Evidence of boosted Co/CO ratio in early-type galaxies in dense environments
We present observations of CO(1–0) in 17 Combined Array for Research in Millimeter Astronomy (CARMA) ATLAS early-type galaxies (ETGs), obtained simultaneously with CO(1–0) observations. The CO in six ETGs is sufficiently bright to create images. In these 6 sources, we do not detect any significant radial gradient in the CO/CO ratio between the nucleus and the outlying molecular gas. Using the CO channel maps as 3D masks to stack the CO emission, we are able to detect 15/17 galaxies to (and 12/17 to at least ) significance in a spatially integrated manner. Overall, ETGs show a wide distribution of CO/CO ratios, but Virgo cluster and group galaxies preferentially show a CO/CO ratio about 2 times larger than field galaxies, although this could also be due to a mass dependence, or the CO spatial extent (). ETGs whose gas has a morphologically-settled appearance also show boosted CO/CO ratios. We hypothesize that this variation could be caused by (i) the extra enrichment of gas from molecular reprocessing occurring in low-mass stars (boosting the abundance of C to C in the absence of external gas accretion), (ii) much higher pressure being exerted on the midplane gas (by the intracluster medium) in the cluster environment than in isolated galaxies, or (iii) all but the densest molecular gas clumps being stripped as the galaxies fall into the cluster. Further observations of CO in dense environments, particularly of spirals, as well as studies of other isotopologues, should be able to distinguish between these hypotheses.
keywords:galaxies: clusters: general – galaxies: elliptical and lenticular, cD – galaxies: ISM
While early-type galaxies (ETGs) were originally assumed to be mainly devoid of molecular gas (e.g., Bower et al. 1992), it has become increasingly apparent that these “red and dead” early-type galaxies often contain non-negligible amounts of cold gas (Wiklind & Rydbeck, 1986; Phillips et al., 1987; Wiklind & Henkel, 1989; Sage & Wrobel, 1989; Welch & Sage, 2003; Sage et al., 2007). Large-scale searches for molecular gas in ETGs have indeed shown that a significant proportion of these galaxies do contain a molecular gas reservoirs (e.g., Combes et al. 2007; Young et al. 2011), though many of the early searches targeted relative small unrepresentative samples of ETGs, rather than unbiased, volume-limited samples.
The ATLAS survey (Cappellari et al., 2011a) provides a complete, unbiased sample of morphologically-selected ETGs in a large local volume. All ATLAS galaxies were searched for molecular gas by Young et al. (2011), with a measured detection rate of %. The detection rate of molecular gas did not appear to be dependent on galaxy environment, with a similar fraction of group and cluster ETGs detected as field ETGs. Follow-up efforts included CO imaging (e.g., Young et al. 2008; Crocker et al. 2011; Alatalo et al. 2013). Davis et al. (2011) found that despite the fact that Virgo galaxies are detected at a similar rate to field ETGs, the origin of the gas within Virgo galaxies appears to be exclusively internal, while a large fraction of field ETGs acquire their molecular gas via accretion of gas-rich companions.
CO(1–0) has long been used as an optically thin tracer of CO, and for the most part has been shown to be a faithful tracer of it (Young & Sanders, 1986; Sage, 1990; Sage & Isbell, 1991; Aalto et al., 1995). The range of CO/CO ratios (hereafter ) seen in past studies (mainly in spirals, starbursts and mergers) was 0.06–0.4, with ongoing major mergers populating the lower end of the distribution (Aalto et al., 1991). Sage (1990) studied the CO(1–0) properties of 3 ETGs (NGC 404, NGC 4710 and NGC 5195) with the National Radio Astronomy Observatory (NRAO) 12m single-dish, and concluded that this small sample of S0 galaxies seems to have a similar value of as spiral galaxies on average. NGC 5195, known to have recently interacted with M51, has a much smaller than the other ETGs, similar to what is seen in NGC 1266 (Alatalo et al., 2011; Crocker et al., 2012), a peculiar ETG with compact, dense molecular gas (Alatalo et al., 2015) currently hosting an active galactic nucleus (AGN)-driven massive molecular outflow (Davis et al., 2012; Alatalo et al., 2014).
|()||(km s)||(km s)||(Jy km s)||(Jy km s)|
|IC 676||1360 – 1520||20||67.95.8|
|IC 719||1670 – 2080||40||–||23.82.9|
|NGC 2764||2570 – 2840||30||79.74.2|
|NGC 3607||680 – 1210||40||53.23.2|
|NGC 3619||1280 – 1770||80||–||15.11.3|
|NGC 3626||1320 – 1670||50||–||40.13.4|
|NGC 3665||1750 – 2400||50||91.24.4|
|NGC 4324||1520 – 1770||40||–||20.02.1|
|NGC 4429||830 – 1370||30||–||70.43.2|
|NGC 4435||560 – 980||30||–||32.62.6|
|NGC 4694||1150 – 1210||30||17.00.8|
|NGC 4710||940 – 1310||30||350.66.0|
|NGC 4753||919 – 1470||50||–||70.24.9|
|NGC 5379||1700 – 1880||30||–||22.92.3|
|NGC 5866||500 – 1010||30||250.98.4|
|PGC 029321||2760 – 2910||30||–||19.61.7|
|PGC 058114||1370 – 1640||30||73.43.1|
Channel width of the CO cube.
