High-redshift Massive Quiescent Galaxies Are as Flat as Star-forming Galaxies: The Flattening of Galaxies and the Correlation with Structural Properties in CANDELS/3D-HST

We investigate the median flattening of galaxies at 0.2 < z < 4.0 in all five CANDELS/3D-HST fields via the apparent axis ratio, q. We separate the sample into bins of the redshift, stellar mass, Sersic index, size, and UVJ-determined star-forming state to discover the most important drivers of the median q (q(med)). Quiescent galaxies at z < 1 and M-* > 10(11) M-circle dot are rounder than those at lower masses, consistent with the hypothesis that they have grown significantly through dry merging. The massive quiescent galaxies at higher redshifts become flatter and are as flat as star-forming massive galaxies at 2.5 < z < 3.5, consistent with formation through direct transformations or wet mergers. We find that in quiescent galaxies, correlations with q(med) and M-*, and z and r(e) are driven by the evolution in the Sersic index (n), consistent with the growing accumulation of minor mergers at lower redshifts. Interestingly, n does not drive these trends fully in star-forming galaxies. Instead, the strongest predictor of q in star-forming galaxies is the effective radius, where larger galaxies are flatter. Our findings suggest that q(med) is the tracing bulge-to-total ratio, which would explain why smaller/more massive star-forming galaxies are rounder than their extended/less massive analogs, although it is unclear why the Sersic index correlates more weakly with flattening for star-forming galaxies than for quiescent galaxies.