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Register a new userThe GALEX Arecibo SDSS Survey V: The Relation between the HI Content of Galaxies and Metal Enrichment at their Outskirts
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Sean M. Moran
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We have obtained long-slit spectra of 174 star-forming galaxies with stellar masses greater than 10^10 M_odot from the GALEX Arecibo SDSS (GASS) survey. These galaxies have both HI and H_2 mass measurements. The average metallicity profile is strikingly flat out to R_90, the radius enclosing 90% of the r-band light. Metallicity profiles which decline steadily with radius are found primarily for galaxies in our sample with low stellar mass (Log(M_*)<10.2), concentration, and/or mean stellar mass density. Beyond ~R_90, however, around 10 percent of the galaxies in our sample exhibit a sharp downturn in metallicity. Remarkably, we find that the magnitude of the outer metallicity drop is well correlated with the total HI content of the galaxy (measured as f_HI=M_HI/M_*). We examine the radial profiles of stellar population ages and star formation rate densities, and conclude that the galaxies with largest outer metallicity drops are actively growing their stellar disks, with mass doubling times across the whole disk only one third as long as a typical GASS galaxy. We also describe a correlation between local stellar mass density and metallicity, which is valid across all galaxies in our sample. We argue that much of the recent stellar mass growth at the edges of these galaxies can be linked to the accretion or radial transport of relatively pristine gas from beyond the galaxies stellar disks.
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The GALEX Arecibo SDSS Survey (GASS) is an ambitious program designed to investigate the cold gas properties of massive galaxies, a challenging population for HI studies. Using the Arecibo radio telescope, GASS is gathering high-quality HI-line spectra for an unbiased sample of ~1000 galaxies with stellar masses greater than 10^10 Msun and redshifts 0.025 < z < 0.05, uniformly selected from the SDSS spectroscopic and GALEX imaging surveys. The galaxies are observed until detected or until a low gas mass fraction limit (1.5-5%) is reached. We present initial results based on the first Data Release, which consists of ~20% of the final GASS sample. We use this data set to explore the main scaling relations of HI gas fraction with galaxy structure and NUV-r colour, and show our best fit plane describing the relation between gas fraction, stellar mass surface density and NUV-r colour. Interesting outliers from this plane include gas-rich red sequence galaxies that may be in the process of regrowing their disks, as well as blue, but gas-poor spirals.
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]
We present the final data release from the GALEX Arecibo SDSS Survey (GASS), a large Arecibo program that measured the HI properties for an unbiased sample of ~800 galaxies with stellar masses greater than 10^10 Msun and redshifts 0.025<z<0.05. This release includes new Arecibo observations for 250 galaxies. We use the full GASS sample to investigate environmental effects on the cold gas content of massive galaxies at fixed stellar mass. The environment is characterized in terms of dark matter halo mass, obtained by cross-matching our sample with the SDSS group catalog of Yang et al. Our analysis provides, for the first time, clear statistical evidence that massive galaxies located in halos with masses of 10^13-10^14 Msun have at least 0.4 dex less HI than objects in lower density environments. The process responsible for the suppression of gas in group galaxies most likely drives the observed quenching of the star formation in these systems. Our findings strongly support the importance of the group environment for galaxy evolution, and have profound implications for semi-analytic models of galaxy formation, which currently do not allow for stripping of the cold interstellar medium in galaxy groups.
The GALEX Arecibo SDSS Survey (GASS) is a large targeted survey that started at Arecibo in March 2008. GASS is designed to measure the neutral hydrogen content of ~1000 massive galaxies (with stellar mass Mstar > 10^10 Msun) at redshift 0.025<z<0.05, uniformly selected from the SDSS spectroscopic and GALEX imaging surveys. Our selected mass range straddles the recently identified transition mass (Mstar ~3x10^10 Msun) above which galaxies show a marked decrease in their present to past-averaged star formation rates. GASS will produce the first statistically significant sample of massive transition galaxies with homogeneously measured stellar masses, star formation rates and gas properties. The analysis of this sample will allow us to investigate if and how the cold gas responds to a variety of different physical conditions in the galaxy, thus yielding insights on the physical processes responsible for the transition between blue, star-forming and red, passively evolving galaxies. GASS will be of considerably legacy value not only in isolation but also by complementing ongoing HI-selected surveys.
We present dynamical scaling relations for a homogeneous and representative sample of ~500 massive galaxies, selected only by stellar mass (>10^10 Msun) and redshift (0.025<z<0.05) as part of the ongoing GALEX Arecibo SDSS Survey. We compare baryonic Tully-Fisher (BTF) and Faber-Jackson (BFJ) relations for this sample, and investigate how galaxies scatter around the best fits obtained for pruned subsets of disk-dominated and bulge-dominated systems. The BFJ relation is significantly less scattered than the BTF when the relations are applied to their maximum samples, and is not affected by the inclination problems that plague the BTF. Disk-dominated, gas-rich galaxies systematically deviate from the BFJ relation defined by the spheroids. We demonstrate that by applying a simple correction to the stellar velocity dispersions that depends only on the concentration index of the galaxy, we are able to bring disks and spheroids onto the same dynamical relation -- in other words, we obtain a generalized BFJ relation that holds for all the galaxies in our sample, regardless of morphology, inclination or gas content, and has a scatter smaller than 0.1 dex. We find that disks and spheroids are offset in the stellar dispersion-size relation, and that the offset is removed when corrected dispersions are used instead. The generalized BFJ relation represents a fundamental correlation between the global dark matter and baryonic content of galaxies, which is obeyed by all (massive) systems regardless of morphology. [abridged]
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