arXiv subjects

Martin Bureau

Publications and source records attributed to Martin Bureau.

At least 73 records · Page 4Linked to original sources

arXiv2019

The AGN fuelling/feedback cycle in nearby radio galaxies - I. ALMA observations and early results

This is the first paper of a series exploring the multi-frequency properties of a sample of eleven nearby low excitation radio galaxies (LERGs) in the southern sky. We are conducting an extensive study of different galaxy components (stars, warm and cold gas, radio jets) with the aim of improving our understanding of the AGN fuelling/feedback cycle in LERGs. We present ALMA Band 6 $^{12}$CO(2-1) and continuum observations of nine sources. Continuum emission from the radio cores was detected in all objects. Six sources also show mm emission from jets on kpc/sub-kpc scales. The jet structures are very similar at mm and cm wavelengths. We conclude that synchrotron emission associated with the radio jets dominates the continuum spectra up to 230 GHz. The $^{12}$CO(2-1) line was detected in emission in six out of nine objects, with molecular gas masses ranging from $2 \times 10^{7}$ to $2 \times 10^{10}$ M$_{\rm \odot}$. The CO detections show disc-like structures on scales from $\approx$0.2 to $\approx$10 kpc. In one case (NGC 3100) the CO disc presents some asymmetries and is disrupted in the direction of the northern radio jet, indicating a possible jet/disc interaction. In IC 4296, CO is detected in absorption against the radio core, as well as in emission. In four of the six galaxies with CO detections, the gas rotation axes are roughly parallel to the radio jets in projection; the remaining two cases show large misalignments. In those objects where optical imaging is available, dust and CO appear to be co-spatial.

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arXiv2018

The Effect of Galaxy Interactions on Molecular Gas Properties

Galaxy interactions are often accompanied by an enhanced star formation rate (SFR). Since molecular gas is essential for star formation, it is vital to establish whether, and by how much, galaxy interactions affect the molecular gas properties. We investigate the effect of interactions on global molecular gas properties by studying a sample of 58 galaxies in pairs and 154 control galaxies. Molecular gas properties are determined from observations with the JCMT, PMO, CSO telescopes, and supplemented with data from the xCOLD GASS and JINGLE surveys at $^{12}$CO(1-0) and $^{12}$CO(2-1). The SFR, gas mass ($M_\mathrm{H_{2}}$), and gas fraction ($f_{gas}$) are all enhanced in galaxies in pairs by $\sim$ 2.5 times compared to the controls matched in redshift, mass, and effective radius, while the enhancement of star formation efficiency (SFE $\equiv$ SFR/$M_{H_{2}}$) is less than a factor of 2. We also find that the enhancements in SFR, $M_{H_{2}}$ and $f_{gas}$ increase with decreasing pair separation and are larger in systems with smaller stellar mass ratio. Conversely, the SFE is only enhanced in close pairs (separation $<$ 20 kpc) and equal-mass systems; therefore most galaxies in pairs lie in the same parameter space on the SFR-$M_{H_{2}}$ plane as controls. This is the first time that the dependence of molecular gas properties on merger configurations is probed statistically with a relatively large sample and with a carefully-selected control sample for individual galaxies. We conclude that galaxy interactions do modify the molecular gas properties, although the strength of the effect is merger configuration dependent.

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arXiv2018

JINGLE, a JCMT legacy survey of dust and gas for galaxy evolution studies: I. Survey overview and first results

JINGLE is a new JCMT legacy survey designed to systematically study the cold interstellar medium of galaxies in the local Universe. As part of the survey we perform 850um continuum measurements with SCUBA-2 for a representative sample of 193 Herschel-selected galaxies with M*>10^9Msun, as well as integrated CO(2-1) line fluxes with RxA3m for a subset of 90 of these galaxies. The sample is selected from fields covered by the Herschel-ATLAS survey that are also targeted by the MaNGA optical integral-field spectroscopic survey. The new JCMT observations combined with the multi-wavelength ancillary data will allow for the robust characterization of the properties of dust in the nearby Universe, and the benchmarking of scaling relations between dust, gas, and global galaxy properties. In this paper we give an overview of the survey objectives and details about the sample selection and JCMT observations, present a consistent 30 band UV-to-FIR photometric catalog with derived properties, and introduce the JINGLE Main Data Release (MDR). Science highlights include the non-linearity of the relation between 850um luminosity and CO line luminosity, and the serendipitous discovery of candidate z>6 galaxies.

