X-RAY-PROPERTIES OF BRIGHT FAR-INFRARED GALAXIES

We have searched the Einstein data base for observations of galaxies in the IRAS Bright Galaxy Sample. Of the 324 galaxies in this sample, 81 were observed with the Imaging Proportional Counter, and 51 were detected at greater than a 3-sigma confidence level. A strong correlation exists between the far-infrared and X-ray luminosities among the normal and starburst (NSB) galaxies with, L(X) is-proportional-to L(fir)0.90-1.01 (90% confidence limits). The X-ray luminosity of the 10 Seyfert galaxies in this sample is a factor of approximately 10 greater than the NSB galaxies with the same far-infrared luminosity. The relation between the X-ray and blue luminosity of the NSB galaxies is also consistent with a linear relation. L(X) is-proportional-to L(B)1.03-1.18 (90% confidence limits) but exhibits increasing scatter with increasing L(B). In fact, the residuals in L(X) around the best-fit relation are strongly correlated with the ratio L(fir)/L(B), which is a measure of the current star formation rate. These correlations in the NSB sample can be understood through a two-component model for the X-ray emission from spirals. One component consists of relatively old objects (Type I supernovae and low-mass X-ray binaries) and is proportional to L(B), while the second component comprises young objects (Type II supernovae, O stars, and high-mass X-ray binaries) and is proportional to the far-infrared luminosity from active regions of star formation. The first component dominates in more quiescent galaxies, while the second component dominates in starburst galaxies. We find that the efficiency of X-ray production through star formation is nearly independent of the star formation rate with 10(47.5) ergs of energy (between 0.5 and 4.5 keV) produced for each solar mass consumed into stars. On energetic grounds alone, we find that the integrated emission from individual Type II supernova remnants and O star atmospheres can only account for approximately 10% and 30% of the total observed X-ray luminosity from starburst galaxies. The observed X-ray luminosity of starbursts can be accounted for by high-mass X-ray binaries if approximately 4% of the newly formed O stars reside in such systems or by gas collectively heated by supernovae. Our results also show that the X-ray emission from bright far-infrared galaxies can make a significant contribution to the observed X-ray background.