Infrared-Faint Radio Sources: A Cosmological View - AGN Number Counts, the Cosmic X-Ray Background and SMBH Formation

TL;DR

This study investigates Infrared Faint Radio Sources (IFRS) as high-redshift AGN, estimating their contribution to the cosmic X-ray background and SMBH formation, revealing their significance in early universe structure formation.

Contribution

It provides the first comprehensive analysis of IFRS number density, their role in SMBH mass density, and their impact on the cosmic X-ray background, linking observations to galaxy evolution models.

Findings

IFRS have a density of about 30.8 per square degree.

They contribute significantly to the SMBH mass density at high redshift.

Results support SMBH formation via halo gas accretion and major mergers.

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 5 sigma sensitivities as low as 1 uJy. Aims. Recent SED-modelling and analysis of their radio properties shows 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) per square degree. 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 about 310 deg^-2, which is equivalent to a SMBH mass density of the order of 10^3 M_sun 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. Concerning the problem of SMBH formation after the Big Bang we find evidence for a scenario involving both halo gas accretion and major mergers.