HI Mass Density Research Articles

Contribution of HI-bearing ultra-diffuse galaxies to the cosmic number density of galaxies

We estimate the cosmic number density of the recently identified class of HI-bearing ultra-diffuse sources (HUDs) based on the completeness limits of the ALFALFA survey. These objects have HI masses approximately in the range 8.5 < logMHI∕M⊙ < 9.5, average r-band surface brightnesses fainter than 24 mag arcsec−2, half-light radii greater than 1.5 kpc, and are separated from neighbours by at least 350 kpc. In this work we demonstrate that they contribute at most ~6% of the population of HI-bearing dwarfs detected by ALFALFA (with similar HI masses), have a total cosmic number density of (1.5 ± 0.6) × 10−3 Mpc−3, and an HI mass density of (6.0 ± 0.8) × 105 M⊙ Mpc−3. We estimate that this is similar to the total cosmic number density of ultra-diffuse galaxies (UDGs) in groups and clusters, and conclude that the relation between the number of UDGs hosted in a halo and the halo mass must have a break below M200 ~ 1012 M⊙ in order to account for the abundance of HUDs in the field. The distribution of the velocity widths of HUDs rises steeply towards low values, indicating a preference for slow rotation rates compared to the global HI-rich dwarf population. These objects were already included in previous measurements of the HI mass function, but have been absent from measurements of the galaxy stellar mass function owing to their low surface brightness. However, we estimate that due to their low number density the inclusion of HUDs would constitute a correction of less than 1%. Comparison with the Santa Cruz semi-analytic model shows that it produces HI-rich central UDGs that have similar colours to HUDs, but that these UDGs are currently produced in a much greater number. While previous results from this sample have favoured formation scenarios where HUDs form in high spin-parameter halos, comparisons with recent results which invoke that formation mechanism reveal that this model produces an order of magnitude more field UDGs than we observe in the HUD population, and these have an occurrence rate (relative to other dwarfs) that is approximately double what we observe. In addition, the colours of HUDs are bluer than predicted, although we suspect this is due to a systematic problem in reproducing the star formation histories of low-mass galaxies rather than being specific to the ultra-diffuse nature of these sources.

Giant Metrewave Radio Telescope observations of neutral atomic hydrogen gas in the COSMOS field atz∼ 0.37

We present the results of HI spectral stacking analysis of Giant Metrewave Radio Telescope (GMRT) observations targeting the COSMOS field. The GMRT data cube contains 474 field galaxies with redshifts known from the zCOSMOS-bright 10k catalogue. Spectra for the galaxies are co-added and the stacked spectrum allows us to make a $\sim 3\sigma$ measurement of the average HI mass. Using this average HI mass along with the integral optical $B$-band luminosity of the galaxies and the luminosity density of the COSMOS field, a volume normalisation is applied to obtain the cosmic HI mass density ($\Omega_{\rm HI}$). We find a cosmic HI mass density of $\Omega_{\rm HI}$ = (0.42 $\pm$ 0.16) $\times$ 10$^{-3}$ at $z \sim 0.37$, which is the highest-redshift measurement of $\Omega_{\rm HI}$ ever made using HI spectral stacking. The value we obtained for $\Omega_{\rm HI}$ at $z \sim 0.37$ is consistent with that measured from large blind 21-cm surveys at $z = 0$ as well as measurements from other HI stacking experiments at lower redshifts. Our measurement in conjunction with earlier measurements indicates that there has been no significant evolution of HI gas abundance over the last 4 Gyr. A weighted mean of $\Omega_{\rm HI}$ from all 21-cm measurements at redshifts $z \lesssim 0.4$ gives $\Omega_{\rm HI}$ = (0.35 $\pm$ 0.01) $\times$ 10$^{-3}$. The $\Omega_{\rm HI}$ measured (from HI 21-cm emission measurements) at $z \lesssim 0.4$ is however approximately half that measured from Damped Lyman-$\alpha$ Absorption (DLA) systems at $z \gtrsim 2$. Deeper surveys with existing and upcoming instruments will be critical to understand the evolution of $\Omega_{\rm HI}$ in the redshift range intermediate between $z \sim 0.4$ and the range probed by DLA observations.

