Compact Nuclei in Galaxies at Moderate Redshift. I. Imaging and Spectroscopy

This study explores the space density and properties of active galaxies toz=0.8. We have investigated the frequency and nature of unresolved nuclei ingalaxies at moderate redshift as indicators of nuclear activity such as ActiveGalactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imagedgalaxies with multi-component models using maximum likelihood estimatetechniques to determine the best model fit. We select those galaxies requiringan unresolved, point source component in the galaxy nucleus, in addition to adisk and/or bulge component, to adequately model the galaxy light. We havesearched 70 WFPC2 images primarily from the Medium Deep Survey for galaxiescontaining compact nuclei. In our survey of 1033 galaxies, the fractioncontaining an unresolved nuclear component greater than 3% of the total galaxylight is 16+/-3% corrected for incompleteness and 9+/-1% for nuclei greaterthan 5% of the galaxy light. Spectroscopic redshifts have been obtained for 35of our AGN/starburst candidates and photometric redshifts are estimated to anaccuracy of sigma_z=0.1 for the remaining sample. In this paper, the first oftwo in this series, we present the selected HST imaged galaxies havingunresolved nuclei and discuss the selection procedure. We also present theground-based spectroscopy for these galaxies as well as the photometricredshifts estimated for those galaxies without spectra.Comment: 56 pages, 22 figures, to appear in ApJ Supplement Series, April 199