CONNECTING TRANSITIONS IN GALAXY PROPERTIES TO REFUELING

Abstract

We relate transitions in galaxy structure and gas content to refueling, here\ndefined to include both the external gas accretion and the internal gas\nprocessing needed to renew reservoirs for star formation. We analyze two z=0\ndata sets: a high-quality ~200-galaxy sample (the Nearby Field Galaxy Survey,\ndata release herein) and a volume-limited ~3000-galaxy sample with reprocessed\narchival data. Both reach down to baryonic masses ~10^9Msun and span\nvoid-to-cluster environments. Two mass-dependent transitions are evident: (i)\nbelow the "gas-richness threshold" scale (V~125km/s), gas-dominated\nquasi-bulgeless Sd--Im galaxies become numerically dominant, while (ii) above\nthe "bimodality" scale (V~200km/s), gas-starved E/S0s become the norm.\nNotwithstanding these transitions, galaxy mass (or V as its proxy) is a poor\npredictor of gas-to-stellar mass ratio M_gas/M_*. Instead, M_gas/M_* correlates\nwell with the ratio of a galaxy's stellar mass formed in the last Gyr to its\npreexisting stellar mass, such that the two ratios have numerically similar\nvalues. This striking correspondence between past-averaged star formation and\ncurrent gas richness implies routine refueling of star-forming galaxies on Gyr\ntimescales. We argue that this refueling underlies the tight M_gas/M_* vs.\ncolor correlations often used to measure "photometric gas fractions."\nFurthermore, the threshold and bimodality scale transitions reflect\nmass-dependent demographic shifts between three refueling regimes --- accretion\ndominated, processing dominated, and quenched. In this picture, gas-dominated\ndwarfs are explained not by inefficient star formation but by overwhelming gas\naccretion, which fuels stellar mass doubling in <~1Gyr. Moreover, moderately\ngas-rich bulged disks such as the Milky Way are transitional, becoming abundant\nonly in the narrow range between the threshold and bimodality scales.\n