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 ($\varepsilon$)
and disky/boxy parameter A$_4$ out to radii of about three times the semi-major
axes for $\sim4,600$ star-forming galaxies (SFGs) at intermediate redshifts
$0.5<z<1.8$ in the CANDELS/GOODS-S and UDS fields. Based on the average size
versus stellar-mass relation in each redshift bin, we divide our galaxies into
Small SFGs (SSFGs), i.e., smaller than average for its mass, and Large SFGs
(LSFGs), i.e., larger than average. We find that, at low masses ($M_{\ast} <
10^{10}M_{\odot}$), the SSFGs generally have nearly flat $\varepsilon$ and
A$_4$ profiles for both edge-on and face-on views, especially at redshifts
$z>1$. 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 $\varepsilon$ with radius and are
dominated by disky values at intermediate radii. These findings at intermediate
redshifts imply that low-mass SSFGs are not disk-like, while low-mass LSFGs
appear to harbour disk-like components flattened by significant rotation. At
high masses ($M_{\ast} > 10^{10}M_{\odot}$), highly-inclined SSFGs and LSFGs
both exhibit a general, distinct trend for both $\varepsilon$ and A$_4$
profiles: increasing values with radius at lower radii, reaching maxima at
intermediate radii, and then decreasing values at larger radii. Such a trend is
more prevalent for more massive ($M_{\ast} > 10^{10.5}M_{\odot}$) galaxies or
those at lower redshifts ($z<1.4$). The distinct trend in $\varepsilon$ and
A$_4$ 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.
