HI-rich Galaxies Research Articles

Ram Pressure Stripping of HI-rich Galaxies Infalling into Massive Clusters

Abstract We estimate the strength of ram pressure stripping (RPS) for Hi-rich galaxies in X-ray detected clusters. We find that galaxies under stronger RPS tend to show more significantly reduced total Hi mass and enhanced central SFR when compared to control galaxies in the field that have similar stellar mass, stellar surface density and integral star formation rate. Galaxies under strong or weak RPS account for ∼40% of the Hi-rich population at R 200, and even beyond R 200 in the most massive clusters. Our results imply the important role of RPS as a channel of environmental processing far before the galaxies reach the core region of clusters.

Cosmic Web Research: Where are the Missing Dwarf Galaxies and the Inter-Galactic Hi Clouds

AbstractWe know that the observed H i (and H2) content cannot explain the SFR observed in galaxies. The only way galaxies can sustain that SFR is by accreting HI-rich dwarf galaxies or Inter-Galactic HI clouds. However, no observation to detect those accretion events has been conclusive so far. Instruments having the necessary sensitivity (e.g. GBT) lack the necessary spatial resolution and those with the proper resolution (e.g. VLA) lack the sensitivity. I will show that both are necessary to detect those illusive Hi clouds. The SKA precursor MeerKAT is starting its operation as we speak and will start the Large Survey Programs at the end of 2018. FAST has started its observations in drift scan mode with CRAFTS (Commensal Radio Astronomy Fast Survey). In the near future (2019-20), the best combination to study low column density H i will be to combine the sensitivity of FAST with the spatial resolution of MeerKAT.

The prevalence of type III disc breaks in H i-rich and low-spin galaxies

We investigate the origin of the type III (up-bending) discs based on a sample of 1808 galaxies from SDSS and a sub-sample of 286 galaxies with HI data from ALFALFA. We examine how the type III fraction f3, the fraction of disc galaxies which host up-bending disc breaks, depends on other galactic properties. We confirm that f3 strongly depends on the stellar concentration of galaxies. We find that HI-rich galaxies with low spins tend to have significantly more type III disc breaks than control galaxies, which are matched in concentration and stellar mass. This trend is independent of the existence of strong bars or environment of the galaxies. This result is broadly consistent with predictions from theoretical simulations, and indicates in-situ star formation fueled by gas accretion to be an important mechanism that builds the outer discs of type III galaxies.

Spatially resolved star formation relation in two HI-rich galaxies with central post-starburst signature

E+A galaxies are post-starburst systems that are identified from their optical spectra. These galaxies contain a substantial young A-type stellar component, but have only little ongoing star formation (SF). HI 21-cm line emission is found in approximately half of the nearby E+A galaxies, indicating that they contain a reservoir of gas that could fuel active SF. Here, we study two HI-rich galaxies, which show a typical E+A spectrum at the centre and SF at larger radii. We present new high spatial resolution radio interferometric observations of the HI 21-cm emission line using the VLA and of the CO(1-0) emission line using ALMA. We combine these data sets to predict the SFR and show that it does not correlate well with the SFR derived from H alpha on sub-kpc scales. We apply a recently developed statistical model for the small scale behaviour of the SF relation to predict and interpret the observed scatter. We find smoothly distributed, regularly rotating HI gas. The CO(1-0) emission line is not detected for both galaxies. The derived upper limit on the CO mass implies a molecular gas depletion time shorter than 20 Myr. However, due to the low metallicity, the CO-to-H2 conversion factor is highly uncertain. In the relations between the H alpha-based SFR and the HI mass, we observe a substantial scatter we demonstrate results from small-number statistics of independent star-forming regions on sub-kpc scales. This finding adds to the existing literature reporting a scale dependence of the molecular SF relation, showing that the atomic and molecular phases are both susceptible to the evolutionary 'cycling' of individual regions. This suggests that the atomic gas reservoirs host substantial substructure, which should be observable with future high-resolution observations. (abridged)

