Angular sizes of the faint blue galaxies

Abstract

We investigate the angular sizes, morphologies and B-1 colours of the 21 :::;; I :::;; 24 galaxies imaged on 10 Wide Field Planetary Camera fields. The number of galaxies with bluer colours (B - 1< 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-1 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 < 1< 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 atz<O.5. At least 16 of these very blue galaxies appear to be interacting, but we find no overall correlation between B-1 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 ~ 25 per 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.