arXiv subjects

C. K. Xu

Publications and source records attributed to C. K. Xu.

At least 19 recordsLinked to original sources

arXiv2026

A radio continuum view of Stephan's Quintet: age, dynamics and origin of the shock

We present a detailed, multi-band radio continuum study of the compact galaxy group Stephan's Quintet (HCG 92). We use a combination of new (MeerKAT, uGMRT) and archival (LOFAR, VLA) observations covering the $\rm 120\,MHz{-}8\,GHz$ frequency range to examine the radio properties of the group, focusing on the famous radio ridge and surrounding diffuse emission. We find filamentary substructure and branching in the southern half of the ridge, confirm an extension of the ridge to the northwest, and identify for the first time a radio counterpart to the gas bridge linking the ridge and NGC 7319. The northern ridge, northwest extension and diffuse emission have relatively steep, curved spectra, with a high-frequency spectral index gradient running north-south along the ridge. We show that the ridge emission primarily arises from a single physical mechanism, probably strong ($\mathcal{M}\simeq 40-100$) shocks in cold gas, caused by the $\rm \sim850-1000\,kms^{-1}$ collision between NGC 7318B and tidal gas filaments produced by past galaxy interactions. Synchrotron spectral age estimates suggest the collision began at the north end of the ridge $\rm \sim20\,Myr$ ago, and finished only $\rm \sim5-6\,Myr$ ago in the south, with the southern end of the shocked ridge likely still within or close to the disk of NGC 7318B. Based on this age gradient, we find that the angle between the intruder galaxy's motion and the tidal filaments was probably only about 15 degrees, and combining this with the lack of a spectral index gradient in the diffuse radio emission suggests that NGC 7318B's direction of motion is probably close to the line of sight.

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arXiv2026

A MeerKAT view of the Neutral Atomic Gas in Stephan's Quintet

We present new MeerKAT 21cm spectral line observations of the neutral hydrogen gas in the compact galaxy group Stephan's Quintet (HCG 92). These data provide a significantly improved view of the atomic gas distribution and kinematics in the group. New features include the first detections of HI associated with member galaxies NGC 7319 and NGC 7320C, the identification of an additional high-velocity HI component associated with SQ-A, and the detection of additional HI at low velocities filling much of the area of the NGC~7318B disk. We also find HI in the previously detected gas bridge linking NGC 7319 and NGC 7318B, and a new northern bridge linking NGC 7319 to the SQ-A star-formation region. We detect HI with a wide range of velocities along the line of sight through the northern half of the famous shock ridge, including in the 6200-6500$\rm\,km\,s^{-1}$ velocity range occupied by shocked H$α$ emission. We examine the morphology and velocity structure of the HI and consider the origins of different components, finding some evidence that while the gas associated with NGC 7318B has been disturbed by its collision with the group, it may still retain a component of disk rotation. We find no gaseous connection between the tidal tails and NGC 7320C, but reaffirm the close connection between the shocked gas in the ridge (traced by X-ray, radio continuum and warm H$_2$ emission) and the southern tidal tail. Based on the integrated spectrum, we find a total HI mass in the group of 3.5$\pm$0.4$\times$10$^{10}\,M_{\odot}$, higher than the VLA estimate and comparable to FAST.

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arXiv2025

Characterizing the star cluster populations in Stephan's Quintet using HST and JWST observations

Stephan's Quintet (SQ) is a local compact galaxy group system that exhibits significant star formation activity. A history of tidal interactions between its four member galaxies and a recent collision between an intruder galaxy and the original group are associated with active star formation, particularly in many shocked regions in the intra-group medium. Using an existing star cluster candidate (SCC) catalog constructed from HST UV/optical images, we integrate flux measurements from five near-infrared filters (F090W, F150W, F200W, F277W, F356W) obtained from JWST NIRCam observations in 2022. Leveraging the extended photometric baseline from HST and JWST, spanning ~300 nm to ~3500 nm, we perform spectral energy distribution (SED) fitting using the CIGALE code to derive reliable estimates of age, mass, and extinction for the 1,588 high-confidence SCCs. We confirm earlier results that very young SCCs (~a few Myr) are predominantly located along previously identified shock regions near the merging galaxies, while older (>100 Myr) and globular clusters are more widely distributed. Our analysis shows that NIR photometry helps break the age-extinction degeneracy, reclassifying many SCCs from older to younger, moderately dust-extincted clusters when added to HST-based SED fits. We also observe a strong spatial correlation between young clusters and CO-traced molecular gas, although active star formation is present in several regions with no detectable CO. We find that the two prominent epochs of star formation, around 5 Myr and 200 Myr, correspond to the two major interaction events in SQ that gave rise to the observed extended tidal features.