Does not include the 20% absolute millimeter flux calibration uncertainty. Errors were calculated as discussed in §3.2.
30m CO integrated flux densities (Crocker et al., 2012).
CO integrated flux densities from the CO cubes described in Alatalo et al. (2013), but derived with the same velocity resolution (and channels) as the CO cubes to match the CO resolution for our stacking analysis.
The CO map of IC 676 is uniformly weighted in Alatalo et al. (2013), whereas the other CO and CO maps in this work and are naturally weighted
from Crocker et al. (2012). Others values are calculated from IRAS fluxes.
IRAS data were unavailable, so the flux ratio was calculated based on the Herschel ratio (T. Bitsakis, private communication).
Dense gas (i.e., gas that is traced by HCN and HCO emission) was observed in a much larger sample of ETGs through the SAURON (Bacon et al., 2001) and ATLAS (Cappellari et al., 2011a) surveys, that targeted the strongest CO-detected ATLAS galaxies with the Institut de Radioastronomie Millimétrique (IRAM) 30m single-dish telescope (Krips et al., 2010; Crocker et al., 2012; Davis et al., 2013). In these works, there was some evidence that while ETGs show similar ratios of dense gas as spirals, the ratios may depend on environment, with Virgo galaxies showing boosted and HCN/CO ratios compared to field galaxies. With the Crocker et al. (2012) sample, statistically significant correlations between and absolute K-band luminosity, stellar population age, molecular to atomic gas ratio, dust morphology and dust temperature were found. The correlation between and environment was not however statistically significant, perhaps due to the low numbers of galaxies in dense environments such as the Virgo cluster.
Here we present the CO(1–0) observations from the CARMA array. In §3, we discuss the simultaneous CO(1–0) and CO(1–0) observations, and the techniques that were used to reduce and analyse the CO(1–0) data, including imaging strong detections and stacking all non-detections. In §2, we describe our sample galaxies and their properties. In §4, we discuss the implications of the CO distributions and how our new detections inform upon what affects in galaxies. In §5, we summarize our conclusions. The ATLAS sample properties are described in detail in Cappellari
et al. (2011a)
2 The Sample
The CARMA sample has been extracted from the ATLAS sample of galaxies, which is a volume-limited sample (D42 Mpc) of massive (M-21.5 mags) morphologically-selected early-type galaxies (Cappellari et al., 2011a). The subsample of galaxies observed with CARMA have been selected among those detected in the CO line by Young et al. (2011), who observed the complete ATLAS sample. CARMA then imaged all previously un-imaged CO-detected ETGs above a flux limit of 18.5 Jy km s (Alatalo et al., 2013). Figure 1a shows the distributions of CO luminosities for CO-detected ETGs from Sage (1990); Crocker et al. (2012) and this work compared to the unbiased, flux-limited survey of Young et al. (2011), indicating that CO-detected ETGs tend to populate the high end of the distribution. Given that CARMA observed the brightest CO ETG detections from Young et al. (2011), the fact that CO detections tend to populate the bright portion of the CO distribution is unsurprising.
Figure 1 shows that when we draw comparisons to the available CO-detected galaxies in the literature (Young & Sanders, 1986; Sage & Isbell, 1991; Aalto et al., 1995), that are either late-type galaxies (LTGs), starbursts or mergers, the CO-detected ETGs fall to the lower end of the distribution of CO luminosities, consistent with what is found in CO-detected ETGs in general (Young et al., 2011). So our CO ETG sample has properties entirely consistent with a population of CO-bright ETGs.
It also appears clear that the galaxies added in this work do not fundamentally differ in their CO properties from the galaxies considered in previous ETG work (Young et al., 2011), and appear to be a robust addition to the CO-detected ETGs. If we look specifically at the representation of Virgo galaxies, we find that 7/26 (268%) are Virgo members within the CO-detected sample, compared to 12/56 (215%) of the CO-detected ETGs from Young et al. (2011). Thus it does not appear that Virgo members are over- or under-represented in our sample. CO-detected ETGs were also selected from this subsample, using identical selection criteria to Virgo and thus are also neither over- nor under-represented in the sample.
