a dusty nuclear
ring morphology. Shapiro et al. (2006) reported a well defined disk of
emission at H with PA 118. The morphology of the soft X-ray contours is quite complex, but a rough agreement with the extended H emission is found (Fig. 6).
image shows a clearly nuclear unresolved source and diffuse emission
following what it appears to be an inclined disk. The strong dust lanes visible in the SD images
obscure the H emission (Fig. 2).
Lauer et al. (2008) suggested that it
contains a dusty outer disk dynamically old which appears to
transition rapidly but smoothly at the center to a second gas disk
that is perpendicular to the first and is seen nearly edge-on. This
inclined disk seems to be settling onto a nuclear ring. Excepting the outermost contours, the soft X-ray emission elongates along the H emission (Fig. 6).
H emission has been detected, extending
130 pc at PA -10.
Inside the more extended
structure an inner disk is appreciated extending 30 parsecs along
PA 53. Large scale dust-lanes clarly appear in the SD image (Fig. 2).
The data (Fig. 2) do not show a well defined nuclear source, most probably due to the dust lane crossing the nuclear region in the direction NS and obscuring
the SE-NW elongated extended emission (see the SD image). Gonzalez-Delgado et al. 2008 reported from HST data that
chaotic dust lanes and several compact sources are identified at the center.
image (Fig. 2) shows a 100 pc extended structure surrounding a compact nucleus.
The major axis of this extension is oriented along a PA=0 (see also
Pogge et al. 2000). The SD image shows
little indication of dust in the nuclear region, in good agreement
with Gonzalez Delgado et al. (2008). Soft X-rays are elongated along the same direction as the H emission (Fig. 6).
The HST H image (Fig. 2)
shows, on top of a well identified nuclei, the existence of a
complicated filamentary and clumpy structure, with an extension of 390
pcs at PA 63, already reported by Pogge et al. (2000) and Dai and
Wang (2009) (see also the SD image). Walsh et al. (2009) have shown
the presence of a gas velocity gradient of 300 km s
across the inner 0.2”, compatible with the outflow-like structure apparent in the ionised gas.
The soft X-ray emission appears to follow the H
emission (Fig. 6).
A rather knotty morphology surrounding a clear nuclear source is
observed, embebed in a diffuse halo. This morphology is interpreted as
a core-halo structure detected at HST resolution both with
medium size filters (Simões Lopes et al. 2007) and narrow band
H data (Dai and Wang, 2009). A crossing dust structure is
seeing perpendicular to the disk main plane (see SD image). Soft X-ray contours are elongated along the same PA as the H emission (Fig. 6).
The H image (Fig. 2) shows both an
unresolved nucleus and a number of
HII regions tracing the star formation ring.
The same features are well traced by the SD image, where the spiral dust lanes associated with the ring are conspicuous (see also Gonzalez Delgado et al. 2008).
At soft energies, its emission follow the
star forming regions observed in H (Fig. 6).
H image (Fig. 2) shows an inclined gas disk surrounding the nucleus.
This emission gas structure takes the form of filaments that extend
roughly east-west and north-south (see also (Pogge et al. 2000).
The dust structure clearly appears in the SD image, where the nucleus is seen in the center of the dust-lane to the South. The soft X-ray contours are roughly aligned with the ionised gas (Fig. 6).
Gonzalez Delgado et al(2008) defined it as galaxy with
very perturbed central morphology and strong dust lanes cross
the center along PA 0 obscuring the eastern side of the galaxy (see the SD image). The H image (Fig. 2) shows a ring-like structure
where a clear knot is seen in the south-east region coincident with the
continuum nucleus. The other side would remain invisible due to obscuration
by dust. Two plumes can be
apreciated to the north and south west extending about 150 pcs in both
directions. This is one of the clearest examples to be a candidate of nuclear
outflow, buble structures, as defined in Veilleux and Brandt (2007). Soft X-rays are aligned with the H emission (Fig. 6).
The H image (Fig. 2) shows a compact source with filaments which
resemble an outflow from the nucleus (see also Pogge et al. 2000; Dai
and Wang 2009). As already noticed by Pogge et al. (2000), the
conpiscous jet clearly visible in the continuum images (see SD)
dissapears in the H continuum substrated map. Soft X-rays are missaligned with respect to H emission, the former following the jet axis (Fig. 6).
At HST resolution the H data show a compact unresolved nuclear
source located at the center of a symmetric extended emission in a disk-like structure.
