Abstract
Optical variability in galaxies at high redshift is a tracer of evolution in AGN activity, and should provide a useful constraint on models of galaxy evolution, AGN structure, and cosmology. We studied optical variability in multiple deep CCD and photographic surveys of blank fields for galaxies with $B_j = 20 - 25$ mag. Weakly variable objects are far more common than strongly variable ones. For objects near $B_j = 22$, $0.74\% \pm 0.2 \%$ vary by 0.026~mag RMS or more, over a decade. This is small compared with previous claims based on photographic surveys, and also small compared with the fraction of bright quasars ($\approx 5\%$ at $B_j = 20$~mag) or Seyferts ($\approx 1-2\%$ for $B_j < 18$). The fraction of objects that vary increases slowly with magnitude. Detection probabilities and error rates were checked by simulations and statistical analysis of fluctuations of sample sky spots.
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