Though the 7 Virgo members have comparable average properties to the non-Virgo CO-detected ETGs (within the scatter), including stellar mass ( vs. ) and molecular gas extent ( vs. ; Davis et al. 2013), they do skew more massive with more compact molecular discs. This is likely due to the small number of Virgo galaxies within this sample, but the scatter is large due to small number statistics. Further CO studies of more Virgo members should reduce the scatter.
3 Observations and Results
CO(1-0) observations were obtained for 17 of the 30 original CARMA CO(1–0) galaxies. The galaxies missing CO(1-0) data are those imaged before Spring 2009, when upgrades to the CARMA receivers allowed for simultaneous imaging of both lines. The total available bandwidth from CARMA in each line varies as well depending on the semester the source was observed, due to upgrades to the correlator. Total bandwidths for each object are listed in Table 2 of Alatalo et al. (2013). All data reduction and calibration were done simultaneously with the CO imaging published in Alatalo et al. (2013). Table 1 presents the galaxies observed in CO, along with CO-specific observing parameters (including beam size, bandwidth and channel widths). The calibrators used, and total observing time for each source, can be found in Alatalo et al. (2013).
3.1 Strong CO detections: CO maps
The CO(1–0) emission was strong enough (emission was detected with signal-to-noise ratio 3) to map 6 of the 17 observed galaxies. Moment maps were constructed based on the methods described in Alatalo et al. (2013), with a full velocity width determined by the CO(1–0) line. Figure 2 shows the six mapped galaxies: NGC 3665, NGC 4429, NGC 4435, NGC 4710, NGC 4753 and NGC 5866. To test the agreement between the CARMA detections and the single dish detections from Crocker et al. (2012), we convolved the images of the three overlapping galaxies with strong CO detections: NGC 3665, NGC 4710 and NGC 5866, to the size of the IRAM 30m beam, and compared , finding them to be in agreement within errors with reported in Crocker et al. (2012).
For these six galaxies, we attempted to investigate whether varies with position. We first regridded both the CO and CO maps so that the area of one square pixel equalled the area of the CO beam, this using the Interactive Data Language (IDL) Astronomer’s Library routine hcongrid
3.2 Weak CO detection: CO spectra
To extract CO integrated flux densities for the fainter galaxies, where CO is not detected in individual beams, we utilize our prior knowledge of the molecular gas distribution from the brighter CO emission. This technique is also used to extract integrated flux densities for the strong CO detections. We used a smoothed CO datacube (created to correspond both to the weighting scheme as well as velocity properties of the CO maps) for each galaxy as an priori dataset, and for each channel determined the two-dimensional region over which the CO line is detected with a signal-to-noise ratio above 1.5. This 3D region then represents a CO detection mask (see Schruba
et al. 2011 for details of a similar method). For each galaxy, we then sum the CO datacube over its detection mask channel by channel, to obtain an integrated spectrum (shown in Fig. 3) and thereafter an line flux
The RMS per channel was determined by calculating the standard deviation of the line flux in an area equivalent to the mask, created by the miriad task restor in “residual” mode, and multiplying by , where is the total size of the CO mask area normalized to the beam area. The RMS on the total line flux was then calculated by multiplying the noise per channel by the width of the channel (shown in Table 1) and multiplying by the square root of the sum of the noise squared in each individual channel (the shaded channels in Fig. 3):
where is the channel number.
There is a further 20% absolute flux calibration uncertainty associated with millimeter observations, due to the time variability of millimeter flux calibration sources. While this 20% uncertainty impacts the total CO line flux measurements, it does not factor into , given that all CO and CO observations were taken simultaneously. A further 30% up-correction was made to the RMS to attempt to account for the oversampling of our interferometric maps. The 30% correction was derived by calculating the noise in the oversampled (1 pixel) maps, and comparing that noise to the RMS derived from maps whose pixel area equals that of the synthesized beam. The RMS per channel for each galaxy is shown in the upper-left corner of each panel.
|CO Type||N||Mean||Std. Deviation|
|Field galaxies (||32||0.112||0.036|
|Field ETGs (||8||0.120||0.066|
The field sample includes all galaxies not in the Virgo cluster
Table 1 lists the galaxies that were observed in CO during the CARMA ATLAS survey, with the derived integrated flux densities and values from the stacked spectra. Of the 17 galaxies, 15 are detected using the a priori masking technique. Figure 3 presents the stacked CO spectra of the 15 detected galaxies. Comparing the line shapes of CO (blue) and CO (red), we see that they are generally in good agreement, supporting our claim that the stacking technique as well as our chosen mask threshold have successfully detected CO emission.
