Flickering faint galaxies : few and far between

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 Bj = 20− 25 mag. Weakly variable objects are far more common than strongly variable ones. For objects near Bj = 22, 0.74% ± 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 (≈ 5% at Bj = 20 mag) or Seyferts (≈ 1− 2% for Bj < 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. Subject headings: galaxies: active — galaxies: nuclei —surveys — galaxies: statistics — galaxies: photometry — techniques: image processing