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 \simeq 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 ( Williams et al . 1996 ) in December 1995 and a second epoch obtained two years later ( Gilliland , Nugent & Phillips 1999 ) .
The second epoch data consists of 36 exposures in F814W with a total integration time of 63000 seconds ( compared to 58 exposures and a total of 123600 seconds in the original HDF-N ) .

We have detected nuclear variability at or above the 3 \sigma level in 8 of 633 HDF galaxies at I _ { 814 } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 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 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } M _ { B } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } -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 \simeq 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 _ { B } \simeq -19 ( H _ { o } =75 km/s/Mpc , q _ { o } =0.5 ) .
From this study , we estimate that \sim 1–3 % of field galaxies with I \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 27 may contain a nuclear AGN .