Silvia Fabello

The GALEX Arecibo SDSS Survey - I. Gas fraction scaling relations of massive galaxies and first data release

We introduce the GALEX Arecibo SDSS Survey (GASS), an on-going large programme that is gathering high quality H i-line spectra using the Arecibo radio telescope for an unbiased sample of ~1000 galaxies with stellar masses greater than 10^(10) M_⊙ and redshifts 0.025 < z < 0.05 , selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging surveys. The galaxies are observed until detected or until a low gas mass fraction limit (1.5–5 per cent) is reached. This paper presents the first Data Release, consisting of ~20 per cent of the final GASS sample. We use this data set to explore the main scaling relations of the H i gas fraction with galaxy structure and NUV−r colour. A large fraction (~60 per cent) of the galaxies in our sample are detected in H i. Even at stellar masses above 10^(11) M_⊙, the detected fraction does not fall below ~40 per cent. We find that the atomic gas fraction M_(HI)/M★ decreases strongly with stellar mass, stellar surface mass density and NUV−r colour, but is only weakly correlated with the galaxy bulge-to-disc ratio (as measured by the concentration index of the r-band light). We also find that the fraction of galaxies with significant (more than a few per cent) H I decreases sharply above a characteristic stellar surface mass density of 10^(8.5) M_⊙ kpc^(−2). The fraction of gas-rich galaxies decreases much more smoothly with stellar mass. One of the key goals of GASS is to identify and quantify the incidence of galaxies that are transitioning between the blue, star-forming cloud and the red sequence of passively evolving galaxies. Likely transition candidates can be identified as outliers from the mean scaling relations between M_(HI)/M★ and other galaxy properties. We have fitted a plane to the two-dimensional relation between the H I mass fraction, stellar surface mass 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 discs, as well as blue, but gas-poor spirals.

COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies – II. The non-universality of the molecular gas depletion time-scale

We study the relation between molecular gas and star formation in a volume-limited sample of 222 galaxies from the COLD GASS survey, with measurements of the CO(1–0) line from the IRAM 30-m telescope. The galaxies are at redshifts 0.025 < z < 0.05 and have stellar masses in the range 10.0 < log M� /M� < 11.5. The IRAM measurements are complemented by deep Arecibo H I observations and homogeneous Sloan Digital Sky Survey and GALEX photometry. A reference sample that includes both ultraviolet (UV) and far-infrared data is used to calibrate our estimates of star formation rates from the seven optical/UV bands. The mean molecular gas depletion time-scale [tdep(H2)] for all the galaxies in our sample is 1 Gyr; however, tdep(H2) increases by a factor of 6 from a value of ∼0.5 Gyr for galaxies with stellar –

The Impact of Interactions, Bars, Bulges, and Active Galactic Nuclei on Star Formation Efficiency in Local Massive Galaxies

Using atomic and molecular gas observations from the GASS and COLD GASS surveys and complementary optical/UV data from the Sloan Digital Sky Survey and the Galaxy Evolution Explorer, we investigate the nature of the variations in the molecular gas depletion time observed across the local massive galaxy population. The large and unbiased COLD GASS sample allows us for the first time to statistically assess the relative importance of galaxy interactions, bar instabilities, morphologies, and the presence of active galactic nuclei (AGNs) in regulating star formation efficiency. We find that both the H2 mass fraction and depletion time vary as a function of the distance of a galaxy from the main sequence traced by star-forming galaxies in the SFR-M * plane. The longest gas depletion times are found in below-main-sequence bulge-dominated galaxies (?*?>5 ? 108 M ??kpc?2, C > 2.6) that are either gas-poor (/M *? 1010 M ?, we derive a global Kennicutt-Schmidt star formation relation of slope 1.18 ? 0.24 and observe structure within the scatter around this relation, with galaxies having low (high) stellar mass surface densities lying systematically above (below) the mean relation, suggesting that is not the only parameter driving the global star formation ability of a galaxy.

The GALEX Arecibo SDSS Survey – VIII. Final data release. The effect of group environment on the gas content of massive galaxies

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 M⊙ 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 M⊙ 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 II: The Star Formation Efficiency of Massive Galaxies

We use measurements of the H I content, stellar mass and star formation rates (SFRs) in ~190 massive galaxies with M_★ > 10^(10) M_⊙, obtained from the GALEX (Galaxy Evolution Explorer) Arecibo SDSS (Sloan Digital Sky Survey) survey described in Paper I to explore the global scaling relations associated with the bin-averaged ratio of the SFR over the H I mass (i.e. ΣSFR/ΣM_(HI)), which we call the H I-based star formation efficiency (SFE). Unlike the mean specific star formation rate (sSFR), which decreases with stellar mass and stellar mass surface density, the SFE remains relatively constant across the sample with a value close to SFE = 10^(−9.5) yr^(−1) (or an equivalent gas consumption time-scale of ~3 × 10^9 yr). Specifically, we find little variation in SFE with stellar mass, stellar mass surface density, NUV −r colour and concentration (R_(90)/R_(50)). 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 per cent 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 SFR. Conversely, we also find a similar fraction of galaxies that appear to be gas-rich for their given sSFR, although these galaxies show both a higher than average gas fraction and lower than average sSFR. Both of these populations are plausible candidates for ‘transition’ galaxies, showing potential for a change (either decrease or increase) in their sSFR in the near future. We also find that 36 ± 5 per cent of the total H I mass density and 47 ± 5 per cent of the total SFR density are found in galaxies with M_★ > 10^(10) M_⊙.

