6.2. Energy from Accretion

As mentioned briefly in Section 1, another significant source of energy at UV to far-IR wavelengths is accretion onto black holes in AGN and quasars. The total bolometric flux from accretion can be estimated from the local mass function of black holes at the centers of galaxies for an assumed radiation efficiency and total accreted mass. Recent surveys find M_bh 0.005M_sph, in which M_sph is the mass of the surrounding spheroid and M_bh is the mass of the central black hole (Richstone et al. 1998, Magorrian et al. 1998, Salucci et al. 1999, and van der Marel 1999). Following Fabian & Iwasawa (1999), the energy density in the universe from accretion is given by

(4)

in which _bh is the radiation efficiency, _sph is the observed mass density in spheroids in units of the critical density, _crit, and (1 + z_e) compensates for the energy lost due to cosmic expansion since the emission redshift z_e. The bolometric flux from accretion is then

(5)

for _bh ~ 0.1, z_e ~ 2, _crit = 2.775 × 10¹¹ h² M Mpc^-3, H₀ = 100h km s^-1 Mpc^-1, and _sph ~ 0.0018^{+ .0012}_-0.00085h^-1 (Fukugita, Hogan, & Peebles 1998, FHP98).

The observed X-ray background (0.1-60 keV) is ~ 0.2 nW m^-2 s^-1. The large discrepancy between the detected X-ray flux and the estimated flux from accretion has led to suggestions that 85% of the energy estimated to be generated from accretion takes place in dust-obscured AGN and is emitted in the thermal IR (see discussions in Fabian 1999). Further support for this view comes from the fact that most of the soft X-ray background (below 2 keV) is resolved into unobscured sources (i.e., optically bright quasars), while most of the hard X-ray background is associated with highly obscured sources (Mushotzky et al. 2000). Photoelectric absorption can naturally account for the selective obscuration of the soft X-ray spectrum. Best estimates for the fraction of the far-IR EBL which can be attributed to AGN are then < 10h nW m^-2 sr^-1, or < 30% of the observed IR EBL. This is in good agreement with estimates of the flux from a growing central black hole relative to the flux from stars in the spheroid based on arguments for termination of both black hole accretion and star formation through wind-driven ejection of cool gas in the spheroid (Silk & Rees 1998, Fabian 1999, Blandford 1999). Together, these studies suggest that < 15% of the bolometric EBL comes from accretion onto central black holes.