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Recent dynamical evidence indicates that supermassive black holes reside at the center of most nearby galaxies. The available data (about 30 objects) show a strong correlation (but with a large scatter) between bulge and black hole mass [38], with Mbh = 0.006 Mbulge as a best-fit. The total mass density in spheroids today is Omegabulge = 0.0036+0.0024-0.0017 [16], implying a mean mass density of dead quasars

Equation 6 (6)

Since the observed energy density from all quasars is equal to the emitted energy divided by the average quasar redshift [49], the total contribution to the EBL from accretion onto black holes is

Equation 7 (7)

where eta0.1 is the efficiency for transforming accreted rest-mass energy into radiation (in units of 10%). Quasars at z ltapprox 2 could then make a significant contribution to the brightness of the night sky if dust-obscured accretion onto supermassive black holes is an efficient process [21], [14] (4).

4 It might be interesting to note in this context that a population of AGNs with strong intrinsic absorption (Type II quasars) is actually invoked in many current models for the X-ray background [33], [7].