Angular sizes and luminosity evolution of faint galaxies

Abstract

We investigate the number counts and the V-I colour, angular size and axis ratio distributions of 5384 faint galaxies detected with the refurbished Hubble Space Telescope (HST) Wide Field Planetary Camera, to limits of I~=25 on a 27-field strip and I~=26 on a single deeper field. These results are compared with non-evolving and pure luminosity evolution (PLE) models, with a steep (alpha=-1.65) luminosity function for late-type galaxies. In these models, we have aimed to incorporate surface brightness selection effects, which can produce a significant bias against detection of higher redshift galaxies with larger angular sizes. We find no deficit of large-angular-size galaxies relative to our non-evolving model. At I<~22 there is a significant (approximately factor of 2) excess of moderately large (0.4<=r_hl<=1.5 arcsec at 22<=I<=24) galaxies with blue (V-I<=1.2) colours over our non-evolving model. The observed angular size and colour distributions are reasonably well-fitted by our PLE model, when detection thresholds appropriate to the data are included. These results appear to provide positive evidence that L~L* galaxies do brighten significantly (by ~1 mag) to z~1-2, increasing the numbers of blue, large-angular-size, high-redshift galaxies seen at I<~22, and that L* evolution, in addition to a steep luminosity function, is needed to explain the steep galaxy number counts. We also investigate faint galaxy axis ratios. At 20<=I<=24 the smaller galaxies tended to be more rounded than galaxies of large angular size, suggesting that a greater proportion are ellipticals. On going faintward, however, the axis ratio distribution of the smaller galaxies appears to shift towards that of a spiral-dominated population, as expected if later-type galaxies have a much steeper luminosity function than ellipticals. The axis ratio distribution of larger-angular-size galaxies remains consistent with a spiral-dominated population over the whole 20<=I<=25 range, suggesting that most of the evolving high-redshift galaxies on these fields are spirals and that the high-redshift ellipticals are significantly dimmed by dust extinction.