2012ApJ...760..131C

The shutdown of star formation in galaxies is generally termed "quenching." Quenching may occur through a variety of processes, e.g., active galactic nucleus (AGN) feedback, stellar feedback, or the shock heating of gas in the dark matter halo. However, which mechanism(s) is, in fact, responsible for quenching is still in question. This paper addresses quenching by searching for traces of possible quenching processes through their effects on galaxy structural parameters such as stellar mass (M_*), M_*/r_e, surface stellar mass density (∼M_*/r ²_e), and Sérsic index (n). We analyze the rest-frame U - B color correlations versus these structural parameters using a sample of galaxies in the redshift range 0.5 ≤ z < 0.8 from the DEEP2/AEGIS survey. In addition to global radii, stellar masses, and Sérsic parameters, we also use "bulge" and "disk" photometric measurements from GIM2D fits to HST/ACS V and I images. We assess the tightness of the color relationships by measuring their "overlap regions," defined as the area in color-parameter space in which red and blue galaxies overlap; the parameter that minimizes these overlap regions is considered to be the most effective color discriminator. We find that Sérsic index (n) has the smallest overlap region among all tested parameters and resembles a step function with a threshold value of n = 2.3. There exists, however, a significant population of outliers with blue colors yet high n values that seem to contradict this behavior; they make up ~40% of n > 2.3 galaxies. We hypothesize that their Sérsic values may be distorted by bursts of star formation, AGNs, and/or poor fits, leading us to consider central surface stellar mass density, Σ*_1 kpc_, as an alternative to Sérsic index. Not only does Σ*_1 kpc_correct the outliers, but it also forms a tight relationship with color, suggesting that the innermost structure of galaxies is most physically linked with quenching. Furthermore, at z ∼ 0.65, the majority of the blue cloud galaxies cannot simply fade onto the red sequence since their GIM2D bulge masses are only half as large on average as the bulge masses of similar red sequence galaxies, thus demonstrating that stellar mass must absolutely increase at the centers of galaxies as they quench. We discuss a two-stage model for quenching in which galaxy star formation rates are controlled by their dark halos while they are still in the blue cloud and a second quenching process sets in later, associated with the central stellar mass buildup. The mass buildup is naturally explained by any non-axisymmetric features in the potential, such as those induced by mergers and/or disk instabilities. However, the identity of the second quenching agent is still unknown. We have placed our data catalog online.