GALAXY STRUCTURE AS A DRIVER of the STAR FORMATION SEQUENCE SLOPE and SCATTER

It is well established that (1) star-forming galaxies follow a relation between their star formation rate (SFR) and stellar mass (M), the "star formation sequence," and (2) the SFRs of galaxies correlate with their structure, where star-forming galaxies are less concentrated than quiescent galaxies at fixed mass. Here, we consider whether the scatter and slope of the star formation sequence is correlated with systematic variations in the Sérsic indices, n, of galaxies across the SFRM plane. We use a mass-complete sample of 23,848 galaxies at 0.5 < z < 2.5 selected from the 3D-HST photometric catalogs. Galaxy light profiles parameterized by n are based on Hubble Space Telescope Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey near-infrared imaging. We use a single SFR indicator empirically calibrated from stacks of Spitzer/MIPS 24 μm imaging, adding the unobscured and obscured star formation. We find that the scatter of the star formation sequence is related in part to galaxy structure; the scatter due to variations in n at fixed mass for star-forming galaxies ranges from 0.14 ± 0.02 dex at z ∼ 2 to 0.30 ±0.04 dex at z <1. While the slope of the log SFR log M relation is of order unity for disk-like galaxies, galaxies with n >2 (implying more dominant bulges) have significantly lower SFR M than the main ridgeline of the star formation sequence. These results suggest that bulges in massive z ∼ 2 galaxies are actively building up, where the stars in the central concentration are relatively young. At z < 1, the presence of older bulges within star-forming galaxies lowers global SFR M, decreasing the slopeand contributing significantly to the scatter of the star formation sequence.