The X-ray properties of luminous infrared galaxies and their contribution to the X-ray background

We present a study of a sample of luminous infrared galaxies (LIGs, L_IR > 10^11 L_sun) observed in the hard (2-10 keV) X rays. The main results are: 1) most LIGs are powered both by AGN and starburst activity; 2) the AGNs in our sample are absorbed in the infrared by a lower N_H than in the X rays or, alternatively, the dust-to-gas ratio is lower than galactic; 3) the study of a subsample of sources observed in the 20-200 keV band indicates that most of the AGNs hosted by the LIGs are heavily obscured up to 100 keV and, therefore, their contribution to the X-ray background must be small.