The hard X-ray emission of luminous infrared galaxies
Abstract
We present a study of the hard X-ray properties of a sample that includes all the Luminous Infrared Galaxies (LIGs, LIR > 1011 Lsun) 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 (NH > 1025 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.
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