The hard X-ray view of bright infrared galaxies

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 Ms 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 7 × 10^-13 erg s^-1 cm^-2. On average, these galaxies do not contain active nuclei brighter than 10⁴¹ erg s^-1 at hard X-rays. The total hard X-ray flux detected from the sample is 4.9 × 10^-9 erg s^-1 cm^-2 (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.