Structure and Star Formation in Galaxies out to<i>z</i>= 3: Evidence for Surface Density Dependent Evolution and Upsizing

Abstract

We present an analysis of galaxies in the CDF-South. We find a tight relation to z = 3 between color and size at a given mass, with red galaxies being small, and blue galaxies being large. We show that the relation is driven by stellar surface density or inferred velocity dispersion: galaxies with high surface density are red and have low specific star formation rates, and galaxies with low surface density are blue and have high specific star formation rates. Surface density and inferred velocity dispersion are better correlated with specific star formation rate and color than stellar mass. Hence stellar mass by itself is not a good predictor of the star formation history of galaxies. In general, galaxies at a given surface density have higher specific star formation rates at higher redshift. Specifically, galaxies with a surface density of (1-3) × 109 M⊙ kpc-2 are "red and dead" at low redshift, approximately 50% are forming stars at z = 1, and almost all are forming stars by z = 2. This provides direct additional evidence for the late evolution of galaxies onto the red sequence. The sizes of galaxies at a given mass evolve like 1/(1 + Z)0.59±0.10, Hence galaxies undergo significant upsizing in their history. The size evolution is fastest for the highest mass galaxies and quiescent galaxies. The persistence of the structural relations from z = 0 to z = 2.5, and the upsizing of galaxies imply that a relation analogous to the Hubble sequence exists out to z = 2.5, and possibly beyond. The star-forming galaxies at z ≥ 1.5 are quite different from star-forming galaxies at z = 0, as they have likely very high gas fractions, and star formation timescales comparable to the orbital time.