Linking gas fractions to bimodalities in galaxy properties

Galaxies over 4 decades in stellar mass are shown to obey a strong correlation between u-K colors and atomic-gas-to-stellar mass ratios (G/S), using stellar mass-to-light ratios derived from optical colors. The cor relation holds for G/S ranging from nearly 10:1 to 1:100 for a sample obtained by merging the SDSS DR2, 2MASS, and HyperLeda H I catalogs. This result implies that u-K colors can be calibrated to provide "photometric gas fractions" for statistical applications. Here this technique is applied to a sample of similar to35,000 SDSS-2MASS galaxies to examine the relationship of gas fractions to observed bimodalities in galaxy properties as a function of color and stellar mass. The recently identified transition in galaxy properties at stellar massessimilar to(2-3)x10(10) M-circle dot, corresponds to a shift in gas richness, dividing low-mass late-type galaxies with G/Ssimilar to1:1 from high-mass galaxies with intermediate-to-low. Early-type galaxies below the transition mass also show elevated G/S, consistent with formation scenarios involving mergers of low-mass gas-rich systems and/or cold-mode gas accretion.