Compact Nuclei in Moderately Redshifted Galaxies

The Hubble Space Telescope WFPC2 is being used to obtain high-resolutionimages in the V and I bands for several thousand distant galaxies as part ofthe Medium Deep Survey (MDS). An important scientific aim of the MDS is toidentify possible AGN candidates from these images in order to measure thefaint end of the AGN luminosity function as well as to study the host galaxiesof AGNs and nuclear starburst systems. We are able to identify candidateobjects based on morphology. Candidates are selected by fitting bulge+diskmodels and bulge+disk+point source nuclei models to HST imaged galaxies anddetermining the best model fit to the galaxy light profile. We present resultsfrom a sample of MDS galaxies with I less than 21.5 mag that have been searchedfor AGN/starburst nuclei in this manner. We identify 84 candidates withunresolved nuclei in a sample of 825 galaxies. For the expected range of galaxyredshifts, all normal bulges are resolved. Most of the candidates are found ingalaxies displaying exponential disks with some containing an additional bulgecomponent. 5% of the hosts are dominated by an r^-1/4 bulge. The V-I colordistribution of the nuclei is consistent with a dominant population ofSeyfert-type nuclei combined with an additional population of starbursts. Ourresults suggest that 10% +/- 1% of field galaxies at z less than 0.6 maycontain AGN/starburst nuclei that are 1 to 5 magnitudes fainter than the hostgalaxies.Comment: 12 pages AASTeX manuscript, 3 separate Postscript figures, to be published in ApJ Letter