The JCMT Nearby Galaxies Legacy Survey – XI. Environmental variations in the atomic and molecular gas radial profiles of nearby spiral galaxies

We present an analysis of the radial profiles of a sample of 43 HI-flux selected spiral galaxies from the Nearby Galaxies Legacy Survey (NGLS) with resolved James Clerk Maxwell Telescope (JCMT) CO $J=3-2$ and/or Very Large Array (VLA) HI maps. Comparing the Virgo and non-Virgo populations, we confirm that the HI disks are truncated in the Virgo sample, even for these relatively HI-rich galaxies. On the other hand, the H$_{2}$ distribution is enhanced for Virgo galaxies near their centres, resulting in higher H$_{2}$ to HI ratios and steeper H$_{2}$ and total gas radial profiles. This is likely due to the effects of moderate ram pressure stripping in the cluster environment, which would preferentially remove low density gas in the outskirts while enhancing higher density gas near the centre. Combined with H$\alpha$ star formation rate data, we find that the star formation efficiency (SFR/H$_{2}$) is relatively constant with radius for both samples, but Virgo galaxies have a $\sim40\%$ lower star formation efficiency than non-Virgo galaxies.

The HIX galaxy survey I: Study of the most gas rich galaxies from HIPASS

We present the HI eXtreme (HIX) galaxy survey targeting some of the most HI rich galaxies in the southern hemisphere. The 13 HIX galaxies have been selected to host the most massive HI discs at a given stellar luminosity. We compare these galaxies to a control sample of average galaxies detected in the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001). As the control sample is matched in stellar luminosity, we find that the stellar properties of HIX galaxies are similar to the control sample. Furthermore, the specific star formation rate and optical morphology do not differ between HIX and control galaxies. We find, however, the HIX galaxies to be less efficient in forming stars. For the most HI massive galaxy in our sample (ESO075-G006, $\rm log\ M_{HI}\ [M_{\odot}] = 10.8$) the kinematic properties are the reason for inefficient star formation and HI excess. Examining the Australian Telescope Compact Array (ATCA) HI imaging and Wide Field Spectrograph (WiFeS) optical spectra of ESO075-G006 reveals an undisturbed galaxy without evidence for recent major, violent accretion events. A tilted-ring fit to the HI disc together with the gas-phase oxygen abundance distribution supports the scenario that gas has been constantly accreted onto ESO07-G006 but the high specific angular momentum makes ESO075-G006 very inefficient in forming stars. Thus a massive HI disc has been built up.

H i-selected galaxies in hierarchical models of galaxy formation and evolution

In this work, we study the basic statistical properties of HI-selected galaxies extracted from six different semi-analytic models, all run on the same cosmological N-body simulation. One model includes an explicit treatment for the partition of cold gas into atomic and molecular hydrogen. All models considered agree nicely with the measured HI mass function in the local Universe, with the measured scaling relations between HI and galaxy stellar mass, and with the predicted 2-point correlation function for HI rich galaxies. One exception is given by one model that predicts very little HI associated with galaxies in haloes above 10^12 Msun: we argue this is due to a too efficient radio-mode feedback for central galaxies, and to a combination of efficient stellar feedback and instantaneous stripping of hot gas for satellites. We demonstrate that treatment of satellite galaxies introduces large uncertainties at low HI masses. While models assuming non instantaneous stripping of hot gas tend to form satellite galaxies with HI masses slightly smaller than those of centrals with the same stellar mass, instantaneous gas stripping does not translate necessarily in lower HI masses. In fact, the adopted stellar feedback and star formation affect the satellites too. We analyze the relation between HI content and spin of simulated haloes: low spin haloes tend to host HI poor galaxies while high spin haloes are populated by galaxies in a wide range of HI mass. In our simulations, this is due to a correlation between the initial gas disk size and the halo spin.

