RADIO X-RAY LUMINOSITY RELATION FOR ADVECTION-DOMINATED ACCRETION - IMPLICATIONS FOR EMISSION-LINE GALAXIES AND THE X-RAY-BACKGROUND/

Recent studies of the cosmic X-ray background (XRB) have suggested the possible existence of a population of relatively faint sources with h ard X-ray spectra; however, the emission mechanism remains unclear. If the hard X-ray emission is from the radiatively inefficient, advectio n-dominated accretion flows (ADAFs) around massive black holes in gala ctic nuclei, X-ray luminosity and radio luminosity satisfy the approxi mate relation L-R similar to 7 x 10(35)(v/15 GHz)(7/5)(M/10(7) M-circl e dot)(L-X/10(40) ergs s(-1))(1/10) ergs s(-1), where L-R = vL(v) is t he radio luminosity at frequency v, M is the mass of the accreting bla ck hole, and 10(40) less than or similar to L-X less than or similar t o 10(42) ergs s(-1) is the 2-10 keV X-ray luminosity. These sources ar e characterized by inverted radio spectra I-v proportional to v(2/5). For example, an ADAF X-ray source with luminosity L-X similar to 10(41 ) ergs s(-1) has a nuclear radio luminosity of similar to 4 x 10(36)(M /3 X 10(7) M-circle dot) ergs s(-1) at similar to 20 GHz, and if it is at a distance of similar to 10(M/3 x 10(7) M-circle dot)(1/2) Mpc, it would be detected as a similar to 1 mJy point radio source. High-freq uency (similar to 20 GHz), high angular resolution radio observations provide an important test of the ADAF emission mechanism. Since L-R de pends strongly on black hole mass and only weakly on X-ray luminosity, the successful measurement of nuclear radio emission could provide an estimate of black hole mass. Because the X-ray spectra produced by AD AFs are relatively hard, sources of this emission are natural candidat es for contributing to the hard(>2 keV) background.