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A direct measurement of the contribution of quasars to the EBL depends on poorly known quantities like the bolometric correction, the faint end of the luminosity function, and the space density of objects at high redshifts. Estimates range from 0.7 to 3 n W m-2sr-1 (Soltan 1982; Chokshi & Turner 1992; Small & Blandford 1992). Another source of uncertainty is the possible existence of a distant population of dusty AGNs with strong intrinsic absorption, as invoked in many models for the X-ray background (e.g. Madau et al. 1994; Comastri et al. 1995). These Type II QSOs, while undetected at optical wavelengths, could contribute significantly to the far-IR background. It is in principle possible to bypass some of the above uncertainties by weighing the local mass density of black holes remnants (Soltan 1982), as recent dynamical evidence indicates that supermassive black holes reside at the center of most nearby galaxies (Richstone et al. 1998). The available data (about 40 objects) show a correlation between bulge and black hole mass, with MBH approx 0.006 Msph as a best-fit (Magorrian et al. 1998). The mass density in old spheroidal populations today is estimated to be Omegasph h = 0.0018+0.0012-0.00085 (Fukugita et al. 1998, hereafter FHP), implying a mean mass density of quasar remnants today

Equation 2 (2)

The observed (comoving) energy density from all quasars is equal to the emitted energy, eta rhoBH c2, divided by the average quasar redshift, < 1 + z >. Here eta is the efficiency of accreted mass to radiation conversion, equal to 5.7% for standard disk accretion onto a Schwarzschild black hole. The total contribution to the EBL from accretion onto black holes can be estimated to be

Equation 3 (3)

(h = 0.5). Therefore, unless dust-obscured accretion onto supermassive black holes is a very efficient process (eta >> 0.05), a population of quasars peaking at z ~ 1.5-2 is expected to make a contribution to the total brightness of the night sky not exceeding 10-20% (Fabian & Iwasawa 1998; Madau 1999).

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