Next Contents Previous


A direct estimate of the contribution of quasars to the EBL depends on the poorly known bolometric correction and the possible existence of a distant population of dusty AGNs with strong intrinsic absorption, as invoked in many models for the X-ray background. 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 [29]. Recent dynamical evidence indicates that supermassive black holes reside at the center of most nearby galaxies. The available data (see Fig. 3) show a correlation (but with a large scatter) between bulge and black hole mass, with MBH approx 0.006 Msph as a best-fit [23]. The mass density in old spheroidal populations today is estimated to be Omegasph h = 0.0018+0.0012-0.00085 [16], implying a mean mass density of quasar remnants today

Equation 2 (2)

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

Equation 3 (3)

(h = 0.5), where eta0.05 is the efficiency of accreted mass to radiation conversion (in units of 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 brightness of the night sky not exceeding 10-20% [14], [21].

Figure 3

Figure 3. Black hole mass distribution against the bulge luminosity of their host galaxies [23]. Arrows indicate upper limits on MBH. The symbols denote different galaxy types: empty circles (E), filled squares (S0), filled circles (Sab), and empty squares (Sbc-Scd).

Next Contents Previous