  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.

Astrophysics