Variable Galaxies in the Hubble Deep Field

We present results from a study to detect variable galaxies in the Hubble Deep Field North. The goal of this project is to investigate the number density of AGN at z=1 through the detection of variable galaxy nuclei. The advantage of HST is the ability to do accurate photometry within smaller apertures, thus allowing us to probe much lower AGN/host galaxy luminosity ratios than can be done from the ground. The primary data sets analyzed for galactic variability follow from the original HDF-N observations in December 1995 and a second epoch obtained two years later. We have detected nuclear variability at or above the 3 sigma level in 8 of 633 HDF galaxies at I<27. Only 2 detections would be expected by chance in a normal distribution. At least one of these 8 has been spectroscopically confirmed as a Seyfert 1 galaxy. Based on the AGN structure function for variability, the estimated luminosity of the varying component in each galaxy lies in the range -19.5<M<-15.0. We construct an upper limit to the luminosity function for the variable nuclei and compare this to the local Seyfert LF and the LF for QSOs at z=1. Assuming we have detected all Seyfert-like nuclei in the HDF-N, we find no evidence for an increase in the number density of AGN at M=-19. We estimate that ~1-3% of field galaxies with I<27 may contain a nuclear AGN.