Galaxy surface brightness and size evolution to z~4

Abstract

Using HST WFPC2 data, we estimate half-light radii, morphological classifications and rest-frame, blue-band absolute magnitudes for 270 galaxies with spectroscopic redshifts from z~0 to z=3.43, and 77 thought from their colours to be at 2.0<=z<=4.5. The mean blue-band surface brightness decreases by ~2 mag along the Hubble sequence from ellipticals to irregulars, but peculiar galaxies (forming ~21 per cent of the sample) tend to have blue colours but a high surface brightness. The mean rest-frame, blue-band surface brightness increases by 0.95+/-0.22 mag between z_mean~=0.2 and z_mean~=0.9 redshift intervals, with similar evolution for all the morphological types. We estimate that galaxies at 2<z<3.5 show 2.79+/-0.31 mag of surface brightness evolution relative to those at z<0.35, which is significantly greater than the luminosity evolution over this redshift range. This can be explained by a size and luminosity evolution (SLE) model, in which the outer regions of spiral galaxies form later and with a longer time-scale than the centres, causing the half-light radius to increase with time. The blue-band luminosity density increases by a factor of ~2.5 from z~0 to z~2.85, but falls back to approximately its zero-redshift value at z~4. Galaxies at z~4 appear to be similar in surface brightness to those at z~3, but smaller in size and lower in luminosity. The angular size distributions of 22<I<26 galaxies in the Hubble Deep Field, divided at z~1.5 using UBI colours, suggest that most size evolution occurs at z>1.5. A small sample of galaxies with I-K>3.8 (passively evolving galaxies at z>1) appeared to be no smaller than the PLE prediction, suggesting that it is primarily star-forming galaxies that evolve in size.