Resolved H I Imaging of a Population of Massive H I-Rich Galaxies With Suppressed Star Formation

Abstract

Despite the existence of well-defined relationships between cold gas and star formation, there is evidence that some galaxies contain large amounts of HI that do not form stars efficiently. By systematically assessing the link between HI and star formation within a sample of galaxies with extremely high HI masses (log M HI /M ⊙ > 10), we uncover a population of galaxies with an unexpected combination of high HI masses and low specific star formation rates that exists primarily at stellar masses greater than log M * /M ⊙ ∼ 10.5. We obtained HI maps of 20 galaxies in this population to understand the distribution of the HI and the physical conditions in the galaxies that could be suppressing star formation in the presence of large quantities of HI. We find that all of the galaxies we observed have low HI surface densities in the range in which inefficient star formation is common. The low HI surface densities are likely the main cause of the low sSFRs, but there is also some evidence that AGN or bulges contribute to the suppression of star formation. The sample's agreement with the global star formation law highlights its usefulness as a tool for understanding galaxies that do not always follow expected relationships. Subject headings: galaxies: formation -galaxies: evolution 1. INTRODUCTION Many theories of galaxy evolution assume that a galaxy's evolutionary stage and its cold gas content are tightly linked. For example, it is often assumed that a depleted cold gas reservoir can explain why massive galaxies are redder and exhibit less active star formation, but recent observations have challenged this picture by identifying populations of galaxies that contain significant amounts of cold gas despite their low or moderate star formation rates (Morganti et al. 2006;Serra et al. 2012;Young et al. 2013). Some attempts have been made to systematically search for these galaxies to understand them as a population. In this paper we pursue this line of reasoning by studying a sample of massive galaxies with an unusual combination of high HI masses and low specific star formation rates.Galaxies that have more cold gas than expected can be placed into two broad categories: red, early-type galaxies with unexpected measurable HI and galaxies that have significantly more HI than their star formation rates (SFRs) would suggest. In the first category, early-type galaxies were expected to be devoid of cold gas because they lack the spiral arms that indicate recent star formation. The first systematic imaging search for HI in early-type galaxies, motivated in part by the evidence for HI in individual early-types, found that HI is common at low levels in early-type galaxies in the field and could play an important role in their evolution (Morganti et al.

GASS 3505: the prototype of H i-excess, passive galaxies

“…Star-forming regions (where the FUV emission is above 3σ) are marked by red dots, and non-star-forming regions (3σ FUV upper limit) are shown as black triangles. We also show the Bigiel et al (2010) and Lemonias et al (2014) samples in grey and green points (respectively) as a comparison. Constant gas depletion timescales of 10 9 − 10 12 yr are marked by black dashed lines.…”

“…In a study by Lemonias et al (2014), the GASS and ALFALFA samples are used to select H I-rich galaxies with gas fractions in the top 5% of the GASS distribution. These galaxies have high gas fractions for their stellar masses, and are H I-massive with M HI > 10 10 M .…”

“…These galaxies have high gas fractions for their stellar masses, and are H I-massive with M HI > 10 10 M . Within this sample, Lemonias et al (2014) identify a population where high H I masses are associated with unexpectedly low specific SFR-s. They show that low surface density gas is likely the reason behind the inefficient SF in these galaxies.…”

“…The Bluedisk and Lemonias et al (2014) sample are massive galaxies with M > 10 10 M like GASS 3505, and for consistency we only consider HIghMass galaxies which meet this high stellar mass selection criteria. The NUV −r color information for Bluedisk and the Lemonias et al (2014) galaxies are taken from their published tables. For the HIghMass sample we extract the NUV magnitudes from GALEX and correct the NUV − r color for Galactic extinction following Wyder et al (2007).…”

Quenching and morphological transformation in semi-analytic models and CANDELS

“…In the models considered here, only gas that is above a critical surface density is allowed to participate in star formation; galaxies with larger than average angular momentum form more extended disks, which have a larger fraction of their gas sitting below this critical surface density. There is observational evidence in the local universe for these gas rich but relatively quiescent disks (Lemonias et al 2014;Schiminovich et al 2010). Thus while the SFSs may be understood as a transient population, a step in the path from SFD to QS, it seems that QDs may be a static population.…”