Non-parametric analysis of the rest-frame UV sizes and morphological disturbance amongst L* galaxies at 4<z<8

TL;DR

This study investigates the sizes and morphologies of high-redshift galaxies (z=4-8) using non-parametric methods, finding little evidence for size or morphological evolution within observational uncertainties.

Contribution

It introduces a non-parametric approach to measure galaxy sizes and disturbances, accounting for biases, and provides new constraints on size and morphological evolution at high redshift.

Findings

Galaxy sizes show no significant evolution from z=4 to 8 within uncertainties.

The disturbed galaxy fraction remains consistent across redshifts 4 to 8.

Size-dependent biases affect the interpretation of high-redshift galaxy size evolution.

Abstract

We present the results of a study investigating the sizes and morphologies of redshift 4 < z < 8 galaxies in the CANDELS GOODS-S, HUDF and HUDF parallel fields. Based on non-parametric measurements and incorporating a careful treatment of measurement biases, we quantify the typical size of galaxies at each redshift as the peak of the log-normal size distribution, rather than the arithmetic mean size. Parameterizing the evolution of galaxy half-light radius as $r_{50} \propto (1+z)^n$, we find $n = -0.20 \pm 0.26$ at bright UV-luminosities ($0.3L_{*(z=3)} < L < L_*$) and $n = -0.47 \pm 0.62$ at faint luminosities ($0.12L_* < L < 0.3L_*$). Furthermore, simulations based on artificially redshifting our z~4 galaxy sample show that we cannot reject the null hypothesis of no size evolution. We show that this result is caused by a combination of the size-dependent completeness of high-redshift galaxy samples and the underestimation of the sizes of the largest galaxies at a given epoch. To explore the evolution of galaxy morphology we first compare asymmetry measurements to those from a large sample of simulated single S\'ersic profiles, in order to robustly categorise galaxies as either `smooth' or `disturbed'. Comparing the disturbed fraction amongst bright ($M_{UV} \leq -20$) galaxies at each redshift to that obtained by artificially redshifting our z~4 galaxy sample, while carefully matching the size and UV-luminosity distributions, we find no clear evidence for evolution in galaxy morphology over the redshift interval 4 < z < 8. Therefore, based on our results, a bright ($M_{UV} \leq -20$) galaxy at z~6 is no more likely to be measured as `disturbed' than a comparable galaxy at z~4, given the current observational constraints.