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

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 ($0.3L_{\ast (z=3)}<L<L_{\ast }$) and $n=-0.47\pm 0.62$ at faint luminosities ($0.12L_{\ast }<L<0.3L_{\ast }$). 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 ($M_{UV}\le -20$) 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.