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arXiv2025

SQ-A: A Collision Triggered Starburst in Intra-Group Medium of Stephan's Quintet

We present new observational evidence supporting the hypothesis that SQ-A, a starburst in the intra-group medium (IGrM) of Stephan's Quintet (SQ), is triggered by a high-speed collision between two gas systems, one associated with the IGrM (v~6900 km/s) and another with the intruder galaxy NGC7318b (v~6000 km/s). The new ALMA CO(2-1) dataset has angular resolutions between 0.2" and 7.0" and the new VLA HI datacube an angular resolution of 6.6" * 7.9". The CO maps show that the two gas systems are bridged by another system with an intermediate velocity of ~6600 km/s, whereas the HI data show that the component of v~6600 km/s fits well into a gap in the more extended v~6000 km/s component, albeit with a displacement of ~5 kpc. Both the bridge and the complementary distributions between different gas systems are common features of starbursts triggered by cloud-cloud collision. An analysis of clumps (sizes of 100--200 pc) reveals very diversified star formation (SF) activity in clumps belonging to different kinematic systems, with the molecular gas depletion time of the v~6900 km/s clumps more than 10 times longer than that of the v~6600 km/s clumps. The results are consistent with a scenario in which the enhanced SF activity (and the starburst) in the system of v~6600 km/s is due to gas compression generated in cloud-cloud collisions, whereas the suppression of SF in the v~6900 km/s system is due to vortices (i.e. gas rotation) generated in more complex collisions involving dense clouds and diffuse intercloud gas accompanied by blast-wave shocks.

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arXiv2025

MeerKAT view of Hickson Compact Groups:I. Data description and release

Context: Hickson Compact Groups (HCGs) are dense gravitationally-bound collections of 4-10 galaxies ideal for studying gas and star formation quenching processes. Aims: We aim to understand the transition of HCGs from possessing complex HI tidal structures (so-called phase 2 groups) to a phase where galaxies have lost most or all their HI (phase 3). We also seek to detect diffuse H i gas that was previously missed by the Very Large Array (VLA). Methods: We observed three phase 2 and three phase 3 HCGs with MeerKAT and reduced the data using the Containerized Automated Radio Astronomy Calibration (CARACal) pipeline. We produced data cubes, moment maps, integrated spectra, and compared our findings with previous VLA and Green Bank Telescope (GBT) observations. Results: Compared with previous VLA observations, MeerKAT reveals much more extended tidal features in phase 2 and some new high surface brightness features in phase 3 groups. However, no diffuse HI component was found in phase 3 groups. We also detected many surrounding galaxies for both phase 2 and phase 3 groups, most of which are normal disk galaxies. Conclusions: The difference between phase 2 and phase 3 groups is still substantial, supporting previous findings that the transition between the two phases must be abrupt.