No trace of dust-lanes is seen in the SD image (Fig. 2). Soft X-rays roughly follow the H emission (Fig. 6).
The H emision (Fig. 2) traces a bright, nuclear point source surrounded
by complex clumpy and filamentary emission (see also Pogge et al. 2000).
The higher ionization gas traced by [OIII] (Fig. 2) is composed of a compact
source and a filamentary, jet-like structure towards the NE.
Walsh et al. (2008) have shown that the gas is not in regular rotation, displaying two
kinematical components with a velocity separation of 450 km s,
being consistent with an outflow from the nucleus. The dust-lanes
seen in the SD image conform a mainly chaotic structure together with a
much stronger, offset, linear feature that goes at
PA45 in the West side. Soft X-ray contorus follow the H emission at large scales. There is a hint of an extension of hard X-rays along the PA of the [OIII] jet-like feature (Fig. 6).
The H image (Fig. 2) shows a compact nuclear source together with
fainter emission extending along the E-W direction in a bar-like
morphology, with two spiral arms emerging from it, with a total
extension of 300 pc. The kinematical data by Walsh et al. (2008) show
organized motion consistent with rotation but with significant
irregularities in the nucleus. A strong velocity gradient
and decoupled kinematics between gas and stars were found
by Emsellem & Ferruit (2000). An overall extension of soft X-rays is seen along the same axis as the extended ionised gas (Fig. 6).
The H data (Fig. 2) show a
central compact source and a very faint ring like structure more clearly visible
in the southern region of the galaxy (see also Simões Lopes et
al. 2007 and Dai and Wang 2009).
Towards the north, a more prominent H emission is seeing with a clear outflow-like morphology. This morphology seems to follow the soft X-ray data (Fig. 6).
NGC 4676A and B. The Mice.
images (Fig. 2)
show in both galaxies a very clumpy and irregular structure. In
galaxy B a more conspicuous knotty structure is visible (one of the knots coincides with
the nucleus). In galaxy A however a more diffuse emission is seen.
Central dust-lanes are much stronger in galaxy B, as can be appreciated in the SD images (see also Laine et al. 2003). The soft X-ray contours are unrelated to the extended H emission in both galaxes (Fig. 6).
NGC 4696. H imaging (Fig. 2) shows a clear nuclear source with elongated extended emission along PA 47, and larger filamentary structures towards the west maybe resembling outflows out of the nucleus.Crawford et al (2005) report also a filamentary structure shared by the H and soft X-ray emission (see also our Fig. 6).
The H image
shows a circumnuclear spiral structure of extension 200 pc. Dust lanes in
spiral arms are traced in the SD image (Fig. 2).
Gonzalez Delgado et al. (2008)
suggest the presence of spiral dust lanes down the nucleus and a compact
nuclear stellar cluster. Despite the complexity of the soft X-ray emission, a rough agreement is found in the overall shape of both images (Fig. 6).
H data (Fig. 2) show a very asymmetric emission with a wide-angle cone-like structure extending to the SE (see Pogge et al. 2000 and Dai Wang 2009),
perpendicular to the major axis of the galaxy.
A strong dust lane crosses the galaxy from East to West, offset from the nucleus (see the SD image and Gonzalez Delgado et al. 2008).
Its H image (Fig. 2) shows
a nuclear source and extended emission along
A floculent spiral structure, stronger to the Souht, is visible in the SD image (see also Gonzalez Delgado et al. 2008). Soft X-rays and H emission are extended along roughly the same direction (Fig. 6).
The H image (Fig. 2) shows a compact nucleus and diffuse emission
resembling a very wide outflow extending up to 2” in the W
direction. This strong asymmetry cannot be explained by dust absorption
(see the SD image). No correaltion is seen between H and soft X-ray emission (Fig. 6).
The H image shows an extremely faint nucleus on top of a very
dusty structure along PA -45 strongly obscuring the nucleus ( see
also the SD image) what hampers either any classification of the emission (Fig. 2) or any comparison with the soft X-ray emission (Fig. 6).
- Also available are the narrow band images of NGC6240 and NGC6241, which are not considered in this paper since the NLR physical sizes for these two galaxies cannot be resolved even with HST data due to their much larger distances.
- IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy (AURA), Inc., under contract with the National Science Foundation.
- The L (2-10 keV) are Compton-thick corrected.
- The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
- Radii smaller than 2 pixels, identified with in Column 7, are not considered.
