The Dependence of Star Formation History and Internal Structure on Stellar Mass for 80,000 Low-Redshift Galaxies

We study the relations between stellar mass, star formation history, size and internal structure for a complete sample of 80,000 galaxies drawn from the Sloan Digital Sky Survey. We show that low-redshift galaxies divide into two distinct families at a stellar mass of 3×10M⊙. Lower mass galaxies have young stellar populations, star formation histories characterized by recent bursts, low surface mass densities, and the low concentrations typical of disks. At given stellar mass their sizes are log-normally distributed with dispersion σ(lnR50) ∼ 0.5, in excellent agreement with the idea that they form with little angular momentum loss through cooling and condensation in a gravitationally dominant dark matter halo (Fall & Efstathiou 1980). Their median stellar surface mass density scales with stellar mass as μ∗ ∝ M0.6 ∗ , suggesting that the stellar mass of a disk galaxy is proportional to the five thirds power of its halo mass. The galaxies with the lowest stellar surface densities most frequently exhibit the spectral signature of a recent starburst. All this points to a picture in which the star formation efficiency increases in proportion to halo mass, perhaps as a result of supernova feedback processes. At stellar masses above 3 × 1010M⊙, there is a rapidly increasing fraction of galaxies with old stellar populations, high surface mass densities and the high concentrations typical of bulges. In this regime, the size distribution remains log-normal, but its dispersion decreases rapidly with increasing stellar mass and the median mass surface density increases more slowly (∝ M0.2 ∗ ). This suggests that the star formation efficiency has saturated, and that little star formation occurs in massive galaxies after they have assembled.