|Annu. Rev. Astron. Astrophys. 2000. 38:
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4.3. Galaxy sizes
One of the interesting and somewhat unexpected findings of HST faint-galaxy surveys has been the small angular diameters of faint galaxies. The sky is peppered with compact high-surface-brightness objects, in contrast to the expectation from M = tot = 1 pure-luminosity evolution (PLE) models (Section 5.1) and models dominated by low-surface-brightness (LSB) galaxies [Ferguson & McGaugh 1995].
Galaxy radii can be measured using image moments (which are subject to severe biases because of isophotal thresholds), growth curves, or profile fitting. [Ferguson 1998b] considered the size distribution derived from image moments, whereas [Roche et al. 1998] discuss the distribution of half-light radii derived from profile fitting. [Roche et al. 1998] find no evidence for evolution in the rest-frame sizes or surface brightnesses of normal spirals and ellipticals out to z = 0.35. At higher redshift they find evidence for strong evolution; galaxies are more compact and of higher surface brightness than expected from a PLE model (see also [Schade et al. 1995b]). The sizes of disk galaxies were shown to be reasonably well matched by a size-luminosity evolution (SLE) model wherein spirals form stars gradually from the inside out. In a study with similar goals, [Simard et al. 1999] analyzed a sample of 190 (non-HDF) field galaxies with spectroscopic redshifts and HST images. They find no evidence for size or luminosity evolution of disk-dominated galaxies out to z ~ 1. The [Simard et al. 1999] analysis includes a more comprehensive treatment of selection biases and an empirical comparison to local samples that makes the results quite compelling. However, [Simard et al. 1999] do identify nine high-surface-brightness objects in their sample at z > 0.9. If these were included as disk galaxies, the results would be closer to those of [Roche et al. 1998]. A comparison of the bivariate distribution of the [Simard et al. 1999] luminosities and scale lengths to a z ~ 0 sample suggests only moderate density evolution out to z ~ 1.2, with a decrease in the number density of bright, large-scale-size galaxies at higher redshift [de Jong & Lacey 1999]. Galaxies in the HDF with spectroscopic or photometric redshifts greater than z = 2 generally appear to be more compact than present-day L* galaxies [Lowenthal et al. 1997, Roche et al. 1998].
At magnitudes fainter than I = 25, morphological classification and profile fitting become quite difficult because most galaxies at these faint magnitudes are so small that even with HST there are few independent resolution elements within the area detected above the background. [Ferguson 1998b] studied the distribution of first-moment radii and modeled the selection boundaries of the HDF in size and magnitude. Because the first-moment radii are measured above a fixed isophote, the interpretation of the observed trends is highly model dependent, and is sensitive to the assumed redshift distribution and morphological-type distribibution of the galaxies. Nevertheless, down to I ~ 27 the HDF counts are clearly dominated by galaxies more compact than local L* spirals. At fainter magnitudes, the locus of HDF galaxy sizes is tightly constrained by selection, and there is very little information on the intrinsic size distribution of the objects.