The axis ratio distribution of local and distant galaxies

Surface photometry from 16 HST/WFPC2 fields in the I-814-band filter is used to derive the distribution of apparent axis ratios for galaxies in progressively fainter magnitude intervals for I-814 less than or equal to 25. We assess the systematic and accidental errors in ellipticity measurements as a function of image resolution and signal-to-noise ratio, and statistically correct for the effect of cosmological surface brightness dimming on our isophotal measurements. The axis ratio distribution for the local galaxy population was computed using log R-25 measurements for 1569 RC3 galaxies with B-T less than or equal to 13 mag. Nonparametric tests are used to show that our distant samples, in the redshift range 0.1 less than or similar to z less than or similar to 1.0-1.5, are not statistically different from the local sample. We present image montages of galaxies selected randomly from different axis ratio and apparent magnitude ranges and discuss the evolutionary consequences of the lack of a strong difference between the ellipticity distributions in near and far data sets. (C) 1997 American Astronomical Society. [S0004-6256(97)00312-9].