Linking Gas Fractions to Bimodalities in Galaxy Properties

As an extension of known correlations, galaxies over four decades in stellar mass are shown to obey a strong correlation between u − K colors and gas-to-stellar mass (G/S) ratios, using neutral H I gas masses and stellar mass-to-light ratios derived from optical colors. The correlation holds for G/S ratios 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. In this paper 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 stellar mass and optical color. The recently identified transition in galaxy properties at stellar masses ∼ 2–3×1010 M⊙ corresponds to a shift in gas richness, which divides low-mass late-type galaxies with G/S ∼ 1:1 from high-mass galaxies with intermediate to low gas fractions. Early-type galaxies below the transition mass also show elevated gas fractions, consistent with formation scenarios involving mergers of low-mass gas-rich systems and/or cold-mode gas accretion. Subject headings: galaxies: evolution