GASS Survey Status

GASS sample

The GASS targets for Arecibo observations are selected according to the following criteria: Since the ALFALFA and GALEX surveys were on-going, we defined a GASS parent sample (based on SDSS DR6 and the maximal ALFALFA footprint) from which the targets were extracted. The final GASS sample includes ~800 galaxies, chosen by randomly selecting a subset which balanced the distribution across stellar mass and RA and which maximized existing GALEX exposure time. The GASS parent sample includes ~10000 galaxies that satisfy all of the above criteria except that the UV photometry comes from either the GALEX All-sky Survey (AIS; ~100s; FUVlim, NUVlim < 21) or the MIS (~1500s; FUVlim, NUVlim < 23).

Sky distribution of galaxies in the GASS parent sample (red). Blue symbols indicate the subset of
GASS galaxies that have been detected by ALFALFA in the Spring sky area cataloged to date.

Observing status

GASS observations started in March 2008 and are completed. A total of 1005 hours of telescope time have been allocated to the project. All observations until end of April 2009 (~170 hours) were carried out remotely from the MPA, Garching. New observers, trained on site in May 2009, have routinely carried out remote observations from MPA, Columbia, JHU and NYU. The observing team included: Barbara Catinella (lead, MPA), Silvia Fabello and Andrew Cooper (MPA), Jenna Lemonias and Cameron Hummels (Columbia), Sean Moran (JHU) and Ronin Wu (Paris). There have been three data releases (see Publications). The combined catalogs and the HI spectra are available on our Data page.

Highlights from first results

Catinella et al. (2010; GP1) presented the initial GASS data release (DR1; ~20% of complete survey) and a first exploration of HI-mass fraction scaling relations as a function of stellar mass, NUV-r color, stellar mass surface density and concentration (a proxy for bulge-to-disk ratio). Contrary to naive expectations, GP1 found that the majority of massive galaxies are not gas-deficient. Roughly 60% of the sample is detected above the gas fraction limit, with a high detection fraction even in the highest stellar mass bins. Despite high detection rates, gas fraction measurements show that the HI-mass fraction is anti-correlated with stellar mass (as shown below), color and stellar mass surface density, with a considerably weaker anti-correlation vs. concentration.
Schiminovich et al. (2010; GP2) measured the mean SFR efficiencies (SFE) across the GASS sample and used this to search for highly efficient or inefficient star-forming galaxies. While the SFE has been discussed extensively in the literature, this work marks the first time that SFEs have been directly measured in a volume-limited sample of massive galaxies. GP2 derived new scaling relations associated with SFR in massive galaxies (shown also below). Unlike the mean sSFR, which decreases with stellar mass, the mean SFE remains nearly constant across the sample with a value close to SFE = 10-9.5 yr-1 (or an equivalent gas consumption timescale of ~3 Gyr).
By combining GASS measurements with recent estimates of the local stellar mass function (Borch et al. 2006), GP2 was also able to obtain a census of the HI mass density and SFR density in massive galaxies. GP2 determined that 36% of the total HI mass density and 47% of the total SFR density is found in galaxies with Mstar > 1010 Msun. In other words, a significant fraction of the HI and SFR in the local universe is found within galaxies in this stellar mass range. These results are consistent with the fact that our sample, while restricted to relatively massive galaxies, includes many galaxies that are blue and/or not quiescent, in contrast with samples selected by color or early-type morphology, the latter of which are known to have low gas fractions and low star formation rates.

Gallery of GASS scaling relations vs. stellar mass, for cold gas in massive galaxies. HI-mass fraction and sSFR are anti-correlated with stellar mass while SFE remains constant. Top: Data points from GASS DR1. Bottom: Mean and median scaling relations. Left: Gas fractions from Catinella et al. (2010). Dashed line indicates GASS detection limit. ALFALFA detections plotted in grey. Middle and Right: sSFR and SFE vs. stellar mass from Schiminovich et al. (2010). Middle top: greyscale from full GALEX+SDSS sample.

Optical follow-up

Sean Moran (P.I.) and others have been pursuing a program of long-slit spectroscopy using the 6.5m MMT and the 3.5m telescope at Apache Point. This optical follow-up of GASS targets aims to relate the HI content to the spatially-resolved recent and ongoing star formation histories of GASS galaxies. Since the first observations in Oct 2008, 120 resolved spectra have been obtained (as of November 2009), out of an eventual goal of 300 galaxies, approximately 1/3 of the GASS sample.

Barbara Catinella
Last modified: Aug 9 2013