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 correlation 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 ~35,000 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 masses ~(2-3) × 10¹⁰ M_☉ corresponds to a shift in gas richness, dividing low-mass late-type galaxies with G/S ~ 1 : 1 from high-mass galaxies with intermediate-to-low G/S. 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.