2003ApJ...599..173S

We present the results of a two-epoch variability survey in the Hubble Deep Field with the goal of investigating the population of active galactic nuclei (AGNs) to z≃1. The high resolution and stability of the Hubble Space Telescope allow accurate photometry to be obtained within subarcsecond apertures, resulting in the ability to probe much lower AGN/host galaxy luminosity ratios than can be done from the ground. The primary data sets analyzed for galactic variability are the original Hubble Deep Field observations obtained in 1995 December and a second V-band (F606W) image obtained almost exactly 5 yr later in 2000. We find evidence for nuclear variability in 16 of 217 galaxies brighter than V_nuc=27.5. Correcting for incompleteness and spurious detections, variable nuclei make up ∼8% of the surveyed galaxies. These sources have a redshift range of 0.09<z<1.8 and cover the full range of galaxy nuclear V-I colors. Seven of our variable sources are coincident with X-ray sources detected in the 2 Ms Chandra survey, six from the main catalog and one from the supplementary catalog. We find that 44% of the variable nuclei are associated with mid-IR detections at 15 µm, and 31% are detected at 1.4 GHz. Optical spectra are available for 13 of the 16 variables. One is a broad-line AGN, and two others show weak evidence of type 2 AGNs. With the assumption that these variables are all active nuclei, we estimate the AGN luminosity function (LF) at 0.4<z<1.1 extending to M_B≃-15. We find evidence for an increase in the number density of faint AGNs when comparing to the local Seyfert luminosity function. The LF for optically varying nuclei appears to rise in number density, with no evidence of turning over at these faint magnitudes.