A compilation of active and normal galaxies observed in both infrared and X-rays

For the largest ensemble of active and normal galaxies to date, we have collected far-infrared and X-ray data from the literature and from the IRAS and Einstein data-bases. We study the relation between emission in these two bands, using survival analysis to exploit the information contained in upper limits. We find that galaxies with soft X-ray to infrared flux ratios greater than about 0.01 are almost certain to show broad-line optical emission. This discriminant will be valuable for finding quasars and Seyfert 1 galaxies from comparisons of IR and X-ray surveys. A sensitive, all-sky X-ray survey like ROSAT is particularly well-suited to such a search in conjunction with the IRAS data. For the full IRAS/Einstein ensemble, we find a significant correlation between luminosities in the 60-µm and 0.5–4.5 keV bands. A strong offset separates broad-line from normal and narrow-line galaxies. We interpret the jump toward higher X-ray emission in broad-line galaxies as evidence for the increasing importance of a non-thermal nuclear source. Among individual galaxy classes, radio-loud quasars show a significant correlation of $${L}_{X}\,\text{to}\,{L}_{60\,\mu m}$$ that is not seen in radio-quiet quasars or Seyferts as individual classes. Since the ratios $${L}_{X}/{L}_{60 \,\mu \text{m}}$$ differ significantly between predominantly thermal and non-thermal nuclear components, we conjecture that the correlations intrinsic to each individual component are lost when the two are strongly mixed in Seyfert galaxies or radio-quiet quasars. Our analysis of the empirical relationship between $${L}_{X}\,\text{and}\,{L}_{60\,\mu \text{m}}$$ for normal and narrow optical emission-line galaxies (excluding Seyfert 2s) allows us to convert published 60-µm IRAS luminosity functions into estimates of the 2-keV X-ray luminosity function of IR-emitting galaxies. We use this luminosity function to estimate the contribution to the soft X-ray background of these lower luminosity IR-emitting galaxies out to $${z}_\text{max}=\,3$$. Depending on the evolutionary model applied, we derive contributions of 5–25 per cent for these galaxies to the soft X-ray background.