Crocker et al. (2012) published IRAM 30m single-dish spectra of CO(1–0) and CO(2–1) for 18 of the ATLAS galaxies. Our sample overlaps the Crocker et al. (2012) sample in the case of 8 galaxies: IC 676, NGC 2764, NGC 3607, NGC 3665, NGC 4694, NGC 4710, NGC 5866 and PGC 058114. We detect all overlapping galaxies at a signal-to-noise ratio . CARMA recovers more emission than the IRAM 30m in all cases except IC0676, equal within the uncertainty (see Table 1). In many cases, this is likely because the CO emission subtends a larger angular size than the 30m beam (as was often the case for the CO emission published in Alatalo et al. 2013). Because CO and CO were measured simultaneously with CARMA, does not suffer from flux calibration uncertainties. We therefore elect to use the CO integrated flux densities from the current work for all overlapping galaxies.
Radio continuum pollution leading to enhanced CO fluxes is not likely to be an issue. NGC 3665 and NGC 4710 are the only two galaxies in this sample with detected continuum, at and 4 mJy, respectively, at 110 GHz. The CO data processing includes a continuum subtraction step (detailed in Alatalo et al. 2013).
Table 2 compares in the nuclear and extended regions of NGC 3665, 4429, 4435, 4710, 4753 and 5866 (see Section 3.1). In the first four, is larger in both the nuclear and extended region than in the stacked spectrum, and vice versa for NGC 4753 and NGC 5866. The discrepancies between stacking and resolved measurements can be explained by many effects. Stacking is able to detect fainter CO emission from a larger area than the resolved data. Compared to well-resolved objects, where the brightest CO peaks were able to fill the beam, the average from stacking would be lower than from the resolved maps. On the other hand, in cases where CARMA has marginally detected CO in the maps, stacking would be able to increase the total CO flux detected, resulting in a higher for stacking than mapping. Deeper CO observations might be able to fully explain this discrepancy, but are beyond the scope of this paper.
4.1 Distribution of CO in the galaxies
Figure 2 shows the spatial distribution of the CO emission, compared to the CO in ATLAS galaxies. Only the galaxies with the brightest CO are detectable in a spatially-resolved way. There are many features worthy of notice in the CO distributions.
NGC 4710 and NGC 5866 have strong, edge-on bars whose kinematic features were detected in the CO emission (see §4.2.2 in Alatalo et al. 2013). In these cases, the CO preferentially traces the bar region, particularly a nuclear disc or ring and an inner ring, the locations of the largest concentrations of molecular gas in barred systems (Athanassoula & Bureau, 1999). NGC 4710 and NGC 5866 were also among the handful of galaxies detected in OH absorption by McBride et al. (2015), confirming high optical depths. The CO detections at the disc edges further confirm the conclusions of Alatalo et al. (2013) and Davis et al. (2013), that these systems are bar+ring systems, as the CO there is likely tracing the portions of the edge-on rings with higher column densities (edge brightening). Topal et al. (in preparation) confirm this and study the spatially-resolved bar signatures using a multitude of high-density molecular tracers in both galaxies.
The Virgo galaxies NGC 4429 and NGC 4435 are robustly detected in CO throughout the extent of the CO detections. In both cases, CO faithfullly traces CO. NGC 4753, known to be at the centre of a developing group (Steiman-Cameron et al., 1992), also seems to have CO that traces the brightest portions of CO, though it is likely that beyond these points the CO emission simply drops below the detection threshold (rather than being absent).
4.2 Radial gradients in
Figure 4 show in nuclear and surrounding gas in the 6 galaxies mapped, with the points representing the nuclear pixels and the open circles representing the extended emission (with resolved values shown in Table 2). As discussed in §3.2, ratios measured from the images differ slightly from those measured with the stacked spectra, likely due to the images missing faint CO emission picked up in the stacked spectra. Within the selected pixels, these ratios are thus likely reasonable, but they are not necessarily representative of the entire galaxy, where the stacking method is better able to recover emission below the local detection threshold.
In spiral galaxies, Paglione et al. (2001) found a gradient, increasing from the nuclear region to the outlying regions on average, though this result had large scatter. Molecular discs in ETGs are much less extended than those in spirals (Davis et al., 2013) in absolute terms, so it is possible that a variation will be much less pronounced in ETG discs, either due to resolution or because the gas is better mixed (i.e. shorter dynamical times). In the 6 spatially-resolved CARMA detections (shown in Fig. 4), there is no decipherable trend, and in 5 of the 6 objects the nuclear and non-nuclear values are consistent within the errors. NGC 3665 is the single object in which the trend is larger than the uncertainties, but follows the opposite trend from spirals, and could be due to sensitivity limitations in detecting fainter CO flux. There might be a slight increase in from the nuclear region to the off-nuclear regions in NGC 4429 and NGC 5866, but remains flat in NGC 4710 and decreases in NGC 3665, NGC 4435 and NGC 4753. It is however possible that we are missing a non-negligible amount of faint extended emission, due to the sensitivity limitations of the CARMA sample. As mentioned above (§3.2), the disagreement in between the image measurements and the stacked measurements is evidence that this might be the case.