Star formation associated with neutral hydrogen in the outskirts of early-type galaxies

About 20 percent of all nearby early-type galaxies ($M_{\star} \gtrsim 6 \times 10^{9}$ M$_{\odot}$) outside the Virgo cluster are surrounded by a disc or ring of low-column-density neutral hydrogen (HI) gas with typical radii of tens of kpc, much larger than the stellar body. In order to understand the impact of these gas reservoirs on the host galaxies, we analyse the distribution of star formation out to large radii as a function of HI properties using GALEX UV and SDSS optical images. Our sample consists of 18 HI-rich galaxies as well as 55 control galaxies where no HI has been detected. In half of the HI-rich galaxies the radial UV profile changes slope at the position of the HI radial profile peak. To study the stellar populations, we calculate the FUV-NUV and UV-optical colours in two apertures, 1-3 and 3-10 R$_{eff}$ . We find that HI -rich galaxies are on average 0.5 and 0.8 mag bluer than the HI-poor ones, respectively. This indicates that a significant fraction of the UV emission traces recent star formation and is associated with the HI gas. Using FUV emission as a proxy for star formation, we estimate the integrated star formation rate in the outer regions (R > 1R$_{eff}$) to be on average $6 \times 10^{-3}$ M$_{\odot}$ yr$^{-1}$ for the HI-rich galaxies. This rate is too low to build a substantial stellar disc and, therefore, change the morphology of the host. We find that the star formation efficiency and the gas depletion time are similar to those at the outskirts of spirals.

The Bluedisk survey: molecular gas distribution and scaling relations in the context of galaxy evolution

One of the key goals of the Bluedisk survey is to characterize the impact of gas accretion in disc galaxies in the context of galaxy evolution. It contains 50 disc galaxies in the stellar mass range 10^10-10^11 Msun, of which half are bluer and more HI-rich galaxies than their HI-normal (control) counterparts. In this paper, we investigate how ongoing disc growth affects the molecular gas distribution and the star-formation efficiency in these galaxies. We present 12CO observations from the IRAM 30-m telescope in 26 galaxies of the Bluedisk survey. We compare the amount and spatial distribution of the molecular gas to key quantities such as atomic gas, stellar mass and surface density, star-formation rate and metallicity. We analyse the star-formation rate per unit gas (SFR/HI and SFR/H2) and relate all those parameters to general galaxy properties (HI-rich/control disc, morphology, etc.). We find that the HI-rich galaxies have similar H2 masses as the control galaxies. In their centres, HI-rich galaxies have lower H2/HI ratios and marginally shorter molecular gas depletion times. However, the main differences between the two samples occur in the outer parts of the discs, with the HI-rich galaxies having slightly smaller CO discs (relative to the optical radius R25) and steeper CO and metallicity gradients than the control galaxies. The ongoing accretion of HI at large radii has thus not led to an appreciable growth of the CO discs in our sample. Based on depletion times, we estimate that this gas will contribute to star formation on time-scales of at least 5 Gyr.

Recent star formation in the Hi dominated outer regions of early-type galaxies

AbstractContextAccording to the ATLAS3D project, about 20 percent of all nearby early-type galaxies (D < 42 Mpc; MK < -21.5 mag; stellar mass Mstars ≳ 6 × 109 M⊙) outside clusters are surrounded by a disc or ring of low-column-density neutral hydrogen (Hi) gas with typical radii of tens of kpc, much larger than the stellar body.AimsOur aim is to understand the impact of these gas systems on the host galaxies, in particular, whether there is any recent star formation related to the Hi and effect of recent star formation on the host early-type galaxies.Methods and sampleWe analyse the distribution of star formation out to large radii by using resolved Hi images together with UV and optical images. We calculate the UV-UV and UV-optical colours in two apertures, 1-3 and 3-10 Reff. Using FUV emission as a proxy for star formation, we also estimate the integrated star formation rate in the outer regions. Our sample consists of 18 Hi-rich galaxies as well as 55 control galaxies where no Hi has been detected. We select the control sample galaxies to match the Hi-rich galaxies in stellar mass, environment, distance and stellar kinematics.ResultsIn half of the Hi-rich galaxies the radial UV profile changes slope at the position of the Hi radial profile peak. We find that the FUV-NUV and UV-optical colours in the first and second apertures of the Hi-rich galaxies are on average 0.5 and 0.8 mag bluer than the Hi-poor ones, respectively. We also find that the Hi-rich early-type galaxies have colour gradients that are almost 2 times stronger than the Hi-poor ones. we estimate the integrated star formation rate in the outer regions (R > 1 Reff) to be on average ~ 6.1×10−3 M⊙ yr−1 for the Hi-rich galaxies. We find that the gas depletion time in the outermost region (3-10 Reff) is ~ 80 Gyrs, which is similar to that estimated for the outskirts of spirals.ConclusionsStudying the stellar populations in early type galaxies with and without Hi, we find that galaxies with Hi generally show UV and UV-Optical colours in the outer parts that are bluer than those of early-type galaxies without Hi. This shows that the Hi is actively involved in recent star formation. The star formation rate in the outer regions is too low to build a stellar disc, and therefore change the morphology of the host even when integrated over several Gyrs. Star formation in outermost regions does not depend on the type of the galaxies.

