Infrared-faint radio sources: a cosmological view

AGN number counts, the cosmic X-ray background and SMBH formation

¹ Astronomical Institute, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany
e-mail: zinn@astro.rub.de
² UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

Received: 6 December 2010
Accepted: 31 March 2011

Abstract

Context. Infrared-faint radio sources (IFRS) are extragalactic emitters clearly detected at radio wavelengths but barely detected or undetected at optical and infrared wavelengths, with 5σ sensitivities as low as 1 μJy.

Aims. Spectral energy distribution (hereafter SED) modelling and analyses of their radio properties indicate that IFRS are consistent with a population of (potentially extremely obscured) high-redshift AGN at 3 ≤ z ≤ 6. We demonstrate some astrophysical implications of this population and compare them to predictions from models of galaxy evolution and structure formation.

Methods. We compiled a list of IFRS from four deep extragalactic surveys and extrapolated the IFRS number density to a survey-independent value of (30.8    ±    15.0) deg^-2. We computed the IFRS contribution to the total number of AGN in the Universe to account for the cosmic X-ray background. By estimating the black hole mass contained in IFRS, we present conclusions for the SMBH mass density in the early universe and compare it to relevant simulations of structure formation after the Big Bang.

Results. The number density of AGN derived from the IFRS density was found to be  ~310 deg^-2, which is equivalent to a SMBH mass density of the order of 10³   M_⊙ Mpc^-3 in the redshift range 3 ≤ z ≤ 6. This produces an X-ray flux of 9 × 10^-16 W m^-2 deg^-2 in the 0.5–2.0 keV band and 3 × 10^-15 W m^-2 deg^-2 in the 2.0–10 keV band, in agreement with the missing unresolved components of the Cosmic X-ray Background. To address SMBH formation after the Big Bang we invoke a scenario involving both halo gas accretion and major mergers.