Barbara Catinella

THE ARECIBO LEGACY FAST ALFA SURVEY. I. SCIENCE GOALS, SURVEY DESIGN, AND STRATEGY

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map ~7000 deg2 of the high Galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range between -1600 and 18,000 km s-1 with ~5 km s-1 resolution. Exploiting Arecibos large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H I mass function in the local universe and will provide a census of H I in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H I diameter function, and enable a first wide-area blind search for local H I tidal features, H I absorbers at z < 0.06, and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H I line sources out to z ~ 0.06, including several hundred with H I masses M < 107.5 M⊙.

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 VELOCITY FUNCTION IN THE LOCAL ENVIRONMENT FROM ΛCDM AND ΛWDM CONSTRAINED SIMULATIONS

Using constrained simulations of the local universe for generic cold dark matter (CDM) and for 1 keV warm dark matter (WDM), we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function (MF) within 20 h –1 Mpc of the Local Group is ~2 times larger than the universal MF in the 109-1013 h –1 M ☉ mass range. Imposing the field of view of the ongoing H I blind survey Arecibo Legacy Fast ALFA (ALFALFA) in our simulations, we predict that the velocity function (VF) in the Virgo-direction region (VdR) exceeds the universal VF by a factor of 3. Furthermore, employing a scheme to translate the halo VF into a galaxy VF, we compare the simulation results with a sample of galaxies from the early catalog release of ALFALFA. We find that our simulations are able to reproduce the VF in the 80-300 km s-1 velocity range, having a value ~10 times larger than the universal VF in the VdR. In the low-velocity regime, 35-80 km s-1, the WDM simulation reproduces the observed flattening of the VF. In contrast, the simulation with CDM predicts a steep rise in the VF toward lower velocities; for V max = 35 km s-1, it forecasts ~10 times more sources than the ones observed. If confirmed by the complete ALFALFA survey, our results indicate a potential problem for the CDM paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.

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.

Template Rotation Curves for Disk Galaxies

A homogeneous sample of ~2200 low-redshift disk galaxies with both high sensitivity long-slit optical spectroscopy and detailed I-band photometry is used to construct average, or template, rotation curves in separate luminosity classes, spanning six magnitudes in I-band luminosity. The template rotation curves are expressed as functions both of exponential disk scale lengths rd and of optical radii Ropt, and extend out to 4.5rd-6.5rd, depending on the luminosity bin. The two parameterizations yield slightly different results beyond Ropt, because galaxies whose Hα emission can be traced to larger extents in the disks are typically of higher optical surface brightness and are characterized by larger values of Ropt/rd. By either parameterization, these template rotation curves show no convincing evidence of velocity decline within the spatial scales over which they are sampled, even in the case of the most luminous systems. In contrast to some previous expectations, the fastest rotators (most luminous galaxies) have, on average, rotation curves that are flat or mildly rising beyond the optical radius, implying that the dark matter halo makes an important contribution to the kinematics also in these systems. The template rotation curves and the derived functional fits provide quantitative constraints for studies of the structure and evolution of disk galaxies, which aim at reproducing the internal kinematics properties of disks at the present cosmological epoch.

The effect of the environment on the H i scaling relations

We use a volume-, magnitude-limited sample of nearby galaxies to investigate the effect of the environment on the Hi scaling relations. We confirm that the Hi-to-stellar mass ratio anti correlates with stellar mass, stellar mass surface density and NUV r colour across the whole range of parameters covered by our sample (10 9 < �M∗ < �10 11 M⊙, 7.5 < �µ∗ < �9.5 M⊙ kpc −2 , 2 < �NUV r < �6 mag). These scaling relations are also followed by galaxies in the Virgo cluster, although they are significantly offset towards lower gas content. Interestingly, the difference between field and cluster galaxies gradually decreases moving towards massive, bulge-dominated systems. By comparing our data with the predictions of chemo-spectrophotometric models of galaxy evolution, we show that starvation alone cannot explain the low gas content of Virgo spirals and that only ram-pressure stripping is able to reproduce our findings. Finally, motivated by previous studies, we investigate the use of a plane obtained from the relations between the Hito-stellar mass ratio, stellar mass surface density and NUV r colour as a proxy for the Hi deficiency parameter. We show that the distance from the ‘Hi gas fraction plane’ can be used as an alternative estimate for the Hi deficiency, but only if carefully calibrated on pre-defined samples of ‘unperturbed’ systems.

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_⊙.

Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations

We study the properties of the cold gas component of the interstellar medium of the Herschel Reference Survey, a complete volumelimited (15. D. 25 Mpc), K-band-selected sample of galaxies spanning a wide range in morphological type (from ellipticals to dwarf irregulars) and stellar mass (10 9 . Mstar. 10 11 M⊙). The multifrequency data in our hands are used to trace the molecular gas mass distribution and the main scaling relations of the sample, which put strong constraints on galaxy formation simulations. We extend the main scaling relations concerning the total and the molecular gas component determined for massive galaxies (Mstar& 10 10 M⊙) from the COLD GASS survey down to stellar masses Mstar≃ 10 9 M⊙. As scaling variables we use the total stellar mass Mstar, the stellar surface density µstar, the specific star formation rate SSFR, and the metallicity o f the target galaxies. By comparing molecular gas masses determined using a constant or a luminosity dependent XCO conversion factor, we estimate the robustness of these scaling relations on the very uncertain assumptions used to transform CO line intensities into molecular gas masses. The molecular gas distribution of a K-band-selected sample is significantly di fferent from that of a far-infrared-selected sample since it includes a significantly smaller number of objects with M(H2). 6 10 9 M⊙. In spiral galaxies the molecular gas phase is only 2530% of the atomic gas. The analysis also indicates that the slope of the main scaling relations depends on the adopted conversion factor. Among the sampled relations, all those concerning M(gas)/Mstar are statistically significant and show little variation wit h XCO. We observe a significant correlation between M(H2)/Mstar and S S FR, M(H2)/M(Hi) and µstar, M(H2)/M(Hi), and 12+log(O/H) regardless of the adopted XCO. The total and molecular gas consumption timescales are anticorrelated with the specific star formation rate. The comparison of HRS and COLD GASS data indicates that some of the observed scaling relations are nonlinear.