The Isophotal Structure of Star-forming Galaxies at 0.5 < z < 1.8 in CANDELS: Implications for the Evolution of Galaxy Structure

We have measured the radial profiles of isophotal ellipticity ( ε ) and disky/boxy parameter A 4 out to radii of about three times the semimajor axes for ∼4600 star-forming galaxies (SFGs) between redshift 0.5 and 1.8 in the CANDELS/GOODS-S and UDS fields. Based on the average size–mass relation in each redshift bin, we divide our galaxies at a given mass into Small SFGs (SSFGs; smaller than the average) and Large SFGs (LSFGs; larger than the average). We show that, at low masses ( ), the SSFGs generally have nearly flat ε and A 4 profiles in both edge-on and face-on views, especially at . Moreover, the median A 4 values at all radii are almost zero. In contrast, the highly inclined low-mass LSFGs in the same mass-redshift bins generally have monotonically increasing ε profiles with radius and disky feature dominated in the intermediate regions. These findings imply that at these redshifts, the low-mass SSFGs are not disk-like, whereas the low-mass LSFGs likely harbour disk-like components flattened by significant rotations. At high masses ( ), both highly inclined SSFGs and LSFGs generally exhibit distinct trends in both ε and A 4 profiles, which increase at lower radii, reach maxima, then decrease at larger radii. Such the feature is more prevalent for more massive ( ) galaxies or at lower redshifts ( ). This feature can be simply explained if galaxies possess all three components: central bulges, disks in the intermediate regions, and halo-like stellar components in the outskirts.