We therefore do not see any evidence for an increase in across galactic discs in this subset of ETGs, in contrast to what has been seen in spirals, but we cannot rule out that such a gradient could exist, given the sensitivity as well as resolution limitations of our observations. Deeper, higher resolution follow-up observations should more robustly establish whether ETGs indeed lack a gradient, whether this is an environmental effect, or whether our result is due to sensitivity issues.
4.3 Variations of based on galaxy type and environment
Figure 5 presents the distributions of as a function of galaxy types, in the new CARMA detections of Virgo and non-Virgo ETGs, the Crocker et al. (2012) ETGs, NGC 404 and NGC 5195 from Sage (1990), as well as literature spirals and mergers (Young & Sanders, 1986; Sage & Isbell, 1991; Aalto et al., 1995). Figure 5 also shows the distribution as a function of environment in the case of ETGs only (as there is no Virgo spiral with CO data currently in the literature). Spirals and mergers tend to have , while ETGs are distributed much more evenly as a function of .
We use the morphological classifications from Alatalo et al. (2013) to differentiate between settled and disturbed gas morphologies, though NGC 1266 was re-classified as disturbed, since the turbulent nature of the molecular gas is visible in the CO and CS(2–1) lines using higher resolution imaging (Alatalo et al., 2011, 2015). The CO in NGC 404 was classified as a ring (Wiklind & Henkel, 1990), and is therefore settled, while that in NGC 5195 would be classified as mildly disrupted (Kohno et al., 2002), and is considered disturbed. The value within ETGs seems to have a strong dependence on the disruption of the molecular gas, with disrupted gas distributions tending toward the lowest Ẇhen ETGs are split by environment, the Virgo ETGs represent the majority of ETGs with , while field ETGs average less than =0.1. The distributions not only show that environment seems to play a role, but also that disrupted molecular gas distributions tend toward lower in ETGs.
Figure 6 compares with the dust temperature, represented by the 60-to-100 m flux ratio. Young & Sanders (1986) discussed the dependence of on the dust temperature, with lower values of corresponding to higher values of , further confirmed by the observations of Sage & Isbell (1991) who increased the sample size. Aalto et al. (1995) drastically increased the number of spirals for which CO observations exist, confirming the trend as well as hypothesizing that turbulence within the molecular gas might influence . Warmer dust temperatures are a reasonable tracer of turbulence in the underlying molecular gas, as an abundance of young stars will provide both UV photons to heat the dust and momentum to stir up the molecular gas. (Ogle et al., 2010). However, the presence of Virgo galaxies at low also having the highest indicate that environment could also be a factor in setting .
Crocker et al. (2012) found evidence that Virgo galaxies and those galaxies in groups have boosted values, but due to small number statistics were not able to confirm that result. The CARMA CO detections add many more ETGs to the Crocker et al. (2012) work, including an additional 3 Virgo galaxies. The addition of these ETGs not only strengthens the trend, but appears to also further support the suggestion that turbulence might influence (Young & Sanders, 1986; Aalto et al., 1995). Magenta outlines in Fig. 6 correspond to ETGs with disturbed molecular gas. The disturbed class of ETGs almost exclusively appears at values of , the only exception being NGC 4694 that also is a member of the Virgo cluster.
In spirals and mergers, there appears to be a ceiling in of around 0.15 for the majority of sources. Once the ETGs are included, a large proportion of them inhabit the region, with Virgo ETGs exclusively lying above this threshold. NGC 4710 (identified as a Virgo member in Cappellari et al. 2011a) is found to have the lowest in the Virgo ETG sample, of 0.15, though it is of note that while NGC 4710 has a velocity similar to other Virgo cluster members, it is not formally within the virial radius of Virgo. in the CARMA sample is on average larger than that in all other samples discussed, due in part to the fact that the CARMA observations include more ETGs with predominantly settled gas distributions (which were not among the brightest CO detections in ATLAS first followed up in CO with the IRAM 30m; Crocker et al. 2012). This inclusion of the ETGs with settled molecular gas seems have increased the population of objects in the region by over a factor of two.
Table 3 gives the mean and standard deviation of each subsample of galaxies shown in Fig. 5, including isolated galaxies based on environment, morphology and dust temperature. Overall, Virgo ETGs have a larger mean value than all other subsamples and are distinct (means inconsistent within the standard deviations) from all other subsamples except for cold dust field ETGs (of which there are only 8). While these two populations agree within the errors, the cold dust field ETG population is much more similar to the complete cold dust field sample than to Virgo ETGs. Thus, while we cannot completely rule out that the variation in that we observe is solely dependent on molecular gas turbulence (as traced by dust temperature), it does appear that in Virgo is boosted.