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arXiv2024

Bird's-eye View of Molecular Gas across Stephan's Quintet Galaxy Group and Intra-group Medium

We present the large-scale distribution and kinematics of cold molecular gas across the compact galaxy group Stephan's Quintet, based on CO(2-1) observations performed with the Atacama Compact Array (ACA) and CO(1-0) data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We find coherent structures of molecular gas associated with the galaxies and intra-group medium, which follow the distribution of warm H$_{2}$ previously seen with the James Webb Space Telescope (JWST). CO is associated with a ridge of shocked gas that crosses the galaxy group, and with a spiral arm of the intruding galaxy NGC7318b, which interacts with the intra-group medium along the ridge. Although the ridge contains widespread shocks, turbulent gas, and warm H$_{2}$, the CO lines are narrower than elsewhere in Stephan's Quintet (FWHM~25-65 km/s), indicative of settled cold gas. At a distinctly different velocity, CO is found in the active galaxy NGC7319 and Northern star-forming region SQ-A. A bridge of turbulent molecular gas connects NGC7319 with the ridge, covering a gap of ~700 km/s between these structures. The gas excitation ranges from $L'_{\rm CO(2-1)}$/$L'_{\rm CO(1-0)}$ ~ 0.3 in the bridge and SQ-A, to ~0.5 along the ridge, to near unity in the center of NGC7319. We also detect either a molecular outflow or turbulent molecular gas associated with the radio source in NGC7319. These ACA data are part of a program with the Atacama Large Millimeter/submillimeter Array (ALMA) and JWST to study molecular gas physics from the largest to the smallest scales across the intra-group medium of Stephan's Quintet.

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arXiv2023

Multi-phase gas interactions on subarcsec scales in the shocked IGM of Stephan's Quintet with JWST and ALMA

We combine JWST and HST imaging with ALMA~CO(2-1) spectroscopy to study the highly turbulent multi-phase intergalactic medium (IGM) in Stephan's Quintet on 25-150 pc scales. Previous Spitzer observations revealed luminous H$_2$ line cooling across a 45 kpc-long filament, created by a giant shock-wave, following the collision with an intruder galaxy NGC~7318b. We demonstrate that the MIRI/F1000W/F770W filters are dominated by 0-0~S(3)~H$_2$ and a combination of PAH and 0-0~S(5)~H$_2$ emission. They reveal the dissipation of kinetic energy as massive clouds experience collisions, interactions and likely destruction/re-cycling within different phases of the IGM. In one kpc-scaled structure, warm H$_2$ formed a triangular-shaped head and tail of compressed and stripped gas behind a narrow shell of cold H$_2$. In another region, two cold molecular clumps with very different velocities are connected by an arrow-shaped stream of warm, probably shocked, H$_2$ suggesting a cloud-cloud collision is occurring. In both regions, a high warm-to-cold molecular gas fraction indicates that the cold clouds are being disrupted and converted into warm gas. We also map gas associated with an apparently forming dwarf galaxy. We suggest that the primary mechanism for exciting strong mid-IR H$_2$ lines throughout Stephan's Quintet is through a fog of warm gas created by the shattering of denser cold molecular clouds and mixing/recycling in the post-shocked gas. A full picture of the diverse kinematics and excitation of the warm H$_2$ will require future JWST mid-IR spectroscopy. The current observations reveal the rich variety of ways that different gas phases can interact with one another.

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arXiv2022

A 0.6 Mpc HI Structure Associated with Stephan's Quintet

Stephan's Quintet (SQ, distance=85$\pm$6 Mpc) is unique among compact groups of galaxies. Observations have previously shown that interactions between multiple members, including a high-speed intruder galaxy currently colliding into the intragroup medium, have likely generated tidal debris in the form of multiple gaseous and stellar filaments, the formation of tidal dwarfs and intragroup-medium starbursts, as well as widespread intergalactic shocked gas. The details and timing of the interactions/collisions remain poorly understood because of the multiple nature. Here we report atomic hydrogen (HI) observations in the vicinity of SQ with a smoothed sensitivity of 1$σ$=4.2 $\times 10^{16}\rm cm^{-2}$ per channel ($Δ$v=20 km s$^{-1}$; angular-resolution=4'), which are about two orders of magnitude deeper than previous observations. The data reveal a large HI structure (linear scale ~0.6 Mpc) encompassing an extended source of size ~0.4 Mpc associated with the debris field and a curved diffuse feature of length ~0.5 Mpc attached to the south edge of the extended source. The diffuse feature was likely produced by tidal interactions in early stages of SQ (>1 Gyr ago), though it is not clear how the low density HI gas (N$_{\rm HI}\leq 10^{18}\rm cm^{-2}$) can survive the ionization by the inter-galactic UV background on such a long time scale. Our observations require a rethinking of gas in outer parts of galaxy groups and demand complex modeling of different phases of the intragroup medium in simulations of group formation.

