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 
0.006 Msph as a best-fit
(Magorrian et al. 1998).
The mass density in old spheroidal populations today is estimated to be
sph h =
0.0018+0.0012-0.00085
(Fukugita et al. 1998,
hereafter FHP), implying a mean mass density of quasar remnants today
The observed (comoving) energy density
from all quasars is equal to the emitted energy,
(h = 0.5). Therefore, unless dust-obscured accretion onto
supermassive black holes is a very efficient process
(
BH
c2,
divided by the average quasar redshift, < 1 + z >. Here
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
>> 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).