H I observations of two new dwarf galaxies: Pisces A and B with the SKA Pathfinder KAT-7

Context. Pisces A & Pisces B are the only two galaxies found via optical imaging and spectroscopy out of 22 HI clouds identified in the GALFAHI survey as dwarf galaxy candidates. Aims. Derive the HI content and kinematics of Pisces A & B. Methods. Aperture synthesis HI observations using the seven dish Karoo Array Telescope (KAT-7), which is a pathfinder instrument for MeerKAT, the South African precursor to the mid-frequency Square Kilometre Array (SKA-MID). Results. The small rotation velocities detected of ~5 km/sec and ~10 km/sec in Pisces A & B respectively, and their HI content show that they are really dwarf irregular galaxies (dIrr). Despite that small rotation component, it is more the random motions ~9-11 km/sec that provide most of the gravitational support, especially in the outer parts. The study of their kinematics, especially the strong gradients of random motions, suggest that those two dwarf galaxies are not yet in equilibrium. Conclusions. These HI rich galaxies may be indicative of a large population of dwarfs at the limit of detectability. However, such gas-rich dwarf galaxies will most likely never be within the virial radius of MW-type galaxies and become sub-halo candidates. Systems such as Pisces A & B are more likely to be found at a few Mpc.s from MW-type galaxies.

The distribution of atomic hydrogen in eagle galaxies: morphologies, profiles, and H i holes

We compare the mass and internal distribution of atomic hydrogen (HI) in 2200 present-day central galaxies with M_star > 10^10 M_Sun from the 100 Mpc EAGLE Reference simulation to observational data. Atomic hydrogen fractions are corrected for self-shielding using a fitting formula from radiative transfer simulations and for the presence of molecular hydrogen using an empirical or a theoretical prescription from the literature. The resulting neutral hydrogen fractions, M_(HI+H2)/M_star, agree with observations to better than 0.1 dex for galaxies with M_star between 10^10 and 10^11 M_Sun. Our fiducial, empirical H2 model based on gas pressure results in galactic HI mass fractions, M_HI/M_star, that agree with observations from the GASS survey to better than 0.3 dex, but the alternative theoretical H2 formula leads to a negative offset in M_HI/M_star of up to 0.5 dex. Visual inspection reveals that most HI disks in simulated HI-rich galaxies are vertically disturbed, plausibly due to recent accretion events. Many galaxies (up to 80 per cent) contain spuriously large HI holes, which are likely formed as a consequence of the feedback implementation in EAGLE. The HI mass-size relation of all simulated galaxies is close to (but 16 per cent steeper than) observed, and when only galaxies without large holes in the HI disc are considered, the agreement becomes excellent (better than 0.1 dex). The presence of large HI holes also makes the radial HI surface density profiles somewhat too low in the centre, at \Sigma_HI > 1 M_Sun pc^-2 (by a factor of <~ 2 compared to data from the Bluedisk survey). In the outer region (\Sigma_HI < 1 M_Sun pc^-2), the simulated profiles agree quantitatively with observations. Scaled by HI size, the simulated profiles of HI-rich (M_HI > 10^9.8 M_Sun) and control galaxies (10^9.1 M_Sun > M_HI > 10^9.8 M_Sun) follow each other closely, as observed. (Abridged)

Gas-phase metallicity profiles of the Bluedisk galaxies: Is metallicity in a local star formation regulated equilibrium?

