The structures of distant galaxies – II. Diverse galaxy structures and local environments at z = 4–6; implications for early galaxy assembly

Abstract

We present an analysis of the structures, sizes, star formation rates and local environmental properties of galaxies at z ~ 4–6 (τ_(universe) < 2 Gyr ), utilizing deep Hubble Space Telescope imaging of the Hubble Ultra Deep Field. The galaxies we study are selected with the Lyman-break drop-out technique, using galaxies which are B-, V- and i-drops, which effectively selects ultraviolet (UV) bright starbursting galaxies between z = 4 and 6. Our primary observational finding is that starbursting galaxies at z > 4 have a diversity in structure, with roughly 30 per cent appearing distorted and asymmetric, while the majority are smooth and apparently undisturbed systems. We utilize several methods to compute the inferred assembly rates for these distorted early galaxies including the CAS (concentration, asymmetry, clumpiness) system and pair counts. Overall, we find a similar fraction of galaxies which are in pairs as the fraction which have a distorted structure. Using the CAS methodology, and our best estimate for merger time-scales, we find that the total number of inferred effective mergers for M_* > 10^(9-10) M_⊙ galaxies at z < 6 is N_m = 4.2^(+4.1)_(−1.4). The more common symmetrical systems display a remarkable scaling relation between the concentration of light and their half-light radii, revealing the earliest known galaxy scaling relationship, and demonstrating that some galaxies at z > 4 are likely in a relaxed state. Systems which are asymmetric do not display a correlation between size and half-light radii, and are generally larger than the symmetric smooth systems. The time-scale for the formation of these smooth systems is 0.5–1 Gyr, suggesting that most of these galaxies are formed through coordinated very rapid gas collapses and star formation over a size of 1–2 kpc, or from merger events at z > 10 . We finally investigate the relation between the UV measured star formation rates for these galaxies and their structures, finding a slight correlation such that more asymmetric systems have slightly higher star formation rates than symmetric galaxies.