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E.C. Moran (Berkeley), D.J. Helfand (Columbia)
ASCA\/ and BeppoSAX\/ have greatly expanded our understanding of the hard X-ray properties of nearby starburst and Seyfert galaxies, allowing, for the first time, detailed estimates of their respective contributions to the hard X-ray background (XRB) to be made. Unfortunately, the sensitivities of these instruments are insufficient to probe either population directly at intermediate and high redshifts, where the majority of the XRB originates. As a result, discrete-source XRB models must typically rely on highly uncertain assumptions about the evolution of potential contributors with cosmic time. Clearly, it would be helpful to identify an observational constraint that minimizes (or eliminates) the need for these assumptions. Since X-ray galaxies of all types produce radio emission in conjunction with their particular brand of activity, we propose that the faint radio source population may provide such a constraint. Existing deep radio surveys, which extend to the microjansky level, should contain both starburst and Seyfert galaxies at cosmological distances. However, optical identification programs carried out to date have revealed that the majority of sub-mJy radio sources are associated with star-forming galaxies rather than AGNs, suggesting that the starburst contribution to the XRB could be significant. By combining hard X-ray and radio data for nearby starburst galaxies with the measured log~N--log~S relation for sub-mJy radio sources, we estimate that starburst galaxies may produce as much as 15--45% of the 5~keV XRB. Preliminary results of a similar analysis for Seyfert galaxies are complimentary, indicating that these objects cannot be responsible for all of the hard XRB.
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