Detection of H i in distant galaxies using spectral stacking

Using the Parkes radio telescope, we study the 21cm neutral hydrogen (HI) properties of a sample of galaxies with redshifts z<0.13 extracted from the optical 2dF Galaxy Redshift Survey (2dFGRS). Galaxies at 0.04<z<0.13 are studied using new Parkes observations of a 42deg^2 field near the South Galactic Pole (SGP). A spectral stacking analysis of the 3,277 2dFGRS objects within this field results in a convincing 12sigma detection. For the low-redshift sample at 0<z<0.04, we use the 15,093 2dFGRS galaxies observed by the HI Parkes All-Sky Survey (HIPASS) and find a 31sigma stacked detection. We measure average HI masses of (6.93 +/- 0.17)*10^9 h^{-2} Msun and (1.48 +/- 0.03)*10^9 h^{-2} Msun for the SGP and HIPASS samples, respectively. Accounting for source confusion and sample bias, we find a cosmic HI mass density of Omega_HI=(3.19_{-0.59} ^{+0.43})*10^{-4} h^{-1} for the SGP sample and (2.82_{-0.59}^{+0.30})*10^{-4} h^{-1} for the HIPASS sample. This suggests no (12 +/- 23%) evolution in the cosmic HI density over the last ~1h^{-1} Gyr. Due to the very large effective volumes, cosmic variance in our determination of Omega_HI is considerably lower than previous estimates. Our stacking analysis reproduces and quantifies the expected trends in the HI mass and mass-to-light ratio of galaxies with redshift, luminosity and colour.

The evolution of H i and C iv quasar absorption line systems at 1.9 &lt;z&lt; 3.2

We have investigated the distribution and evolution of ~3100 intergalactic HI absorbers with HI column densities log N(HI) = [12.75, 17.0] at 1.9 < z < 3.2, using 18 high resolution, high S/N quasar spectra obtained from the ESO VLT/UVES archive. We used two sets of Voigt profile fitting analysis, one including all the available high-order Lyman lines to obtain reliable HI column densities of saturated lines, and another using only the Ly-alpha lines. There is no significant difference between the results from the two fits. Combining our results with literature data, the mean number density at 0 < z < 4 is not well described by a single power law and strongly suggests that its evolution slows down at z < 1.5 at the high and low column density ranges. We also divided our entire HI absorbers at 1.9 < z < 3.2 into two samples, the unenriched forest and the CIV-enriched forest, depending on whether HI lines are associated with CIV at log N(CIV) > 12.2 within a given velocity range. The entire HI column density distribution function (CDDF) can be described as the combination of these two well-characterised populations which overlap at log N(HI) ~ 15. At log N(HI) < 15, the unenriched forest dominates, showing a similar power-law distribution to the entire forest. The CIV-enriched forest dominates at log N(HI) > 15, with its distribution function proportional to N(HI)^(-1.45). However, it starts to flatten out at lower N(HI), since the enriched forest fraction decreases with decreasing N(HI). The deviation from the power law at log N(HI) = [14, 17] shown in the CDDF for the entire HI sample is a result of combining two different HI populations with a different CDDF shape. The total HI mass density relative to the critical density is Omega(HI) ~ 1.6 x 10^(-6) h^(-1), where the enriched forest accounts for ~40% of Omega(HI).

UPDATED NEARBY GALAXY CATALOG

We present an all-sky catalog of 869 nearby galaxies, having individual distance estimates within 11 Mpc or corrected radial velocities V_{LG} < 600 km/s. The catalog is a renewed and expanded version of the "Catalog of Neighboring Galaxies" by Karachentsev et al. (2004). It collects data on the following observables for the galaxies: angular diameters, apparent magnitudes in FUV-, B-, and K_s- bands, H_alpha and HI fluxes, morphological types, HI-line widths, radial velocities and distance estimates. In this Local Volume (LV) sample 108 dwarf galaxies remain to be still without measured radial velocities. The catalog yields also calculated global galaxy parameters: linear Holmberg diameter, absolute B-magnitude, surface brightness, HI-mass, stellar mass estimated via K-band luminosity, HI rotational velocity corrected for galaxy inclination, indicative mass within the Holmberg radius, and three kinds of "tidal index", which quantify the local density environment. The catalog is supplemented with the data based on the local galaxies (http://www.sao.ru/lv/lvgdb), which presents their optical and available H_alpha images, as well as other service. We briefly discuss the Hubble flow within the LV, and different scaling relations that characterize galaxy structure and global star formation in them. We also trace the behavior of the mean stellar mass density, HI-mass density and star formation rate density within the considered volume.