Figure 7 isolates the ETGs and spirals in the ATLAS parent sample (Cappellari et al., 2011a), performing Spearman rank statistics as was done for the original sample from Crocker et al. (2012). The Spearman rank correlation coefficient and the probability that the null hypothesis (no correlation) is true are given. Low probabilities for the null hypothesis (we define ‘low’ to be less than 0.03) suggest the data are correlated. In most cases, the correlations that were originally seen in Crocker et al. (2012) (including those with (1) dust temperature, (2) stellar mass and (3) environment) persist within the larger sample, and a new correlation is observed with the (4) molecular gas extent ( from Davis et al. 2013), which was only performed on the ETGs. The dependence on (the 3 Mpc number density; a proxy for environment from Cappellari et al. 2011a) has strengthened with the increased number statistics, but so has the dependence on galaxy mass. We are thus unable to determine whether the variation in is independently correlated with stellar mass, environment, or molecular extent, or if our CO-detected Virgo ETGs are simply more massive and compact than the CO-detected field ETGs (discussed in §2) or spirals, causing the correlation.
When we differentiate between types of molecular gas in ETGs, either using the CO morphological classifications from Alatalo et al. (2013), molecular gas that is disturbed morphologically preferentially has a much lower than that of settled or aligned discs. In fact, on average morphologically-settled molecular discs have values almost two times higher than those of disturbed molecular distributions. When we compared the to the kinematic misalignment between the gas and the stars (; Davis et al. 2011), the probability of a null hypothesis was 24%, meaning that it appears that the disruption of the molecular gas matters far more than whether the gas was externally accreted. Figure 6 outlines in magenta ETGs that have been classified as “disrupted” by Alatalo et al. (2013), and shows that in general those galaxies have lower , (in addition to high CO integrated flux densities, given they were part of the flux-limited dense gas survey from Crocker et al. 2012).
This fits quite well with the work of Aalto et al. (1995), who see similarly low values in mergers, compared to normal spirals. It is difficult to parse whether the differences in distributions we see are because galaxies in Virgo tend to have much more settled molecular discs, or if there are other effects at play. Further studies of CO in a large number of ETGs with settled molecular discs, in a wide variety of environments, will likely shed light on the relative importance of morphology and environment. No previous CO study includes observations of Virgo spirals.
4.4 Possible causes of the variation in , optical depth or abundance?
In the giant molecular clouds (GMCs) of our Milky Way, typical values are 0.2 for whole GMCs, and 0.3 for GMC centres (Polk et al., 1988). So an ensemble dominated by cold GMC-like clouds should have a high . Ratios above this can be understood as arising from optically thick and very cold gas, perhaps like a GMC but with its surrounding envelope stripped away. We know that the CO/CO line ratio correlates strongly with the dust colour (or temperature; Young & Sanders 1986; Aalto et al. 1995), as is clearly seen in Figure 6. Alternatively, a high value could be partially a result of enhanced C over C, although this would still require very optically thick gas. An illustration of the latter is the fact that the C/C abundance ratio decreases strongly with radius in the Milky Way (Milam et al., 2005), but has a strong opposite trend (Digel et al., 1990; Carpenter et al., 1990; Sodroski, 1991). This is because the CO optical depth in Galactic centre clouds is significantly lower due to both temperature effects and the presence of unbound gas.
For the CO/CO line ratio, optical depth effects may either be caused by temperature/density changes (e.g. Aalto et al. 1997), or by increased line-widths in diffuse, non self-gravitating gas (e.g. Aalto et al. 2010). These do not require a secondary boost in the C isotope compared to C, that has been suggested as taking place during stellar processing, specifically in low-mass stars (Casoli et al., 1991). Crocker et al. (2012) considered abundance, fractionation and optical depth explanations for the trends seen in their smaller sample of ETGs, and concluded that optical depth (mediated by the gas dynamical state) dominates the ratio changes. Here, with additional sample galaxies and more information about the gas dynamics from the interferometric maps, we return to this question. In addition, we consider a new option, the evaporation of lower-mass GMCs, or “the survival of the densest.”
Carbon-bearing molecules are susceptible to a process called fractionation. This is particularly true for CO, and therefore CO/CO abundance ratios may deviate from the C/C ratio from nucleosynthesis. There are two competing processes. The deviation may be in the form of an enhancement or a reduction depending on which process dominates. The first process is selective photodissociation, which will enhance CO since it is the more abundant isotopic variant and self-shields more effectively (e.g. van Dishoeck & Black 1988). The second process operates when gas kinetic temperatures are low to moderate (below 35 K) and elevates CO through the isotopic charge exchange reaction:
This results in CO/CO abundances above the C/C ratio from nucleosynthesis by up to a factor of five (e.g. Sonnentrucker et al. 2007), without also requiring an enhancement in the C/C abundance.