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arXiv2018

CO Tully-Fisher Relation of Star-forming Galaxies at z=0.05-0.3

The Tully-Fisher relation (TFR) is an empirical relation between galaxy luminosity and rotation velocity. We present here the first TFR of galaxies beyond the local Universe that uses carbon monoxide (CO) as the kinematic tracer. Our final sample includes 25 isolated, non-interacting star-forming galaxies with double-horned or boxy CO integrated line profiles located at redshifts z <= 0.3, drawn from a larger ensemble of 67 detected objects. The best reverse Ks-band, stellar mass and baryonic mass CO TFRs are respectively M_Ks = (-8.4+/-2.9) [log(W_50/sini)-2.5]+(-23.5 +/- 0.5), log (Mstar/Msun) =(5.2+/-3.0) [log(W_50/sini)-2.5]+(10.1+/-0.5) and log (Mb/Msun) = (4.9+/-2.8) [log(W_50/sini)-2.5]+(10.2+/-0.5), where M_Ks is the total absolute Ks-band magnitude of the objects, Mstar and Mb their total stellar and baryonic masses, and W_50 the width of their line profile at 50% of the maximum. Dividing the sample into different redshift bins and comparing to the TFRs of a sample of local (z = 0) star-forming galaxies from the literature, we find no significant evolution in the slopes and zero-points of the TFRs since z = 0.3, this in either luminosity or mass. In agreement with a growing number of CO TFR studies of nearby galaxies, we more generally find that CO is a suitable and attractive alternative to neutral hydrogen (HI). Our work thus provides an important benchmark for future higher redshift CO TFR studies.

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arXiv2018

Shape of LOSVD\lowercase{s} in barred disks: Implications for future IFU surveys

The shape of LOSVDs (line-of-sight velocity distributions) carries important information about the internal dynamics of galaxies. The skewness of LOSVDs represents their asymmetric deviation from a Gaussian profile. Correlations between the skewness parameter ($h_3$) and the mean velocity (\vm) of a Gauss-Hermite series reflect the underlying stellar orbital configurations of different morphological components. Using two self-consistent $N$-body simulations of disk galaxies with different bar strengths, we investigate \hv\ correlations at different inclination angles. Similar to previous studies, we find anticorrelations in the disk area, and positive correlations in the bar area when viewed edge-on. However, at intermediate inclinations, the outer parts of bars exhibit anticorrelations, while the core areas dominated by the boxy/peanut-shaped (B/PS) bulges still maintain weak positive correlations. When viewed edge-on, particles in the foreground/background disk (the wing region) in the bar area constitute the main velocity peak, whereas the particles in the bar contribute to the high-velocity tail, generating the \hv\ correlation. If we remove the wing particles, the LOSVDs of the particles in the outer part of the bar only exhibit a low-velocity tail, resulting in a negative \hv\ correlation, whereas the core areas in the central region still show weakly positive correlations. We discuss implications for IFU observations on bars, and show that the variation of the \hv\ correlation in the disk galaxy may be used as a kinematic indicator of the bar and the B/PS bulge.

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arXiv2017

WISDOM Project - III: Molecular gas measurement of the supermassive black hole mass in the barred lenticular galaxy NGC4429

As part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM) project we present an estimate of the mass of the supermassive black hole (SMBH) in the nearby fast-rotating early-type galaxy NGC4429, that is barred and has a boxy/peanut-shaped bulge. This estimate is based on Atacama Large Millimeter/submillimeter Array (ALMA) cycle-2 observations of the 12CO(3-2) emission line with a linear resolution of ~13 pc (0"18x0"14). NGC4429 has a relaxed, flocculent nuclear disc of molecular gas that is truncated at small radii, likely due to the combined effects of gas stability and tidal shear. The warm/dense 12CO(3-2) emitting gas is confined to the inner parts of this disc, likely again because the gas becomes more stable at larger radii, preventing star formation. The gas disc has a low velocity dispersion of 2.2$^{+0.68}_{-0.65}$ km/s. Despite the inner truncation of the gas disc, we are able to model the kinematics of the gas and estimate a mass of (1.5$\pm0.1^{+0.15}_{-0.35}$)$\times$10$^8$ Msun for the SMBH in NGC4429 (where the quoted uncertainties reflect the random and systematic uncertainties, respectively), consistent with a previous upper limit set using ionised gas kinematics. We confirm that the V-band mass-to-light ratio changes by ~30% within the inner 400 pc of NGC4429, as suggested by other authors. This SMBH mass measurement based on molecular gas kinematics, the sixth presented in the literature, once again demonstrates the power of ALMA to constrain SMBH masses.

