Compact nuclei in galaxies at moderate redshift. II. Their nature and implications for the active galactic nucleus luminosity function

This study explores the space density and properties of active galaxies to z similar or equal to 0.8. We have investigated the frequency and nature of unresolved nuclei in galaxies at moderate redshift as indicators of nuclear activity such as active galactic nuclei (AGNs) or starbursts. Candidates are selected by fitting imaged galaxies with multicomponent models using maximum-likelihood estimate techniques to determine the best model fit. We select those galaxies requiring unresolved point-source components in the galaxy nuclei, in addition to disk and/or bulge components, to adequately model the galaxies' light. We have searched 70 WFPC2 images, primarily from the Medium Deep Survey, for galaxies containing compact nuclei. In our survey of 1033 galaxies, the fraction containing unresolved nuclear components contributing greater than or equal to 5% of the total galaxy light is 9% +/- 1% corrected for incompleteness. In this second of two papers in this series, we discuss the nature of the compact nuclei and their hosts. We present the upper limit luminosity function (LF) for low-luminosity AGNs (LLAGNs) in two redshift bins to z = 0.8. Mild number-density evolution of the form phi proportional to (1 + z)(1.9) is detected for nuclei at -18 less than or similar to M-B less than or similar to - 14. The LFs appear to flatten at M-B greater than or equal to -16 and this flatness, combined with the increase in number density, is inconsistent with pure luminosity evolution. Based on the amount of density evolution observed for these objects, we find that almost all present-day spiral galaxies could have hosted LLAGNs at some point in their lives. We also comment on the likely contribution of these compact nuclei to the soft X-ray background.