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V. L. Sarajedini (Wesleyan U.), R. L. Gilliland (STScI), M. M. Phillips (Carnegie Inst. of Washington, Las Campanas Obs.)
We present results from a study to detect variable galaxies in the Hubble Deep Field. 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 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).
We have detected nuclear variability at the 3 sigma level in 8 of 633 HDF galaxies having I < 27.5. At least one of these 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 -16 > MB > -21. We present the results of this study and discuss the evolution in number density of intrinsically faint AGN to z ~ 1.
Support for this work was provided by NASA through grant number AR-07984.01-96A from STScI which is operated by AURA, Inc. under NASA contract NAS5-26555.
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