astro-ph/9907268

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ABSTRACT. 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 UBVIJHK 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 n W m^-2 sr^-1, 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 _* > 0.005 I₅₀ (at Hubble constant 50 km s^-1 Mpc^-1) is derived for the visible (processed gas + stars) mass density needed to generate an extragalactic background light (EBL) at a level of 50 I₅₀ n W m^-2 sr^-1. The current ``best-guess'' estimate to _* 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%.

Table of Contents

INTRODUCTION

GALAXY COUNTS

THE EXTRAGALACTIC BACKGROUND LIGHT

EBL FROM QUASAR ACTIVITY

THE STELLAR MASS DENSITY TODAY

REFERENCES