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arXiv2021

Extremely broad Lyman-alpha line emission from the molecular intra-group medium in Stephan's Quintet: evidence for a turbulent cascade in a highly clumpy multi-phase medium?

We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line spectroscopy and integral-field unit (IFU) observations of the intra-group medium in Stephan's Quintet (SQ). SQ hosts a 30 kpc long shocked ridge triggered by a galaxy collision at a relative velocity of 1000 km/s, where large amounts of molecular gas coexist with a hot, X-ray emitting, plasma. COS spectroscopy at five positions sampling the diverse environments of the SQ intra-group medium reveals very broad (2000 km/s) Ly$α$ line emission with complex line shapes. The Ly$α$ line profiles are similar to or much broader than those of H$β$, [CII]$\lambda157.7μ$m and CO~(1-0) emission. The extreme breadth of the Ly$α$ emission, compared with H$β$, implies resonance scattering within the observed structure. Scattering indicates that the neutral gas of the intra-group medium is clumpy, with a significant surface covering factor. We observe significant variations in the Ly$α$/H$β$ flux ratio between positions and velocity components. From the mean line ratio averaged over positions and velocities, we estimate the effective escape fraction of Ly$α$ photons to be 10-30%. Remarkably, over more than four orders of magnitude in temperature, the powers radiated by X-rays, Ly$α$, H$_2$, [CII] are comparable within a factor of a few, assuming that the ratio of the Ly$α$ to H$_2$ fluxes over the whole shocked intra-group medium stay in line with those observed at those five positions. Both shocks and mixing layers could contribute to the energy dissipation associated with a turbulent energy cascade. Our results may be relevant for the cooling of gas at high redshifts, where the metal content is lower than in this local system, and a high amplitude of turbulence is more common.

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arXiv2019

The galaxy luminosity function in the LAMOST Complete Spectroscopic Survey of Pointing Area at the Southern Galactic Cap

We present optical luminosity functions (LFs) of galaxies in the $^{0.1}g$, $^{0.1}r$, and $^{0.1}i$ bands, calculated using data in $\sim 40$ $deg^{2}$ sky area of LAMOST Complete Spectroscopic Survey of Pointing Area (LaCoSSPAr) in Southern Galactic Cap. Redshifts for galaxies brighter $r = 18.1$ were obtained mainly with LAMOST. In each band, LFs derived using both parametric and non-parametric maximum likelihood methods agree well with each other. In the $^{0.1}r$ band, our fitting parameters of the Schechter function are $ϕ_{*}=(1.65\pm0.36)\times10^{-2}h^{3}Mpc^{-3}$, $M_{*}=-20.69\pm0.06$ mag, and $α=-1.12\pm0.08$, in agreements with previous studies. Separate LFs are also derived for emission line galaxies and absorption line galaxies, respectively. The LFs of absorption line galaxies show a dip at $^{0.1}r \sim 18.5$ and can be well fitted by a double-Gaussian function, suggesting a bi-modality in passive galaxies.

