The nature of composite Seyfert/star-forming galaxies revealed by X-ray observations

This paper presents new Chandra and BeppoSAX observations aimed at investigating the optical-X-ray mismatch in the enigmatic class of the composite galaxies discovered by a cross-correlation of IRAS and ROSAT all-sky survey catalogs. These galaxies have been classified as star-forming objects on the basis of their optical spectra, while the detection of weak broad wings in the H alpha emission in a few of them and their high X-ray luminosity in the ROSAT band indicated the presence of an active nucleus. The analysis of Chandra observations for four composite galaxies has revealed nuclear pointlike sources, with a typical AGN spectrum (Gamma similar to 1.7-1.9) and little intrinsic absorption. A strong flux variability has been observed on different timescales; in particular, most of the sources were brighter at the ROSAT epoch. Although it is of relatively low luminosity for the AGN class (L2-10keV similar to 3-60x10(41) ergs s(-1)), the active nucleus is nevertheless dominant in the X-ray domain. At other wavelengths it appears to be overwhelmed by the starburst and/or host galaxy light, yielding the composite classification for these objects.