How can we test if our ratios are caused by ion molecule exchange reactions? The dust temperatures are low, so if they are related to the gas kinetic temperatures fractionation seems possible. Ritchey et al. (2011) point out that because CO is the most abundant carbon-bearing molecule in the ISM, an enhancement in CO will result in a depletion of the carbon reservoir of C. Other molecules originating from the remaining carbon should therefore have a C/C ratio that behaves in the opposite sense compared to , i.e. we would expect to see smaller C/C ratios in these species for Virgo galaxies. Ritchey et al. (2011) suggest CN and CH as comparison species, and for our purposes CN should work well. CN has the additional advantage that the two adjacent spin groups can be used for an accurate measure of the optical depth. HCN and HCO should also work quite well. We note that, for homogeneous clouds, the enhancement of CO should occur mostly in the outer regions where there is sufficient C. If the clouds have structure the enhancement will leak further into the clouds.
While fractionation is a compelling possibility to create the enhanced ratios seen, Crocker et al. (2012) did not see a correlation between CO/HCN (compared to CO/HCN) and morphological type, or temperature, potential signs that fractionation is an important driver of the variations. Observations of dense gas across a larger sample of molecular gas-containing ETGs would further clarify whether or not we can completely rule out fractionation, but current data suggest it is not a dominant effect.
Enhanced C abundances from stellar reprocessing
A possible origin for the distribution of ratios is that the molecular gas in Virgo galaxies has resided in those galaxies for far longer than that of average field ETGs. Davis et al. (2011) discovered that the angular momentum of ionised and molecular gas in Virgo galaxies is always aligned with the stellar angular momentum, which was posited to mean that the molecular gas within Virgo ETGs is not externally accreted, and either is due to stellar mass loss (Faber & Gallagher, 1976) or remains from aligned molecular gas when the galaxy first entered Virgo, thus supporting this claim.
The observation that some Virgo spirals may have significantly higher O/H abundances within their ionised gas than field galaxies (Shields et al., 1991) also support this claim, although more recent metallicity studies contradict it (Hughes et al., 2013). Casoli et al. (1991); Casoli et al. (1992a, b) discuss low ratios as likely being an abundance effect, with more primordial (and largely unprocessed) molecular gas containing fewer C atoms, since this isotope is formed via low-mass stellar nucleosynthesis. The interstellar medium (ISM) in Virgo galaxies would have existed sufficiently long for this enrichment to occur. NGC 4710, which is on the outskirts of Virgo, has =0.15, at the junction between most spirals, interacting galaxies, and Virgo galaxies. If NGC 4710 is on its first approach (suggested by its presence outside of the Virgo virial radius), and therefore just losing the ability to accrete new primordial gas, then this enrichment has just started to boost the C abundance and is beginning to impact .
Objects with morphological signs of recent accretion events are the most likely to also have low values, as is expected if the gas in the objects that recently accreted is more primordial and less enhanced. Though this is the case, kinematic misalignments with stars, consistent with external accretion, did not show such a distinctive difference, especially compared to the disruption seen in the gas (see §4.3). If we assume that on average, the accreted molecular gas has a lower C abundance, then there should be a trend toward lower from externally acquired gas, not solely morphologically disrupted gas.
Studying isotopic line ratio in other gas species (HCN vs. HCN, for instance) might be able to confirm whether enrichment plays a critical role in the ratio. Alternatively, studying at higher excitation, and getting a different value of in the settled discs, could rule out enrichment (though not necessarily confirm it).
Rather than being an abundance effect, the elevated in Virgo ETGs could also be the result of midplane pressure, that can modify the optical depth of the molecular gas. First posited by Crocker et al. (2012) to describe the correlations seen in the original sample of ATLAS ETGs, it is possible that the hot intracluster medium (ICM) within Virgo produces a larger than normal midplane pressure in galaxies. This excess pressure on the molecular disc would compactiy it, creating a disc with higher optical depth (and thus higher ) than is seen in field galaxies. The fact that the resolved maps of NGC 4429 and NGC 4435 do not appear to show any significant spatial variation in seems to support the possibility of a high pressure throughout.
However, high optical depths within the discs of these galaxies would likely imply a higher fraction of dense gas, and thus impact their star formation efficiencies (Gao & Solomon, 2004), but Davis (2014) have shown that the star formation efficiencies of Virgo ETGs are similar to those of all ETGs. Since it also appears that the molecular gas distribution plays a role in this ratio for ETGs, the high in non-Virgo settled ETGs seems to indicate that midplane pressure is not the dominant effect in these systems.