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arXiv2018

Star formation and gas in the minor merger UGC 10214

UGC 10214 is a minor merger in which a dwarf galaxy has interacted with a large spiral galaxy $\sim$250 Myr ago and produced a perturbed disk and a giant tidal tail. We use a multiwavelength dataset in order to study the present and past star formation rate (SFR) and its relation to the gas and stellar mass at a spatial resolution down to 4 kpc. UGC 10214 is a very massive (stellar mass $M_{\rm \star}$ = $1.28\times 10^{11}$$M_\odot$) galaxy with a low gas fraction ($M_{\rm gas}$/$M_{\rm \star}$ = 0.24), a high molecular gas fraction ($M_{{\rm H}_2}$/$M_{\rm HI}$ = 0.4) and a modest SFR (2-5 $M_\odot$ yr$^{-1}$). The comparison of the molecular gas mass and current SFR gives a molecular gas depletion time of about $\sim$ 2 Gyr (based on H$α$), comparable to those of normal spiral galaxies. Both from a comparison of the H$α$ emission, tracing the current SFR, and far-ultraviolet (FUV) emission, tracing the recent SFR during the past tens of Myr, as well as from spectral energy distribution (SED) fitting with CIGALE, we find that the SFR has increased by a factor of about 2-3 during the recent past. This increase is particularly noticeable in the centre of the galaxy. A pixel-to-pixel comparison of the SFR, molecular gas mass and stellar mass shows that the central region has had a depressed FUV-traced SFR, both compared to the molecular gas and the stellar mass, whereas the H$α$-traced SFR shows a normal level. The atomic and molecular gas distribution is asymmetric, but the position-velocity diagram along the major axis shows a pattern of regular rotation. We conclude that the minor merger has most likely caused variations in the SFR in the past resulting in a moderate increase of the SFR, but it has not perturbed the gas significantly so that the molecular depletion time remains normal.

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arXiv2018

Herschel Spectroscopy of the Taffy Galaxies (UGC 12914/12915 = VV 254): Enhanced [C II] emission in the collisionally-formed bridge

Using the PACS and SPIRE spectrometers on-board Herschel, we obtained observations of the Taffy galaxies (UGC 12914/12915) and bridge. The Taffy system is believed to be the result of a face-on collision between two gas-rich galaxies, in which the stellar disks passed through each other, but the gas was dispersed into a massive H I and molecular bridge between them. Emission is detected and mapped in both galaxies and the bridge in the [C II]157.7 $μ$m and [O I]63.2 $μ$m fine-structure lines. Additionally, SPIRE FTS spectroscopy detects the [C I] $^3$P$_2$$\rightarrow$$^3$P$_1$(809.3 GHz) and [C I] $^3$P$_1$$\rightarrow$$3$P$_0$(492.2 GHz) neutral carbon lines, and weakly detects high-J CO transitions in the bridge. These results indicate that the bridge is composed of a warm multi-phase medium consistent with shock and turbulent heating. Despite low star formation rates in the bridge, the [C II] emission appears to be enhanced, reaching [C II]/FIR ratios of 3.3% in parts of the bridge. Both the [C II] and [O I] lines show broad intrinsic multi-component profiles, similar to those seen in previous CO 1-0 and H I observations. The [C II] emission shares similar line profiles with both the double-peaked H I profiles and shares a high-velocity component with single-peaked CO profiles in the bridge, suggesting that the [C II] emission originates in both the neutral and molecular phases. We show that it is feasible that a combination of turbulently heated H$_2$ and high column-density H I, resulting from the galaxy collision, is responsible for the enhanced [C II] emission.

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arXiv2017

Imaging the Molecular Outflows of the Prototypical ULIRG NGC 6240 with ALMA

We present 0".97 $\times$ 0".53 (470 pc $\times$ 250 pc) resolution CO ($J$ = 2-1) observations toward the nearby luminous merging galaxy NGC 6240 with the Atacama Large Millimeter/submillimeter Array. We confirmed a strong CO concentration within the central 700 pc, which peaks between the double nuclei, surrounded by extended CO features along the optical dust lanes ($\sim$11 kpc). We found that the CO emission around the central a few kpc has extremely broad velocity wings with full width at zero intensity $\sim$ 2000 km s$^{-1}$, suggesting a possible signature of molecular outflow(s). In order to extract and visualize the high-velocity components in NGC 6240, we performed a multiple Gaussian fit to the CO datacube. The distribution of the broad CO components show four extremely large linewidth regions ($\sim$1000 km s$^{-1}$) located 1-2 kpc away from both nuclei. Spatial coincidence of the large linewidth regions with H$α$, near-IR H$_2$, and X-ray suggests that the broad CO (2-1) components are associated with nuclear outflows launched from the double nuclei.