- Catalog of AGB in the XMM-Newton archive, Bianchi et al. (2009).
- Barth, A. J., Ho, L. C., Filippenko, A. V., & Sargent, W. L. W. 1998, ApJ, 496, 133
- Barth, A. J., 2002, ASP Conf. Ser. vol. 258, 147
- Bennert, N., Falcke, H., Schulz, H., Wilson, A. S., & Wills, B. J. 2002, ApJ, 574, L105
- Bianchi, S., Guainazzi, M., & Chiaberge, M. 2006, A&A, 448, 499
- Bianchi, S., Guainazzi, M. Matt, G. Fonseca Bonilla, N. & Ponti, G. 2009, A&A, 495, 421
- Bianchi, S., Chiaberge, M., Evans, D. A., Guainazzi, M., Baldi, R. D., Matt, G., & Piconcelli, E. 2010, MNRAS, 418
- Crawford, F., Hessels, J. W. T., & Kaspi, V. M. 2007, ApJ, 662, 1183
- Chiaberge, M., Capetti, A., & Macchetto, F. D. 2005, ApJ, 625, 716
- Dai, H.-F., & Wang, T.-G. 2008, Chinese Journal of Astronomy and Astrophysics, 8, 245
- Donato, D., Sambruna, R. M., & Gliozzi, M. 2004, ApJ, 617, 915
- Dudik, R. P., Satyapal, S., Gliozzi, M., & Sambruna, R. M. 2005, ApJ, 620, 113
- Dunn, R. J. H., Allen, S. W., Taylor, G. B., Shurkin, K. F., Gentile, G., Fabian, A. C.& Reynolds, C. S. 2010 MNRAS, 404, 780
- Elvis, M. 2000, ApJ, 545, 63
- Emsellem, E., & Ferruit, P.W., 2000, A&A, 357, 111
- Eracleous, M., Shields, J. C., Chartas, G., & Moran, E. C. 2002, ApJ, 565, 108
- Eracleous, M., Lewis, K. T., & Flohic, H. M. L. G. 2009, New Astronomy Review, 53, 133
- Eracleous, M., Hwang, J. A., & Flohic, H. M. L. G. 2010, ApJ, 711, 796
- Eracleous, M., Hwang, J. A., & Flohic, H. M. L. G. 2010, ApJS, 187, 135
- Filho, M. E., Barthel, P. D., & Ho, L. C. 2002, A&A, 385, 425
- González Delgado, R. M., Cid Fernandes, R., Pérez, E. et al. 2004, ApJ, 605, 127
- González Delgado, R. M., Pérez, E., Cid Fernandes, R., & Schmitt, H. 2008, AJ, 135, 747
- González-Martín et al. (2010, in preparation)
- González-Martín, O., Masegosa, J., Márquez, I., & Guainazzi, M. 2009b, ApJ, 704, 1570
- González-Martín, O., Masegosa, J., Márquez, I., Guainazzi, M., & Jiménez-Bailón, E. 2009a, A&A, 506, 1107
- González-Martín, O., Masegosa, J., Márquez, I., Guerrero, M. A., & Dultzin-Hacyan, D. 2006, A&A, 460, 45
- Goudfrooij, P.; de Jong, T.; Norgaard-Nielsen, H. U.; Hansen, L.; Jorgensen, H. E. 1990, A&A, 228, L9
- Greene, J. E.; Hood, Carol E.; Barth, Aaron J.; Bennert, Vardha N.; Bentz, Misty C.; Filippenko, Alexei V.; Gates, Elinor; Malkan, Matthew A.; Treu, Tommaso; Walsh, Jonelle L.; Woo, Jong-Hak 2010, arXiv:1009.0532.
