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Astron. Astrophys. 357, 13-23 (2000)

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The hard X-ray emission of luminous infrared galaxies

G. Risaliti 1, R. Gilli 1, R. Maiolino 2 and M. Salvati 2

1 Università di Firenze, Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 5, 50125 Firenze, Italy (risaliti, gilli@arcetri.astro.it)
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy (maiolino, salvati@arcetri.astro.it)

Received 12 October 1999 / Accepted 7 February 2000


We present a study of the hard X-ray properties of a sample that includes all the Luminous Infrared Galaxies (LIGs, [FORMULA] [FORMULA]) observed in the 2-10 keV energy band (new and archival data). We find that a significant fraction of the sources optically classified as AGNs do not show any indication of nuclear activity in the X rays, thus suggesting heavy absorption along our line of sight. The absence of strong emission in the 20-200 keV band in a subsample of LIGs observed with BeppoSAX suggests that in many cases these sources are completely Compton thick ([FORMULA] cm-2). From a comparison between the infrared and the X-ray emission we deduce that the mid-IR emission is absorbed by a lower column density than the X-ray emission or, alternatively, that the dust-to-gas ratio is lower than Galactic. We describe a simple model that reproduces the IR-X correlation by means of mixed AGN and starburst contributions and we compare the predictions of this model with the observational data at X-ray and optical wavelengths. Finally, we discuss the biases that affect the currently available samples of LIGs and briefly analyze a small unbiased sample, finding that at least 50% of the sources host a (weak) AGN.

Key words: galaxies: active – galaxies: starburst – infrared: galaxies – X-rays: galaxies

Send offprint requests to: G. Risaliti

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000