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arXiv2017

A star formation study of the ATLAS3D early-type galaxies with the AKARI all-sky survey

The star formation properties of early-type galaxies (ETGs) are currently the subject of considerable interest, particularly whether they differ from those of gas-rich spirals. We perform a systematic study of star formation in a large sample of local ETGs using polycyclic aromatic hydrocarbon (PAH) and dust emission, focusing on the galaxies' star formation rates (SFRs) and star formation efficiencies (SFEs). Our sample is composed of the 260 ETGs from the ATLAS3D survey, from which we use the cold gas measurements (HI and CO). The SFRs are estimated from stellar, PAH and dust fits to spectral energy distributions created from new AKARI measurements and literature data from WISE and 2MASS. The mid-infrared luminosities of non-CO-detected galaxies are well correlated with their stellar luminosities, showing that they trace (circum)stellar dust emission. CO-detected galaxies show an excess above these correlations, uncorrelated with their stellar luminosities, indicating that they likely contain PAHs and dust of interstellar origin. PAH and dust luminosities of CO-detected galaxies show tight correlations with their molecular gas masses, and the derived current SFRs are typically 0.01-1 Msun/yr. These SFRs systematically decrease with stellar age at fixed stellar mass, while they correlate nearly linearly with stellar mass at fixed age. The majority of local ETGs follow the same star-formation law as local star-forming galaxies, and their current SFEs do not depend on either stellar mass or age. Our results clearly indicate that molecular gas is fueling current star formation in local ETGs, that appear to acquire this gas via mechanisms regulated primarily by stellar mass. The current SFEs of local ETGs are similar to those of local star-forming galaxies, indicating that their low SFRs are likely due to smaller cold gas fractions rather than a suppression of star formation.

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arXiv2017

WISDOM Project - II: Molecular gas measurement of the supermassive black hole mass in NGC4697

As part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM) project, we present an estimate of the mass of the supermassive black hole (SMBH) in the nearby fast-rotating early-type galaxy NGC4697. This estimate is based on Atacama Large Millimeter/submillimeter Array (ALMA) cycle-3 observations of the 12CO(2-1) emission line with a linear resolution of 29 pc (0.53"). We find that NGC4697 hosts a small relaxed central molecular gas disc with a mass of 1.6x10^7 Msun, co-spatial with the obscuring dust disc visible in optical Hubble Space Telescope (HST) imaging. We also resolve thermal 1mm continuum emission from the dust in this disc. NGC4697 is found to have a very low molecular gas velocity dispersion, $σ_{gas}=1.65^{+0.68}_{-0.65}$ km/s. This seems to be partially because the giant molecular cloud mass function is not fully sampled, but other mechanisms such as chemical differentiation in a hard radiation field or morphological quenching also seem to be required. We detect a Keplerian increase of the rotation of the molecular gas in the very centre of NGC4697, and use forward modelling of the ALMA data cube in a Bayesian framework with the KINematic Molecular Simulation (KinMS) code to estimate a SMBH mass of ($1.3_{-0.17}^{+0.18})\times10^8$ Msun and an i-band mass-to-light ratio of $2.14_{-0.05}^{+0.04}$ Msun/Lsun (at the 99% confidence level). Our estimate of the SMBH mass is entirely consistent with previous measurements from stellar kinematics. This increases confidence in the growing number of SMBH mass estimates being obtained in the ALMA era.

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arXiv2016

The growth of the central region by acquisition of counter-rotating gas in star-forming galaxies

Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Based on new measurements with integral field spectroscopy of a large representative galaxy sample, we find an appreciable fraction of counter-rotators among blue galaxies (9 out of 489 galaxies). The central regions of blue counter-rotators show younger stellar populations and more intense, ongoing star formation than their outer parts, indicating ongoing growth of the central regions. The result offers observational evidence that the acquisition of external gas in blue galaxies is possible; the interaction with pre-existing gas funnels the gas into nuclear regions (< 1 kpc) to form new stars.

