Angular sizes of the faint blue galaxies

Abstract

We investigate the angular sizes, morphologies and B-I colours of the 21≮I≮24 galaxies imaged on 10 Wide Field Planetary Camera fields. The number of galaxies with bluer colours (B-I<2.0) and moderately large angular sizes exceeds non-evolving predictions by a factor of ~2, but is consistent with pure L* evolution (PLE) models. There is also an excess of galaxies of smaller angular size with very blue B-I colours, suggesting the presence of a lower redshift starburst dwarf galaxy population, with a steep luminosity function. The bluest (B-I<1.2) subsample, consisting of 57 galaxies at 21<I<23, contained galaxies with a wide range of angular size and morphology, including spirals, chain galaxies, irregulars and bulge-profile nucleated galaxies. The blue spirals and chain galaxies tend to have the lowest apparent surface brightness, suggesting that they are mostly evolving L* galaxies at higher (z~0.8-2.0) redshifts, while many of the others, with smaller angular sizes, may be starburst dwarf galaxies at z<0.5. At least 16 of these very blue galaxies appear to be interacting, but we find no overall correlation between B-I colour and the presence of a near neighbour. The angular size distribution of the bluest galaxies is peaked at smaller angular sizes than the PLE model predictions, suggesting that the mean half-light radius of spiral galaxies has increased with time by ~25per cent since z~1.2. Blue galaxies of spiral morphology tend to have larger (by ~0.2 dex) half-light radii in the blue band than in the I band, indicating strong radial colour gradients.