Deep galaxy counts, extragalactic background light and the stellar baryon budget

Abstract

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 15nWm^-2sr^-1, comparable to the intensity of the far-IR background from COBE data. Assuming a Salpeter initial mass function with a lower cut-off consistent with observations of M subdwarf disc stars, we set a lower limit of Ω_g+sh²>0.0013I₅₀ to the visible (processed gas+stars) mass density required to generate an EBL at a level of 50I₅₀nWm^-2sr^-1 our `best-guess' value is Ω_g+sh²~0.0031I₅₀. Motivated by the recent microlensing results of the MACHO collaboration, we consider the possibility that massive dark haloes around spiral galaxies are composed of faint white dwarfs, and show that only a small fraction (<~5per cent) of the nucleosynthetic baryons can be locked in the remnants of intermediate-mass stars forming at z_F<~5, as the bright early phases of such haloes would otherwise overproduce the observed EBL.