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arXiv2016

Large-scale filamentary structures around the Virgo cluster revisited

We revisit the filamentary structures of galaxies around the Virgo cluster, exploiting a larger dataset based on the HyperLeda database than previous studies. In particular, this includes a large number of low-luminosity galaxies, resulting in better sampled individual structures. We confirm seven known structures in the distance range 4~$h^{-1}$~Mpc~$<$ SGY~$<$ 16~$h^{-1}$ Mpc, now identified as filaments, where SGY is the axis of the supergalactic coordinate system roughly along the line of sight. The Hubble diagram of the filament galaxies suggests they are infalling toward the main-body of the Virgo cluster. We propose that the collinear distribution of giant elliptical galaxies along the fundamental axis of the Virgo cluster is smoothly connected to two of these filaments (Leo~II~A and B). Behind the Virgo cluster (16~$h^{-1}$~Mpc~$<$ SGY~$<$ 27~$h^{-1}$~Mpc), we also identify a new filament elongated toward the NGC 5353/4 group ("NGC 5353/4 filament") and confirm a sheet that includes galaxies from the W and M clouds of the Virgo cluster ("W-M sheet"). In the Hubble diagram, the NGC 5353/4 filament galaxies show infall toward the NGC 5353/4 group, whereas the W-M sheet galaxies do not show hints of gravitational influence from the Virgo cluster. The filamentary structures identified can now be used to better understand the generic role of filaments in the build-up of galaxy clusters at z~$\approx$~0.

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arXiv2016

Star Formation in Nearby Early-Type Galaxies: The Radio Continuum Perspective

We present a 1.4 GHz Karl G. Jansky Very Large Array (VLA) study of a sample of early-type galaxies (ETGs) from the volume- and magnitude-limited ATLAS-3D survey. The radio morphologies of these ETGs at a resolution of 5" are diverse and include sources that are compact on sub-kpc scales, resolved structures similar to those seen in star-forming spiral galaxies, and kpc-scale radio jets/lobes associated with active nuclei. We compare the 1.4 GHz, molecular gas, and infrared (IR) properties of these ETGs. The most CO-rich ATLAS-3D ETGs have radio luminosities consistent with extrapolations from H_2-mass-derived star formation rates from studies of late-type galaxies. These ETGs also follow the radio-IR correlation. However, ETGs with lower molecular gas masses tend to have less radio emission relative to their CO and IR emission compared to spirals. The fraction of galaxies in our sample with high IR-radio ratios is much higher than in previous studies, and cannot be explained by a systematic underestimation of the radio luminosity due to the presence extended, low-surface-brightness emission that was resolved-out in our VLA observations. In addition, we find that the high IR-radio ratios tend to occur at low IR luminosities, but are not associated with low dynamical mass or metallicity. Thus, we have identified a population of ETGs that have a genuine shortfall of radio emission relative to both their IR and molecular gas emission. A number of mechanisms may conspire to cause this radio deficiency, including a bottom-heavy stellar initial mass function, weak magnetic fields, a higher prevalence of environmental effects compared to spirals and enhanced cosmic ray losses.

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arXiv2016

Molecular Gas Kinematics and Line Diagnostics in Early-type Galaxies: NGC4710 and NGC5866

We present interferometric observations of CO lines (12CO(1-0, 2-1) and 13CO(1-0, 2-1)) and dense gas tracers (HCN(1-0), HCO+(1-0), HNC(1-0) and HNCO(4-3)) in two nearby edge-on barred lenticular galaxies, NGC 4710 and NGC 5866, with most of the gas concentrated in a nuclear disc and an inner ring in each galaxy. We probe the physical conditions of a two-component molecular interstellar medium in each galaxy and each kinematic component by using molecular line ratio diagnostics in three complementary ways. First, we measure the ratios of the position-velocity diagrams of different lines, second we measure the ratios of each kinematic component's integrated line intensities as a function of projected position, and third we model these line ratios using a non-local thermodynamic equilibrium radiative transfer code. Overall, the nuclear discs appear to have a tenuous molecular gas component that is hotter, optically thinner and with a larger dense gas fraction than that in the inner rings, suggesting more dense clumps immersed in a hotter more diffuse molecular medium. This is consistent with evidence that the physical conditions in the nuclear discs are similar to those in photo-dissociation regions. A similar picture emerges when comparing the observed molecular line ratios with those of other galaxy types. The physical conditions of the molecular gas in the nuclear discs of NGC4710 and NGC5866 thus appear intermediate between those of spiral galaxies and starbursts, while the star formation in their inner rings is even milder.