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arXiv2017

VALES: IV. Exploring the transition of star formation efficiencies between normal and starburst galaxies using APEX/SEPIA Band-5 and ALMA at low redshift

In this work we present new APEX/SEPIA Band-5 observations targeting the CO ($J=2\text{-}1$) emission line of 24 Herschel-detected galaxies at $z=0.1-0.2$. Combining this sample {with} our recent new Valparaíso ALMA Line Emission Survey (VALES), we investigate the star formation efficiencies (SFEs = SFR/$M_{\rm H_{2}}$) of galaxies at low redshift. We find the SFE of our sample bridges the gap between normal star-forming galaxies and Ultra-Luminous Infrared Galaxies (ULIRGs), which are thought to be triggered by different star formation modes. Considering the $\rm SFE'$ as the SFR and the $L'_{\rm CO}$ ratio, our data show a continuous and smooth increment as a function of infrared luminosity (or star formation rate) with a scatter about 0.5 dex, instead of a steep jump with a bimodal behaviour. This result is due to the use of a sample with a much larger range of sSFR/sSFR$_{\rm ms}$ using LIRGs, with luminosities covering the range between normal and ULIRGs. We conclude that the main parameters controlling the scatter of the SFE in star-forming galaxies are the systematic uncertainty of the $α_{\rm CO}$ conversion factor, the gas fraction and physical size.

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arXiv2017

Powerful H2 Line Cooling in Stephan's Quintet II. Group-wide Gas and Shock Modeling of the Warm H2 and a Comparison with [CII]157.7um Emission and Kinematics

We map for the first time the two-dimensional H_2 excitation of warm intergalactic gas in Stephan's Quintet on group-wide (50 x 35 kpc^2) scales to quantify the temperature, mass and warm-H_2 mass fraction as a function of position using Spitzer. Molecular gas temperatures are seen to rise (to T > 700K) and the slope of the power--law density-temperature relation flattens along the main ridge of the filament, defining the region of maximum heating. We also performed MHD modeling of the excitation properties of the warm gas, to map the velocity structure and energy deposition rate of slow and fast molecular shocks. Slow magnetic shocks were required to explain the power radiated from the lowest--lying rotational states of H_2, and strongly support the idea that energy cascades down to small scales and low velocities from the fast collision of NGC 7318b with group-wide gas. The highest levels of heating of the warm H_2 is strongly correlated with the large-scale stirring of the medium as measured by [CII] spectroscopy with Herschel. H_2 is also seen associated with a separate bridge which extends towards the Seyfert nucleus in NGC 7319, both from Spitzer and CARMA CO observations. This opens up the possibility that both galaxy collisions and AGN outflows can turbulently heat gas on large-scales in compact groups. The observations provide a laboratory for studying the effects of turbulent energy dissipation on group-wide scales that may provide clues about the heating and cooling of gas at high-z in early galaxy and protogalaxy formation.

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arXiv2016

Deep ALMA imaging of the merger NGC1614 - Is CO tracing a massive inflow of non-starforming gas?

Observations of the molecular gas over scales of 0.5 to several kpc provide crucial information on how gas moves through galaxies, especially in mergers and interacting systems, where it ultimately reaches the galaxy center, accumulates, and feeds nuclear activity. Studying the processes involved in the gas transport is an important step forward to understand galaxy evolution. 12CO, 13CO and C18O1-0 high-sensitivity ALMA observations were used to assess properties of the large-scale molecular gas reservoir and its connection to the circumnuclear molecular ring in NGC1614. The role of excitation and abundances were studied in this context. Spatial distributions of the 12CO and 13CO emission show significant differences. 12CO traces the large-scale molecular gas reservoir, associated with a dust lane that harbors infalling gas. 13CO emission is - for the first time - detected in the large-scale dust lane. Its emission peaks between dust lane and circumnuclear molecular ring. A 12CO-to-13CO1-0 intensity ratio map shows high values in the ring region (~30) typical for the centers of luminous galaxy mergers and even more extreme values in the dust lane (>45). This drop in ratio is consistent with molecular gas in the dust lane being in a diffuse, unbound state while being funneled towards the nucleus. We find a high 16O-to-18O abundance ratio in the starburst region (>900), typical of quiescent disk gas - by now, the starburst is expected to have enriched the nuclear ISM in 18O relative to 16O. The massive inflow of gas may be partially responsible for the low 18O/16O abundance since it will dilute the starburst enrichment with unprocessed gas from greater radii. The 12CO-to-13CO abundance is consistent with this scenario. It suggests that the nucleus of NGC1614 is in a transient phase of evolution where starburst and nuclear growth are fuelled by returning gas from the minor merger event.

