On radio-bright active galactic nuclei in a complete spectroscopic redshift survey

Analysis of the frequency and physical properties of galaxies with star formation and active galactic nucleus (AGN) activity in different environments in the local universe is a cornerstone for understanding structure formation and galaxy evolution. We have built a new multiwavelength catalog for galaxies in a complete redshift survey (the 15R Survey), gathering information on their H alpha, R-band, radio, far-infrared, and X-ray emission, as well as their radio and optical morphologies, and developed a classification scheme to compare different selection methods and accurately select samples of radio-emitting galaxies with AGN and star-forming activity. While alternative classification schemes do not lead to major differences for star-forming galaxies, we show that spectroscopic and photometric classifications of AGNs lead to incomplete samples. In particular, a large population of AGN-containing galaxies with absorption-line spectra, and in many cases extended radio structures (jets, lobes), is missed in the standard Baldwin-Phillips-Terlevich emission-line classification of active galaxies. This missed class of objects accounts for roughly half of the radio AGN population. Similarly, for X-ray-selected AGNs in our sample, we find that absorption-line AGNs account for half of the sample. Spectroscopically unremarkable, passive galaxies with AGN activity are not an exception, but the norm, and we show that although they exist in all environments, these systems preferentially reside in higher density regions. Because of the existence of this population, the fractional abundance of AGNs increases with increasing density, in contrast to the results based on emission-line AGNs extracted from the 15R, SDSS, and 2dF redshift surveys. Since emission-line radio AGNs are mostly associated with late-type galaxies and absorption-line radio AGNs with early-type galaxies, the trends found are connected to the well-known but poorly understood density-morphology relation.