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arXiv2016

The Tully-Fisher Relation of COLD GASS Galaxies

We present the stellar mass (M_*) and Wide-Field Infrared Survey Explorer (WISE) absolute Band 1 magnitude (M_W1) Tully-Fisher relations (TFRs) of subsets of galaxies from the CO Legacy Database for the Galex Arecibo SDSS Survey (COLD GASS). We examine the benefits and drawbacks of several commonly used fitting functions in the context of measuring CO(1-0) line widths (and thus rotation velocities), favouring the Gaussian Double Peak function. We find the M_W1 and M_* TFR, for a carefully selected sub-sample, to be M_W1 = (-7.1 +/- 0.6) [log(W_50/sin i) - 2.58] - 23.83 +/- 0.09 and log(M_*/M_Sun) = (3.3 +/- 0.3) [log(W_50/sin i) - 2.58] + 10.51 +/- 0.04, respectively, where W_50 is the width of a galaxy's CO(1-0) integrated profile at 50% of its maximum and the inclination i is derived from the galaxy axial ratio measured on the SDSS r-band image. We find no evidence for any significant offset between the TFRs of COLD GASS galaxies and those of comparison samples of similar redshifts and morphologies. The slope of the COLD GASS M_* TFR agrees with the relation of Pizagno et al. (2005). However, we measure a comparatively shallower slope for the COLD GASS M_W1 TFR as compared to the relation of Tully & Pierce (2000). We attribute this to the fact that the COLD GASS sample comprises galaxies of various (late-type) morphologies. Nevertheless, our work provides a robust reference point with which to compare future CO TFR studies.

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arXiv2016

The KMOS Redshift One Spectroscopic Survey (KROSS): The Tully-Fisher Relation at z ~ 1

We present the stellar mass ($M_{*}$), and K-corrected $K$-band absolute magnitude ($M_{K}$) Tully-Fisher relations (TFRs) for sub-samples of the 584 galaxies spatially resolved in H$α$ emission by the KMOS Redshift One Spectroscopic Survey (KROSS). We model the velocity field of each of the KROSS galaxies and extract a rotation velocity, $V_{80}$ at a radius equal to the major axis of an ellipse containing 80% of the total integrated H$α$ flux. The large sample size of KROSS allowed us to select 210 galaxies with well measured rotation speeds. We extract from this sample a further 56 galaxies that are rotationally supported, using the stringent criterion $V_{80}/σ> 3$, where $σ$ is the flux weighted average velocity dispersion. We find the $M_{K}$ and $M_{*}$ TFRs for this sub-sample to be $M_{K} / \rm{mag}= (-7.3 \pm 0.9) \times [(\log(V_{80}/\rm{km\ s^{-1}})-2.25]- 23.4 \pm 0.2$ , and $\log(M_{*} / M_{\odot})= (4.7 \pm 0.4) \times [(\log(V_{80}/\rm{km\ s^{-1}}) - 2.25] + 10.0 \pm 0.3$, respectively. We find an evolution of the $M_{*}$ TFR zero-point of $-0.41 \pm 0.08$ dex over the last $\sim $8 billion years. However, we measure no evolution in the $M_{K}$ TFR zero-point over the same period. We conclude that rotationally supported galaxies of a given dynamical mass had less stellar mass at $z \sim 1$ than the present day, yet emitted the same amounts of $K$-band light. The ability of KROSS to differentiate, using integral field spectroscopy with KMOS, between those galaxies that are rotationally supported and those that are not explains why our findings are at odds with previous studies without the same capabilities.

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arXiv2016

The Atlas3D project -- XXXI. Nuclear radio emission in nearby early-type galaxies

We present the results of a high-resolution, 5 GHz, Karl G. Jansky Very Large Array study of the nuclear radio emission in a representative subset of the Atlas3D survey of early-type galaxies (ETGs). We find that 51 +/- 4% of the ETGs in our sample contain nuclear radio emission with luminosities as low as 10^18 W/Hz. Most of the nuclear radio sources have compact (< 25-110 pc) morphologies, although < 10% display multi-component core+jet or extended jet/lobe structures. Based on the radio continuum properties, as well as optical emission line diagnostics and the nuclear X-ray properties, we conclude that the majority of the central 5 GHz sources detected in the Atlas3D galaxies are associated with the presence of an active galactic nucleus (AGN). However, even at sub-arcsecond spatial resolution, the nuclear radio emission in some cases appears to arise from low-level nuclear star formation rather than an AGN, particularly when molecular gas and a young central stellar population is present. This is in contrast to popular assumptions in the literature that the presence of a compact, unresolved, nuclear radio continuum source universally signifies the presence of an AGN. Additionally, we examine the relationships between the 5 GHz luminosity and various galaxy properties including the molecular gas mass and - for the first time - the global kinematic state. We discuss implications for the growth, triggering, and fueling of radio AGNs, as well as AGN-driven feedback in the continued evolution of nearby ETGs.

