The hard X-ray view of bright infrared galaxies

TL;DR

This study investigates whether bright infrared galaxies host hidden Compton-thick active nuclei and assesses their contribution to the cosmic X-ray background, finding that non-Seyfert galaxies contribute minimally and lack such nuclei.

Contribution

It provides the first extensive hard X-ray analysis of a large sample of bright infrared galaxies, constraining their role in the cosmic X-ray background.

Findings

Seyfert galaxies are detected at hard X-rays.

Non-Seyfert galaxies show no significant hard X-ray emission.

Non-Seyfert galaxies lack the Compton-thick nuclei needed for CXB synthesis.

Abstract

Aims. The synthesis of the cosmic X-ray background (CXB) requires a large population of Compton-thick active galactic nuclei that have not been detected so far. We probe whether bright infrared galaxies could harbor a population of Compton-thick nuclei and if they could contribute significantly. Methods. We analyzed 112 Msec of INTEGRAL observations obtained on 613 galaxies from the IRAS Revised Bright Galaxy Sample. We derived the average hard X-ray (18-80 keV) emission of Seyfert and various non Seyfert galaxy subsamples to estimate their relative contribution to the locally emitted CXB. Results. The Seyfert 1 & 2 are detected at hard X-rays. None of the other galaxy subsamples were detected. ULIRGs are at least 5 times under-luminous at hard X-rays when compared to Seyferts. The upper limit obtained for the average non Seyfert galaxies is as low as 7E-13 erg/s cm2. On average, these galaxies do not contain active nuclei brighter than 10E41 erg/s at hard X-rays. The total hard X-ray flux detected from the sample is 4.9E-9 erg/s cm2 (about 1% of the CXB), and 64% of this originates in absorbed active nuclei. Local non-Seyfert galaxies contribute for less than 7% and do not harbor the Compton-thick nuclei assumed to synthesize the locally emitted CXB.