Precursor active galaxies and the cosmic X-ray background

Abstract

Data on the isotropic extragalactic X-ray background (>3 keV) obtained with the HEAO 1 A2 instrument¹ yield a spectrum over the energy band of maximum flux that is well described by a thermal model involving an optically thin hot plasma at a temperature of a half-billion degrees (kT = 40±5 keV). The spectra for known strong X-ray sources in the present epoch (active galaxies, clusters of galaxies) obtained with the same HEAO 1 instrument^2,3 indicate that such objects at high redshifts cannot account for most of this background. On the other hand, deep-field exposures to soft X rays (<3 keV) with the HEAO 2 Einstein Observatory⁴ suggest that discrete sources could be a sufficient explanation. This implies either (1) that these dim sources are generally unrelated to relatively bright nearby objects or (2) that they represent an earlier stage in their evolution. Short-lived thermal X-ray emission by galactic winds peculiar to an earlier epoch has been suggested⁵ as an explanation of the first kind, but the need for postulating new sources can be obviated if, as Leiter has argued⁶, prolific X-ray sources evolve in connection with active galactic nuclei involving massive compact objects. Here, we pursue the suggestion by Carr⁷ that most of the cosmic X-ray background (CXB) arises from thermal X-ray emission associated with black hole accretion disks for compact objects formed before galaxies but we favour a model where the onset of significant X-rays takes place within the epoch of galaxy formation at z≲4, when accretion disks are created by the interaction between massive black holes of pregalactic origin and young galaxies.