- Heckman, T. M. 1980, A&A, 87, 152
- Ho, L. C., Filippenko, A. V., & Sargent, W. L. W. 1997, ApJ, 487, 568
- Ho, L. C. 1999, ApJ, 516, 672
- Ho, Luis C.; Feigelson, Eric D.; Townsley, Leisa K.; Sambruna, Rita M.; Garmire, Gordon P.; Brandt, W. N.; Filippenko, Alexei V.; Griffiths, Richard E.; Ptak, Andrew F.; Sargent, Wallace L. W.Ho, L. C., et al. 2001, ApJ, 549, L51
- Ho, L. C. 2002, ApJ, 564, 120
- Ho, L. C. 2008, ARA&A, 564, 120
- Laine, S., van der Marel, R. P., Rossa, J., Hibbard, J. E., Mihos, J. C., Böker, T., & Zabludoff, A. I. 2003, AJ, 126, 2717
- Maiolino, R.; Comastri, A.; Gilli, R.; Nagar, N. M.; Bianchi, S.; BÃ¶ker, T.; Colbert, E.; Krabbe, A.; Marconi, A.; Matt, G.; Salvati, M. 2003, MNRAS, 344, L59
- Malizia, A.; Stephen, J. B.; Bassani, L.; Bird, A. J.; Panessa, F.; & Ubertini, P. 2009, MNRAS, 399, 944
- Maoz, D., Filippenko, A. V., Ho, L. C., Rix, H.-W., Bahcall, J. N., Schneider, D. P., & Macchetto, F. D. 1995, ApJ, 440, 91
- Maoz, D., Nagar, N. M., Falcke, H., & Wilson, A. S. 2005, ApJ, 625, 699
- Maoz, D. 2007, MNRAS, 377, 1696
- Márquez, I., Masegosa, J., Moles, M., Varela, J., Bettoni, D., & Galletta, G. 2003, Ap&SS, 284, 711
- Nagar, N. M., Falcke, H., Wilson, A. S., & Ho, L. C. 2000, ApJ, 542, 186
- Nagar, N. M., Falcke, H., Wilson, A. S., & Ulvestad, J. S. 2002, A&A, 392, 53
- Nagar, N. M., Falcke, H., & Wilson, A. S. 2005, A&A, 435, 521
- Panessa, F., Bassani, L., Cappi, M., Dadina, M., Barcons, X., Carrera, F. J., Ho, L. C., & Iwasawa, K. 2006, A&A, 455, 173
- Panessa, F.; Bassani, L.; de Rosa, A.; Bird, A. J.; Dean, A. J.; Fiocchi, M.; Malizia, A.; Molina, M.; Ubertini, P.; Walter, R. 2008 A&A, 483, 151
- Peterson, B. M.; Berlind, P.; Bertram, R.; Bischoff, K.; Bochkarev, N. G. et al. 2002, ApJ, 581, 197
- Pian, E., Romano, P., Maoz, D., Cucchiara, A., Pagani, C., & Parola, V. L. 2010, MNRAS, 401, 677
- Pogge, R. W., Maoz, D., Ho, L. C., & Eracleous, M. 2000, ApJ, 532, 323
- Sarzi, M.; Falcón-Barroso, J.; Davies, R.L.; Bacon, R.; Bureau, M.et al. 2006, MNRAS, 366, 1151
- Schmitt, H. R., Donley,J. L., Antonucci, R. R. J., Hutchings, J. B., Kinney, A. L., & Pringle, J. E. 2003, ApJ, 597, 768
- Shapiro, K. L., Cappellari, M., de Zeeuw, T., McDermid, R. M., Gebhardt, K., van den Bosch, R. C. E., & Statler, T. S. 2006, MNRAS, 370, 559
- Shields, J. C.; Rix, H.-W.; Sarzi, M.; Barth, A.J.; Filippenko, A.V. et al. 2007, ApJ, 654, 125
- Simões Lopes, R. D., Storchi-Bergmann, T., de Fátima Saraiva, M., & Martini, P. 2007, ApJ, 655, 718
- Starling, R. L. C.,Page, M. J., Branduardi-Raymont, G., Breeveld, A. A., Soria, R., & Wu, K. 2005, MNRAS, 356, 727
- Urry, C. M., Padovani, P. 1995 PASP, 107, 803
- van Dokkum, P.G. 2001 PASP, 113, 1420
- Veilleux, Sylvain; Cecil, Gerald; Bland-Hawthorn, Joss 2005 ARA&A, 43, 769
- Verdoes Kleijn, G. A., Baum, S. A., de Zeeuw, P. T., & O’Dea, C. P. 1999, AJ, 118, 2592
- Walsh, J. L., Barth, A. J., Ho, L. C., Filippenko, A. V., Rix, H.-W., Shields, J. C., Sarzi, M., & Sargent, W. L. W. 2008, AJ, 136, 1677
- Worrall, D. M., Birkinshaw, M., Laing, R. A., Cotton, W. D., & Bridle, A. H. 2007, MNRAS, 380, 2
- Worrall, D. M., Birkinshaw, M., & Hardcastle, M. J. 2003, MNRAS, 343, L73
- Wu, Q. & Cao, X. 2005 ApJ, 621, 130