The optical/far–IR extragalactic background light ( EBL ) from both resolved and unresolved extragalactic sources is an indicator of the total luminosity of cosmic structures , as the cumulative emission from young and evolved galactic systems , as well as from active galactic nuclei ( AGNs ) , is recorded in this radiation .
This is a brief review of some of the implications of the observed brightness of the night sky for the stellar mass density and average metallicity of the universe today , and of the possible contribution of MACHO progenitors and QSOs to the EBL .
Assuming a Salpeter initial mass function with a cutoff below 0.6 { M _ { \odot } } , a lower limit of \Omega _ { g + s } h ^ { 2 } > 0.0015 I _ { 60 } can be derived to the visible ( recycled gas + stars ) mass density required to generate an EBL at a level of I _ { EBL } = 60 I _ { 60 } { nW m ^ { -2 } sr ^ { -1 } } : our latest , ‘ best–guess ’ estimate is \Omega _ { g + s } h ^ { 2 } \approx 0.0023 I _ { 60 } , which implies a mean metallicity at the present–epoch of y _ { Z } \Omega _ { g + s } / \Omega _ { b } \approx 0.2 Z _ { \odot } .
If massive dark halos around spiral galaxies are partially composed of faint , old white dwarfs , i.e .
if a non–negligible fraction ( \sim a few percent ) of the nucleosynthetic baryons is locked in the remnants ( MACHOs ) of intermediate–mass stars forming at very high redshifts , then the bright early phases of such halos should contribute significantly to the observed EBL .
Assuming a standard black hole accretion model for quasar activity and using recent observations of the quasar population and new synthesis models for the cosmic X–ray background , we estimate a present mass density of QSO remnants of \rho _ { BH } \approx 3 \times 10 ^ { 5 } { M _ { \odot } Mpc ^ { -3 } } for a 10 % efficiency of accreted mass–to–radiation conversion .
The quasar contribution to the brightness of the night sky is I _ { QSO } \approx 2 { nW m ^ { -2 } sr ^ { -1 } } .