We assess the constraints imposed by the observed extragalactic background light ( EBL ) on the cosmic history of star formation and the stellar mass density today .
The logarithmic slope of the galaxy number-magnitude relation from the Southern Hubble Deep Field imaging survey is flatter than 0.4 in all seven UBVIJHK optical bandpasses , i.e .
the light from resolved galaxies has converged from the UV to the near-IR .
We find a lower limit to the surface brightness of the optical extragalactic sky of about 15 { nW m ^ { -2 } sr ^ { -1 } } , comparable to the intensity of the far-IR background from COBE data .
Assuming a Salpeter initial mass function with a lower cutoff consistent with observations of M subdwarf disk stars , we set a lower limit of \Omega _ { g + s } h ^ { 2 } > 0.0013 I _ { 50 } to the visible ( processed gas + stars ) mass density required to generate an EBL at a level of 50 I _ { 50 } { nW m ^ { -2 } sr ^ { -1 } } ; our ‘ best-guess ’ value is \Omega _ { g + s } h ^ { 2 } \approx 0.0031 I _ { 50 } .
Motivated by the recent microlensing results of the MACHO collaboration , we consider the possibility that massive dark halos around spiral galaxies are composed of faint white dwarfs , and show that only a small fraction ( \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 316 $ } } 5 \% ) of the nucleosynthetic baryons can be locked in the remnants of intermediate-mass stars forming at z _ { F } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 % pt \hbox { $ \mathchar 316 $ } } 5 , as the bright early phases of such halos would otherwise overproduce the observed EBL .