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arXiv2016

The KMOS Redshift One Spectroscopic Survey (KROSS): Dynamical properties, gas and dark matter fractions of typical z~1 star-forming galaxies

The KMOS Redshift One Spectroscopic Survey (KROSS) is an ESO guaranteed time survey of 795 typical star-forming galaxies in the redshift range z=0.8-1.0 with the KMOS instrument on the VLT. In this paper we present resolved kinematics and star formation rates for 584 z~1 galaxies. This constitutes the largest near-infrared Integral Field Unit survey of galaxies at z~1 to date. We demonstrate the success of our selection criteria with 90% of our targets found to be Halpha emitters, of which 81% are spatially resolved. The fraction of the resolved KROSS sample with dynamics dominated by ordered rotation is found to be 83$\pm$5%. However, when compared with local samples these are turbulent discs with high gas to baryonic mass fractions, ~35%, and the majority are consistent with being marginally unstable (Toomre Q~1). There is no strong correlation between galaxy averaged velocity dispersion and the total star formation rate, suggesting that feedback from star formation is not the origin of the elevated turbulence. We postulate that it is the ubiquity of high (likely molecular) gas fractions and the associated gravitational instabilities that drive the elevated star-formation rates in these typical z~1 galaxies, leading to the ten-fold enhanced star-formation rate density. Finally, by comparing the gas masses obtained from inverting the star-formation law with the dynamical and stellar masses, we infer an average dark matter to total mass fraction within 2.2$r_e$ (9.5kpc) of 65$\pm$12%, in agreement with the results from hydrodynamic simulations of galaxy formation.

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arXiv2015

On the depletion and accretion timescales of cold gas in local early-type galaxies

We consider what can be learnt about the processes of gas accretion and depletion from the kinematic misalignment between the cold/warm gas and stars in local early-type galaxies. Using simple analytic arguments and a toy model of the processes involved, we show that the lack of objects with counter-rotating gas reservoirs strongly constrains the relaxation, depletion and accretion timescales of gas in early-type galaxies. Standard values of the accretion rate, star formation efficiency and relaxation rate are not simultaneously consistent with the observed distribution of kinematic misalignments. To reproduce that distribution, both fast gas depletion ($t_{\rm dep} <10^8$ yr; e.g. more efficient star formation) and fast gas destruction (e.g. by active galactic nucleus feedback) can be invoked, but both also require a high rate of gas-rich mergers ($>1$ Gyr$^{-1}$). Alternatively, the relaxation of misaligned material could happen over very long timescales ($\simeq100$ dynamical times or $\approx1$-$5$ Gyr). We explore the various physical processes that could lead to fast gas depletion and/or slow gas relaxation, and discuss the prospects of using kinematic misalignments to probe gas-rich accretion processes in the era of large integral-field spectroscopic surveys.

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arXiv2015

Evidence of boosted 13CO/12CO ratio in early-type galaxies in dense environments

We present observations of $^{13}$CO(1-0) in 17 Combined Array for Research in Millimeter Astronomy (CARMA) Atlas3D early-type galaxies (ETGs), obtained simultaneously with $^{12}$CO(1-0) observations. The $^{13}$CO in six ETGs is sufficiently bright to create images. In these 6 sources, we do not detect any significant radial gradient in the $^{13}$CO/$^{12}$CO ratio between the nucleus and the outlying molecular gas. Using the $^{12}$CO channel maps as 3D masks to stack the $^{13}$CO emission, we are able to detect 15/17 galaxies to $>3σ$ (and 12/17 to at least 5$σ$) significance in a spatially integrated manner. Overall, ETGs show a wide distribution of $^{13}$CO/$^{12}$CO ratios, but Virgo cluster and group galaxies preferentially show a $^{13}$CO/$^{12}$CO ratio about 2 times larger than field galaxies, although this could also be due to a mass dependence, or the CO spatial extent ($R_{\rm CO}/R_{\rm e}$). ETGs whose gas has a morphologically-settled appearance also show boosted $^{13}$CO/$^{12}$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 $^{13}$C to $^{12}$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 $^{13}$CO in dense environments, particularly of spirals, as well as studies of other isotopologues, should be able to distinguish between these hypotheses.

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