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

TL;DR

This study analyzes the isophotal shapes of star-forming galaxies at intermediate redshifts, revealing different structural components and their evolution with mass and redshift, which informs galaxy formation models.

Contribution

It provides the first detailed analysis of isophotal ellipticity and A4 profiles for a large sample of star-forming galaxies at $0.5<z<1.8$, highlighting structural differences based on mass and inclination.

Findings

Low-mass SSFGs are not disk-like at intermediate redshifts.

Low-mass LSFGs show disk-like components with rotation.

High-mass galaxies exhibit combined bulge, disk, and halo structures.

Abstract

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.