ROSAT All-Sky Survey observations of IRAS galaxies

Abstract

A catalog of 244 IRAS galaxies positionally coincident with Rosat x-ray sources, which results from the correlation of a sample of 14708 extragalactic IRAS sources selected from the Point Source Catalog via statistical classification with the Rosat All Sky Survey, is presented. For 222 of them, optical counterparts could be identified by comparison with the NASA/IPAC Extragalactic Database. The sample of x-ray emitting IRAS galaxies comprises infrared luminous nearby galaxies, spirals and ellipticals, Seyfert galaxies, and Quasi Stellar Objects (QSO's). It is dominated by galaxies with active nuclei, unlike the sample of IRAS galaxies which have not been detected at x-ray wavelengths, at least in part due to the flux limitations of the Rosat survey. An unexpected result is the discovery of several spiral galaxies with luminosities up to 10 to the 43rd power erg/s, well above those found with the Einstein satellite. This suggests as a possible explanation that the x-ray luminous spirals are starbursts which are characterized by steep spectra and thus are more easily detected by Rosat. The x-ray properties are studied as a function of IR and optical fluxes for the different subgroups of galaxies in the sample. In general, a positive correlation is found between the x-ray luminosity and the IR and optical luminosities, respectively, which is steepest for Seyfert galaxies. The latter also appear to be separated from the other classes in correlations involving x- ray to IR or optical flux ratios. The x-ray to IR or optical flux ratios will be valuable for preselecting Seyfert galaxies when comparing x-ray to IR or optical surveys. By taking Seyfert galaxies into account, the trend of an increasing ratio f(sub x)/f(sub opt) with decreasing optical absolute magnitude, found for x- ray bright QSOs alone, is removed. (ESA)