Variable galaxies in the Hubble Deep Field

We present results from a study to detect variable galaxies in the Hubble Deep Field North (HDF-N). The goal of this project is to investigate the number density of active galactic nuclei (AGNs) at z ≃ 1 through the detection of variable galaxy nuclei. The advantage of the Hubble Space Telescope is its 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 1995 December and a second epoch obtained 2 years later. The second-epoch data consist of 36 exposures in F814W with a total integration time of 63,000 s (compared with 58 exposures and a total of 123,600 s in the original HDF-N). We have detected nuclear variability at or above the 3 τ level in eight of 633 HDF galaxies at I⁸¹⁴814 ≲ 27. Only two detections would be expected by chance in a normal distribution. At least one of these eight 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_B ≲ - 15.0. We construct an upper limit to the luminosity function for the variable nuclei and compare this with the local Seyfert luminosity function (LF) and the LFs for quasi-stellar objects 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 AGNs at M_B ≃ -19 (H₀ = 75 km s^-1 Mpc^-1, q₀ = 0.5). From this study, we estimate that ∼ 1%-3% of field galaxies with I ≲ 27 may contain a nuclear AGN.