Faint Gigahertz Sources and the Giga-gigahertz Background

The origin of the giga-gigahertz background radiation (also known as the hard X-ray background, or XRB) has puzzled astronomers since its discovery nearly four decades ago. While the debate as to whether the XRB arises from diffuse or discrete sources has been settled, the class of objects with the properties required to explain the observed characteristics of the XRB remains elusive. Recent research has identified two promising candidates: Seyfert galaxies and star-forming galaxies. Unfortunately, the sensitivities of X-ray instrumentation available to date have been insufficient to probe either population directly at intermediate and high redshifts, where the majority of the XRB originates. As a result, models for the XRB must typically rely on highly uncertain assumptions about the evolution of these sources with cosmic time. Given that X-ray galaxies of all types produce gigahertz radio emission in conjunction with their particular brand of activity, the faint radio source population should provide constraints that reduce the need for such assumptions. Indeed, existing deep radio surveys, which extend to the microjansky level, contain both star-forming and Seyfert galaxies at cosmological distances. I have combined hard X-ray and centimeter radio data for nearby starburst and Seyfert galaxies with the measured log N - log S relation for sub-mJy radio sources to estimate the contributions of these objects to the hard XRB. The faint radio source statistics suggest that the XRB traces the history of both accretion power and star formation in the young universe.