Cosmic Star Formation History and the Brightness of the Night Sky

I review the constraints imposed by the observed extragalactic background light (EBL) on the history of the stellar birthrate in galaxies. At faint magnitudes, the logarithmic slope of the galaxy counts is flatter than 0.4 in all seven UBV IJHK optical bandpasses of the Hubble Deep Field-South imaging survey. The integration of the number counts provides a lower limit to the surface brightness of the optical extragalactic sky of 15 nWm sr, comparable to the intensity of the far-IR background from COBE data. If the initial mass function has a Salpeter slope with a lower mass cutoff consistent with observations of M subdwarf disk stars, a lower limit of Ω∗ > 0.005 I50 (at Hubble constant 50 kms −1 Mpc) is derived for the visible (processed gas + stars) mass density needed to generate an extragalactic background light (EBL) at a level of 50 I50 nWm −2 sr. The current ‘best-guess’ estimate to Ω∗ is 0.012 I50, about 16% of the nucleosynthetic baryon density. The contribution of quasar activity to the observed EBL is unlikely to exceed 20%.