The GALEX Arecibo SDSS Survey - VI. Second data release and updated gas fraction scaling relations

We present the second data release from the GALEX Arecibo SDSS Survey (GASS), an ongoing large Arecibo program to measure the Hi properties for an unbiased sample of∼1000 galaxies with stellar masses greater than 10 10 M⊙ and redshifts 0.025< z< 0.05. GASS targets are selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX)imaging surveys, and are observed until detected or until a gas mass fraction limit of a few per cent is reached. This second data installment includes new Arecibo observations of 240 galaxies, and marks the 50% of the complete survey. We present catalogs of the Hi, optical and ultraviolet parameters for these galaxies, and their Hi-line profiles. Having more than doubled the size of the sampl e since the first data release, we also revisit the main scaling relations of t he Hi mass fraction with galaxy stellar mass, stellar mass surfac e density, concentration index, and NUV−r color, as well as the gas fraction plane introduced in our ear lier work.

The clustering of galaxies as a function of their photometrically estimated atomic gas content

We introduce a new photometric estimator of the H i mass fraction () in local galaxies, which is a linear combination of four parameters: stellar mass, stellar surface mass density, NUV-r colour and g-i colour gradient. It is calibrated using samples of nearby galaxies (0.025 < z < 0.05) with H i line detections from the GALEX Arecibo SDSS Survey (GASS) and Arecibo Legacy Fast ALFA (ALFALFA) surveys, and it is demonstrated to provide unbiased estimates even for H i-rich galaxies. We apply this estimator to a sample of similar to 24 000 galaxies from the Sloan Digital Sky Survey (SDSS)/Data Release 7 (DR7) in the same redshift range. We then bin these galaxies by stellar mass and H i mass fraction and compute projected two-point cross-correlation functions with respect to a reference galaxy sample. Results are compared with predictions from current semi-analytic models of galaxy formation. The agreement is good for galaxies with stellar masses larger than 1010 M?, but not for lower mass systems. We then extend the analysis by studying the bias in the clustering of H i-poor or H i-rich galaxies with respect to galaxies with normal H i content on scales between 100 kpc and similar to 5 Mpc. For the H i-deficient population, the strongest bias effects arise when the H i deficiency is defined in comparison to galaxies of the same stellar mass and size. This is not reproduced by the semi-analytic models, where the quenching of star formation in satellites occurs by starvation and does not depend on their internal structure. H i-rich galaxies with masses greater than 1010 M? are found to be antibiased compared to galaxies with normal H i content. Interestingly, no such effect is found for lower mass galaxies.

The GALEX Arecibo SDSS Survey IV: baryonic mass-velocity-size relations of massive galaxies

We present dynamical scaling relations for a homogeneous and representative sample of �500 massive galaxies, selected only by stellar mass (> 10 10 M⊙) 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 (for the BTF, only galaxies with Hi detections), 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 compare the velocity-size relation for the three dynamical indicators used in this work, i.e., rotational velocity, observed and concentration-corrected stellar dispersion. 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.

Hα3: an Hα imaging survey of HI selected galaxies from ALFALFA - I. Catalogue in the Local Supercluster

Context. We present Hα3 (acronym for Hα − αα), an Hα narrow-band imaging survey of ∼400 galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Local Supercluster, including the Virgo cluster. Aims. By using hydrogen recombination lines as a tracer of recent star formation, we aim to investigate the relationships between atomic neutral gas and newly formed stars in different environments (cluster and field), morphological types (spirals and dwarfs), and over a wide range of stellar masses (∼10 7.5 −10 11.5 M� ).

Gas-Bearing Early-Type Dwarf Galaxies in Virgo: Evidence for Recent Accretion

We investigate the dwarf (M_B> -16) galaxies in the Virgo cluster in the radio, optical, and ultraviolet regimes. Of the 365 galaxies in this sample, 80 have been detected in HI by the Arecibo Legacy Fast ALFA survey. These detections include 12 early-type dwarfs which have HI and stellar masses similar to the cluster dwarf irregulars and BCDs. In this sample of 12, half have star-formation properties similar to late type dwarfs, while the other half are quiescent like typical early-type dwarfs. We also discuss three possible mechanisms for their evolution: that they are infalling field galaxies that have been or are currently being evolved by the cluster, that they are stripped objects whose gas is recycled, and that the observed HI has been recently reaccreted. Evolution by the cluster adequately explains the star-forming half of the sample, but the quiescent class of early-type dwarfs is most consistent with having recently reaccreted their gas.