As part of the Bluedisk survey we analyse the radial gas-phase metallicity profiles of 50 late-type galaxies We compare the metallicity profiles of a sample of HI-rich galaxies against a control sample of HI-'normal' galaxies. We find the metallicity gradient of a galaxy to be strongly correlated with its HI mass fraction (M(HI) / Mstar). We note that some galaxies exhibit a steeper metallicity profile in the outer disc than in the inner disc. These galaxies are found in both the HI-rich and control samples. This contradicts a previous indication that these outer drops are exclusive to HI-rich galaxies. These effects are not driven by bars, although we do find some indication that barred galaxies have flatter metallicity profiles. By applying a simple analytical model we are able to account for the variety of metallicity profiles that the two samples present. The success of this model implies that the metallicity in these isolated galaxies may be in a local equilibrium, regulated by star formation. This insight could provide an explanation of the observed local mass-metallicity relation.

The outer stellar populations and environments of unusually H i-rich galaxies

We investigate the nature of HI-rich galaxies from the ALFALFA and GASS surveys, which are defined as galaxies in the top 10th percentile in atomic gas fraction at a given stellar mass. We analyze outer (R>1.5 Re) stellar populations for a subset of face-on systems using optical g-r versus r-z colour/colour diagrams. The results are compared with those from control samples that are defined without regard to atomic gas content, but are matched in redshift, stellar mass and structural parameters. HI-rich early-type (C>2.6) and late-type (C<2.6) galaxies are studied separately. When compared to the control sample, the outer stellar populations of the majority of HI-rich early-type galaxies are shifted in the colour/colour plane along a locus consistent with younger stellar ages, but similar metallicities. The outer colours of HI-rich late-type galaxies are much bluer in r-z than the HI-rich early types, and we infer that they have outer disks which are both younger and more metal-poor. We then proceed to analyze the galaxy environments of HI-rich galaxies on scales of 500 kpc. HI-rich early-type galaxies with low (log M* < 10.5) stellar masses differ significantly from the control sample in that they are more likely to be central rather than satellite systems. Their satellites are also less massive and have younger stellar populations. Similar, but weaker effects are found for low mass HI-rich late-type galaxies. In addition, we find that the satellites of HI-rich late-types exhibit a greater tendency to align along the major axis of the primary. No environmental differences are found for massive (log M* > 10.5) HI-rich galaxies, regardless of type.

H i scaling relations of galaxies in the environment of H i-rich and control galaxies observed by the Bluedisk project

Our work is based on the "Bluedisk" project, a program to map the neutral gas in a sample of 25 HI-rich spirals and a similar number of control galaxies with the Westerbork Synthesis Radio Telescope (WSRT). In this paper we focus on the HI properties of the galaxies in the environment of our targeted galaxies. In total, we extract 65 galaxies from the WSRT cubes with stellar masses between $10^8M_{\odot}$ and $10^{11}M_{\odot}$. Most of these galaxies are located on the same HI mass-size relation and "HI-plane" as normal spiral galaxies. We find that companions around HI-rich galaxies tend to be HI-rich as well and to have larger R90,HI/R50,HI. This suggests a scenario of "HI conformity", similar to the colour conformity found by Weinmann et al. (2006): galaxies tend to adopt the HI properties of their neighbours. We visually inspect the outliers from the HI mass-size relation and galaxies which are offset from the HI plane and find that they show morphological and kinematical signatures of recent interactions with their environment. We speculate that these outliers have been disturbed by tidal or ram-pressure stripping processes, or in a few cases, by accretion events.