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arXiv2015

The HerMES sub-millimetre local and low-redshift luminosity functions

We used wide area surveys over 39 deg$^2$ by the HerMES collaboration, performed with the Herschel Observatory SPIRE multi-wavelength camera, to estimate the low-redshift, $0.02<z<0.5$, monochromatic luminosity functions (LFs) of galaxies at 250, 350 and 500$\,μ$m. SPIRE flux densities were also combined with Spitzer photometry and multi-wavelength archival data to perform a complete SED fitting analysis of SPIRE detected sources to calculate precise k-corrections, as well as the bolometric infrared (8-1000$\,μ$m) luminosity functions and their low-$z$ evolution from a combination of statistical estimators. Integration of the latter prompted us to also compute the local luminosity density (LLD) and the comoving star formation rate density (SFRD) for our sources, and to compare them with theoretical predictions of galaxy formation models. The luminosity functions show significant and rapid luminosity evolution already at low redshifts, $0.02<z<0.2$, with L$_{IR}^* \propto (1+z)^{6.0\pm0.4}$ and $Φ_{IR}^* \propto (1+z)^{-2.1\pm0.4}$, L$_{250}^* \propto (1+z)^{5.3\pm0.2}$ and $Φ_{250}^* \propto (1+z)^{-0.6\pm0.4}$ estimated using the IR bolometric and the 250$\,μ$m LFs respectively. Converting our IR LD estimate into an SFRD assuming a standard Salpeter IMF and including the unobscured contribution based on the UV dust-uncorrected emission from local galaxies, we estimate a SFRD scaling of SFRD$_0+0.08 z$, where SFRD$_0\simeq (1.9\pm 0.03)\times 10^{-2} [\mathrm{M}_\odot\,\mathrm{Mpc}^{-3}]$ is our total SFRD estimate at $z\sim0.02$.

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arXiv2015

Star Formation Suppression in Compact Group Galaxies: A New Path to Quenching?

We present CO(1-0) maps of 12 warm H$_2$-selected Hickson Compact Groups (HCGs), covering 14 individually imaged warm H$_2$ bright galaxies, with CARMA. We found a variety of molecular gas distributions within the HCGs, including regularly rotating disks, bars, rings, tidal tails, and possibly nuclear outflows, though the molecular gas morphologies are more consistent with spirals and early-type galaxies than mergers and interacting systems. Our CO-imaged HCG galaxies show star formation suppression of $\langle$S$\rangle$=10$\pm$5, distributed bimodally, with five objects exhibiting suppressions of S$\gtrsim$10 and depletion timescales $\gtrsim$10Gyr. This star formation inefficiency is also seen in the efficiency per freefall time. We investigate the gas-to-dust ratios of these galaxies to determine if an incorrect conversion caused the apparent suppression and find that HCGs have normal ratios. It is likely that the cause of the suppression in these objects is associated with shocks injecting turbulence into the molecular gas. Galaxies with high star formation suppression (S$\gtrsim$10) also appear to be those in the most advanced stages of transition across optical and infrared color space. This supports the idea that some galaxies in HCGs are transitioning objects, where a disruption of the existing molecular gas in the system suppresses star formation by inhibiting the molecular gas from collapsing and forming stars efficiently. These observations, combined with recent work on poststarburst galaxies with molecular reservoirs, indicates that galaxies do not need to expel their molecular reservoirs prior to quenching star formation and transitioning from blue spirals to red early-type galaxies. This may imply that star formation quenching can occur without the need to starve a galaxy of cold gas first.

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