1996A&A...309..431M

We have analyzed the multiwavelength (soft X-rays to far infrared) energy distribution of IRAS 15564+6359 and NGC 4619, two galaxies previously not classified as active galaxies but found during the ROSAT All Sky Survey to be very luminous in the X-ray and far infrared (FIR) ranges. By comparison with population synthesis evolutionary models we have found that a scenario consisting in very intense starbursts on dusty preexisting galaxies can reproduce the energy output at ultraviolet (UV), optical and far infrared wavelengths of both galaxies, as well as their narrow emission lines. The starburst models alone fail in principle to explain the rapid X-ray variability in IRAS 15564+6359, underestimate its soft-X-ray emission when in high-state, and fail also to explain the presence of broad Lyα and Hα lines in NGC 4619, unless the presence of compact supernova remnants is assumed. On the other hand, if these galaxies are hosting active sources, the excess in far infrared emission compared to Seyfert 1 galaxies would indicate that in any case additional intense starbursts processes should be taking place there. The detection of non-absorbed X-ray emission (IRAS 15564+6359) as well as a strong Lyα emission line (NGC 4619) on otherwise dusty galaxies points out the existence of a very patchy interstellar medium, with holes through which the high-energy emission can escape. This scenario suggests the existence of similar galaxies in which the active source could be completely blocked by gas and dust, being unveiled possibly only at very hard X-rays.