HIGHz: a survey of the most H i-massive galaxies at z ∼ 0.2

We present the results of the HIGHz Arecibo survey, which measured the HI content of 39 galaxies at redshift $z>0.16$ selected from the Sloan Digital Sky Survey. These are all actively star-forming, disk-dominated systems in relatively isolated environments, with stellar and HI masses larger than $10^{10}$ M$_\odot$ and redshifts $0.17\leq z\leq 0.25$. Our sample includes not only the highest-redshift detections of HI emission from individual galaxies to date, but also some of the most HI-massive systems known. Despite being exceptionally large, the HI reservoirs of these galaxies are consistent with what is expected from their ultraviolet and optical properties. This, and the fact that the galaxies lie on the baryonic Tully-Fisher relation, suggests that HIGHz systems are rare, scaled-up versions of local disk galaxies. We show that the most HI-massive galaxies discovered in the Arecibo Legacy Fast ALFA survey are the local analogues of HIGHz, and discuss the possible connection between our sample and the turbulent, gas-rich disks identified at $z\sim 1$. The HIGHz sample provides a first glimpse into the properties of the massive, HI-rich galaxies that will be detected at higher redshifts by the next-generation HI surveys with the Square Kilometer Array and its pathfinders.

SMALL-SCALE PROPERTIES OF ATOMIC GAS IN EXTENDED DISKS OF GALAXIES

We present high-resolution HI 21 cm observations with the Karl G. Jansky Very Large Array (VLA) for three HI rich galaxies in absorption against radio quasars. Our sample contains six sightlines with impact parameters from 2.6 to 32.4 kpc. We detected a narrow HI absorber of FWHM 1.1 km/s at 444.5 km/s towards J122106.854+454852.16 probing the dwarf galaxy UCG 7408 at an impact parameter of 2.8 kpc. The absorption feature was barely resolved and its width corresponds to a maximum kinetic temperature, $\rm T_k \approx 26~K$. We estimate a limiting peak optical depth of 1.37 and a column density of $\rm 6\times 10^{19}~cm^{-2}$. The physical extent of the absorber is $\rm 0.04~kpc^2$ and covers $\sim$25-30\% of the background source. A comparison between the emission and absorption strengths suggests the cold-to-total HI column density in the absorber is ~30%. Folding in the covering fraction, the cold-to-total HI mass is ~10%. This suggest that condensation of warm HI ($\rm T_s\sim 1000~K$) to cold phase ($\rm T_s < 100~K$) is suppressed in UGC 7408. The unusually low temperature of the HI absorber also indicates inefficiency in condensation of atomic gas into molecular gas. The suppression in condensation is likely to be the result of low-metal content in this galaxy. The same process might explain the low efficiency of star formation in dwarf galaxies despite their huge of gas reservoirs. We also report the non-detection of HI in absorption in five other sightlines. This indicates that either the cold gas distribution is highly patchy or the gas is much warmer ($\rm T_s~>1000~K$) towards these sightlines.

An observational and theoretical view of the radial distribution of H I gas in galaxies

We analyze the radial distribution of HI gas for 23 disk galaxies with unusually high HI content from the Bluedisk sample, along with a similar-sized sample of "normal" galaxies. We propose an empirical model to fit the radial profile of the HI surface density, an exponential function with a depression near the center. The radial HI surface density profiles are very homogeneous in the outer regions of the galaxy; the exponentially declining part of the profile has a scale-length of $\sim 0.18$ R1, where R1 is the radius where the column density of the HI is 1 M$_{\odot}$ pc$^{-2}$. This holds for all galaxies, independent of their stellar or HI mass. The homogenous outer profiles, combined with the limited range in HI surface density in the non-exponential inner disk, results in the well-known tight relation between HI size and HI mass. By comparing the radial profiles of the HI-rich galaxies with those of the control systems, we deduce that in about half the galaxies, most of the excess gas lies outside the stellar disk, in the exponentially declining outer regions of the HI disk. In the other half, the excess is more centrally peaked. We compare our results with existing smoothed-particle hydrodynamical simulations and semi-analytic models of disk galaxy formation in a $\Lambda$ Cold Dark Matter universe. Both the hydro simulations and the semi-analytic models reproduce the HI surface density profiles and the HI size-mass relation without further tuning of the simulation and model inputs. In the semi-analytic models, the universal shape of the outer HI radial profiles is a consequence of the {\em assumption} that infalling gas is always distributed exponentially. The conversion of atomic gas to molecular form explains the limited range of HI surface densities in the inner disk. These two factors produce the tight HI mass-size relation.