THE H I MASS DENSITY IN GALACTIC HALOS, WINDS, AND COLD ACCRETION AS TRACED BY Mg II ABSORPTION

It is well established that MgII absorption lines detected in background quasar spectra arise from gas structures associated with foreground galaxies. The degree to which galaxy evolution is driven by the gas cycling through halos is highly uncertain because their gas mass density is poorly constrained. Fitting the MgII equivalent width (W) distribution with a Schechter function and applying the N(HI)-W correlation of Menard & Chelouche, we computed Omega(HI)_MgII ~ Omega(HI)_halo =(1.41 +0.75 -0.44)x10^-4 for 0.4<z<1.4. We exclude DLAs from our calculations so that Omega(HI)_halo comprises accreting and/or outflowing halo gas not locked up in cold neutral clouds. We deduce the cosmic HI gas mass density fraction in galactic halos traced by MgII absorption is Omega(HI)_halo/Omega(HI)_DLA=15% and Omega(HI)_halo/Omega_b=0.3%. Citing several lines of evidence, we propose infall/accretion material is sampled by small W whereas outflow/winds are sampled by large W, and find Omega(HI)_infall is consistent with Omega(HI)_outflow for bifurcation at W=1.23^{+0.15}_{-0.28}\AA; cold accretion would then comprise no more than ~7% of of the total HI mass density. We discuss evidence that (1) the total HI mass cycling through halos remains fairly constant with cosmic time and that the accretion of HI gas sustains galaxy winds, and (2) evolution in the cosmic star formation rate depends primarily on the rate at which cool HI gas cycles through halos.

The GALEX Arecibo SDSS Survey - II. The star formation efficiency of massive galaxies

We use measurements of the HI content, stellar mass and star formation rates in ~190 massive galaxies with stellar masses greater than 10^10 Msun, obtained from the Galex Arecibo SDSS Survey (GASS) described in Paper I (Catinella et al. 2010) to explore the global scaling relations associated with the bin-averaged ratio of the star formation rate over the HI mass, which we call the HI-based star formation efficiency (SFE). Unlike the mean specific star formation rate, which decreases with stellar mass and stellar mass surface density, the star formation efficiency remains relatively constant across the sample with a value close to SFE = 10^-9.5 yr^-1 (or an equivalent gas consumption timescale of ~3 Gyr). Specifically, we find little variation in SFE with stellar mass, stellar mass surface density, NUV-r color and concentration. We interpret these results as an indication that external processes or feedback mechanisms that control the gas supply are important for regulating star formation in massive galaxies. An investigation into the detailed distribution of SFEs reveals that approximately 5% of the sample shows high efficiencies with SFE > 10^-9 yr^-1, and we suggest that this is very likely due to a deficiency of cold gas rather than an excess star formation rate. Conversely, we also find a similar fraction of galaxies that appear to be gas-rich for their given specific star-formation rate, although these galaxies show both a higher than average gas fraction and lower than average specific star formation rate. Both of these populations are plausible candidates for "transition" galaxies, showing potential for a change (either decrease or increase) in their specific star formation rate in the near future. We also find that 36+/-5% of the total HI mass density and 47+/-5% of the total SFR density is found in galaxies with stellar mass greater than 10^10 Msun. [abridged]

A method to measure the mass of damped Lyα absorber host galaxies using fluctuations in 21-cm emission

Observations of damped Ly-alpha absorbers (DLA) indicate that the fraction of hydrogen in its neutral form (HI) is significant by mass at all redshifts. This gas represents the reservoir of material that is available for star formation at late times. As a result, observational identification of the systems in which this neutral hydrogen resides is an important missing ingredient in models of galaxy formation. Precise identification of DLA host mass via traditional clustering studies is not practical owing to the small numbers of known systems being spread across sparsely distributed sight lines. However following the completion of reionization, 21cm surface brightness fluctuations will be dominated by neutral hydrogen in DLAs. Observations of these fluctuations will measure the combined clustering signal from all DLAs within a large volume. We show that measurement of the spherically averaged power-spectrum of 21cm intensity fluctuations due to DLAs could be used to measure the galaxy bias for DLA host galaxies when combined with an independent measurement of the cosmological HI mass density from quasar absorption studies. Utilising this technique, the low frequency arrays now under construction could measure the characteristic DLA host mass with a statistical precision as low as 0.3 dex at z~4. In addition, high signal-to-noise observations of the peculiar-motion induced anisotropy of the power-spectrum would facilitate measurement of both the DLA host mass and the cosmic HI density directly from 21cm fluctuations. By exploiting this anisotropy, a second generation of low frequency arrays with an order of magnitude increase in collecting area could measure the values of cosmic HI density and DLA host mass, with uncertainties of a few percent and a few tens of percent respectively.

A Search for HI Absorption in the Radio Spectra of the Quasi-Stellar Objects 4C 05.34 and PKS 0237-23

The ratio of the mass density in visible galaxies to the upper limit for the HI mass density determined from the lack of continuous Ly-α absorption in QSO spectra is 3 × 104. Thus, if galaxies form by condensation from an intergalactic medium, the process must be extremely efficient. Although no absorption due to a continuous distribution of intergalactic HI has been detected, many QSOs have been discovered that have absorption lines in their spectra, which have redshifts very different from the emission lines, and may be produced in clouds of integalactic HI. As would be expected if this were the case, the proportion of QSOs with absorption lines in their spectra is greatest for those which have large emission line redshifts. Another possibility, especially for those absorption lines in redshift systems with Zabs nearly equal to Zem , is that the absorption lines are produced in clouds surrounding the QSO.