The hard X-ray view of bright infrared galaxies

¹ INTEGRAL Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland e-mail: Roland.Walter@obs.unige.ch
² Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
³ Université de Bordeaux 1, UFR de Physique, 351 Cours de la Libération, 33405 Talence Cedex, France

Received: 29 August 2008
Accepted: 30 January 2009

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.