EVOLUTION IN THE H I GAS CONTENT OF GALAXY GROUPS: PRE-PROCESSING AND MASS ASSEMBLY IN THE CURRENT EPOCH

We present an analysis of the neutral hydrogen (HI) content and distribution of galaxies in groups as a function of their parent dark matter halo mass. The Arecibo Legacy Fast ALFA survey alpha.40 data release allows us, for the first time, to study the HI properties of over 740 galaxy groups in the volume of sky common to the SDSS and ALFALFA surveys. We assigned ALFALFA HI detections a group membership based on an existing magnitude/volume-limited SDSS DR7 group/cluster catalog. Additionally, we assigned group "proximity" membership to HI detected objects whose optical counterpart falls below the limiting optical magnitude--thereby not contributing substantially to the estimate of the group stellar mass, but significantly to the total group HI mass. We find that only 25% of the HI detected galaxies reside in groups or clusters, in contrast to approximately half of all optically detected galaxies. Further, we plot the relative positions of optical and HI detections in groups as a function of parent dark matter halo mass to reveal strong evidence that HI is being processed in galaxies as a result of the group environment: as optical membership increases, groups become increasingly deficient of HI rich galaxies at their center and the HI distribution of galaxies in the most massive groups starts to resemble the distribution observed in comparatively more extreme cluster environments. We find that the lowest HI mass objects lose their gas first as they are processed in the group environment, and it is evident that the infall of gas rich objects is important to the continuing growth of large scale structure at the present epoch, replenishing the neutral gas supply of groups. Finally, we compare our results to those of cosmological simulations and find that current models cannot simultaneously predict the HI selected halo occupation distribution for both low and high mass halos.

The ESO UVES advanced data products quasar sample

Quasar foreground damped absorbers, associated with HI-rich galaxies allow to estimate the neutral gas mass over cosmic time, which is a possible indicator of gas consumption as star formation proceeds. The DLAs and sub-DLAs are believed to contain a large fraction of neutral gas mass in the Universe. In Paper I of the series, we present the results of a search for DLAs and sub-DLAs in the ESO-UVES Advanced Data Products dataset of 250 quasars. Here we use an unbiased sub-sample of sub-DLAs from this dataset. We build a subset of 122 quasars ranging from 1.5 <z_em < 5.0, suitable for statistical analysis. The statistical sample is analyzed in conjunction with other sub-DLA samples from the literature. This makes up a combined sample of 89 sub-DLAs over a redshift path of $\Delta z=193$. Redshift evolution of the number density and the line density are derived for sub-DLAs and compared with the LLSs and DLAs measurements from the literature. The results indicate that these three classes of absorbers are evolving in the redshift interval 1 < z < 5. The column density distribution, f(N,z), down to the sub-DLA limit is determined. The flattening of f_(N,z) in the sub-DLA regime is present in the observations. The redshift evolution of f_(N,z) down to sub-DLA regime is also presented, indicating the presence of more sub-DLAs at high-redshift as compared to low-redshift. f_(N,z) is further used to determine the neutral gas mass density, Omega_g, at 1.5 < z < 5.0. The complete sample shows that sub-DLAs contribute 8-20% to the total Omega_g from 1.5 < z < 5.0. In agreement with previous studies, no evolution of Omega_g is seen from low-redshift to high-redshift, suggesting that star formation solely cannot explain this non-evolution and replenishment of gas and/or recombination of ionized gas is needed. (Abridged)