Evolution of Star Forming and Active Galaxies in Nearby Clusters

We have used optical spectroscopy to investigate the active galaxy populations in a sample of 20 nearby Abell clusters. The targets were identified on the basis of 1.4 GHz radio emission, which identifies them as either AGN or galaxies forming stars at rates comparable to or greater than that of the Milky Way. The spectra were used to characterize the galaxies via their emission and absorption features. The spectroscopy results reveal a significant population of star forming galaxies with large amounts of nuclear dust extinction. This extinction eliminates bluer emission lines such as [OII] from the spectra of these galaxies, meaning their star formation could easily be overlooked in studies which focus on such features. Around 20% of the cluster star forming galaxies have spectra of this type. The radial distributions of active galaxies in clusters show a strong segregation between star forming galaxies and AGN, with star forming galaxies broadly distributed and AGN preferentially in the cluster cores. The radial distribution of the dusty star forming galaxies is more centrally-concentrated than the star forming galaxies in general, which argues that they are a consequence of some cluster environmental effect. Furthermore, we note that such galaxies may be identified using their 4000Å break strengths. We find that discrepancies in reported radio luminosity functions for AGN are likely the result of classification differences. There exists a large population of cluster galaxies whose radio fluxes, far-infrared fluxes, and optical magnitudes suggest their radio emission may be powered by stars, yet their spectra lack emission lines. Understanding the nature of these galaxies is critical to assessing the importance of AGN in the radio luminosity function at low luminosities. We also find that regardless of this population, the crossover point where the radio luminosity function is comprised equally of star forming galaxies and AGN occurs at lower luminosities in clusters than the field. This is likely a simple consequence of the reduction in star formation in cluster galaxies and the morphological mix in clusters compared to the field. Subject headings: galaxies: clusters: general — galaxies: evolution — galaxies: radio continuum Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with