Preferential evaporation of lower density GMCs
It is possible that the observed enhancement of is nothing more than “survival of the densest” (GMCs). As Virgo galaxies move through the ICM, and the low density clumps are stripped (leaving only the densest clumps, likely with high ), one would expect our measurements to probe only the highest end of the usual density distribution of GMCs. Pineda et al. (2008) show that the highest values in local clumps ( 0.4), with derived densities of cm, are indeed comparable to the average of the Virgo galaxies. The mild correlation of the HCN/CO ratio with seen by Crocker et al. (2012) also supports this hypothesis. In fact, results from the Herschel Virgo Cluster Survey (Davies et al., 2012) suggest that dust and gas densities are higher in Virgo cluster galaxies than in field galaxies, likely requiring denser constituent molecular clouds. Higher resolution studies of GMCs in Virgo galaxies with ALMA would further elucidate whether the properties of the dust transfer directly to the molecular gas properties in these systems.
4.5 Investigating the origins of the variation
To fully understand the dependence of on environment, as well as what mechanism drives that dependence, further CO observations of Virgo galaxies, including Virgo spirals, will likely be required, hopefully shedding light on what is causing the large spread in values observed (including the Virgo–field galaxy dichotomy). If C abundance is the most important factor, then the gas phase metallicity (a proxy for “primordality”) should correlate with . If midplane pressure is the dominant factor for the variation in , then the ratio in varying environments (including galaxy groups and possibly the Coma Cluster) should correlate. Finally, if entry into the hot X-ray gas halo of the Virgo cluster has stripped all but the densest clumps, then a measure of how long each galaxy has been within the cluster should correlate with . A high resolution census of the GMCs in Virgo (similar to the investigation of Utomo et al. 2015) might also provide evidence for whether “survival of the densest” is able to explain the variations we see in .
The CARMA ATLAS CO survey of Alatalo et al. (2013) included 17 galaxies simultaneously observed in CO(1–0), increasing the number of ATLAS ETGs observed in CO by 50%, and including far fainter CO detections (and more objects with settled molecular gas) than the original work of Krips et al. (2010) and Crocker et al. (2012).
We presented here the CO(1–0) resolved maps of 6 ETGs from the CARMA ATLAS survey, those detected with sufficient significance to derive spatial-resolved CO information. An a priori masking analysis based on the method of Schruba et al. (2011) allowed us to detect CO in 15/17 of the observed sources, this in a spatially-integrated manner.
There does not appear to be evidence for spatial variations in in the 6 ETGs we were able to map, though it is likely we would not have been able to detect a gradient at the level found in Paglione et al. (2001), given the signal-to-noise and resolution of our data.
Combining these new observations with those from Crocker et al. (2012) and other data from the literature, we find that Virgo ETGs have a significantly higher than both field ETGs, and spirals and interacting galaxies, although ETGs with settled molecular discs also have preferentially higher than both spirals and ETGs with morphological disruptions. It is also possible that the variations of are driven by mass or molecular extent, as Virgo ETGs are, on average, more massive and contain more compact molecular discs than field ETGs.
We hypothesize that the higher ratio in Virgo could be caused by extra low-mass stellar enrichment taking place in Virgo cluster galaxies (boosting the C elemental abundance in the absence of external gas accretion), by increased midplane pressure due to the ICM, or by the survival of only the densest clumps of molecular gas (as the galaxies enter the Virgo ICM). Studies of CO in more Virgo galaxies, particularly spirals, as well as studies of other C and C isotopologues, should be able to distinguish between these hypotheses.
K.A. thanks Erik Rosolowsky, Mark Lacy, Francoise Combes, Adam Leroy, Asunción Fuente and Jeff Kenney for insightful conversations that have improved this work, Theodoros Bitsakis for Herschel related questions and Selcuk Topal for a detailed investigation of the single dish data from NGC 5866. K.A. would also like to thank the anonymous referee for insightful comments and thoughtful recommendations. K.A. is supported by funding through Herschel, a European Space Agency Cornerstone Mission with significant participation by NASA, through an award issued by JPL/Caltech. M.B. is supported by the rolling grants ‘Astrophysics at Oxford’ PP/E001114/1 and ST/H002456/1 from the UK Research Councils. L.M.Y. is supported by the National Science Foundation under Grant No. NSF AST-1109803. Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the James S. McDonnell Foundation, the Associates of the California Institute of Technology, the University of Chicago, the states of California, Illinois, and Maryland, and the National Science Foundation. Ongoing CARMA development and operations are supported by the National Science Foundation under a cooperative agreement, and by the CARMA partner universities. 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.
- pagerange: Evidence of boosted CO/CO ratio in early-type galaxies in dense environments–References
- pubyear: 2015
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