Evolution of Star-forming and Active Galaxies in Nearby Clusters

Abstract

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 active galactic nuclei (AGNs) 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 [O II] from the spectra of these galaxies, meaning their star formation could easily be overlooked in studies that 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 AGNs, with star-forming galaxies broadly distributed and AGNs 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 4000 A break strengths. We find that discrepancies in reported radio luminosity functions for AGNs 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 whose spectra lack emission lines. Understanding the nature of these galaxies is critical to assessing the importance of AGNs 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 composed equally of star-forming galaxies and AGNs occurs at lower luminosities in clusters than in the field. This is likely a simple consequence of the reduction in star formation in cluster galaxies and the morphological mix in clusters compared with the field.