The Evolution of the Ultraluminous Infrared Galaxy Population from Redshift

We use redshift observations of two deep 1.4 GHz fields to probe the evolution of the bright end of the radio galaxy luminosity function to z = 1.5. We show that the number of galaxies with radio power that would correspond locally to an ultraluminous infrared galaxy (ULIG) evolves rapidly over this redshift range. The optical spectra and X-ray luminosities are used to refine this result by separating the sources with clear active galactic nucleus (AGN) signatures from those that may be dominated by star formation. Both populations show extremely rapid evolution over this redshift range. We find that the number of sources with ULIG radio power and no clear AGN signatures evolves as (1 + z). Subject headings: cosmology: observations — galaxies: distances and redshifts — galaxies: active — X-rays: galaxies — galaxies: formation — galaxies: evolution