Lyman Break Galaxies and the Reionization of the Intergalactic Medium

Near-infrared observations of Lyman-break galaxies at redshifts z~3 arebeginning to provide constraints on ages, star-formation histories, dustcontent, metallicities, and stellar masses. At present, uncertainties of morethan an order of magnitude are typical for many of these parameters. It isnonetheless interesting to ask what the stellar-population models imply for theexistence and luminosities of Lyman-break galaxies at higher redshift. To thisend we examine the inferred star-formation rates in two well-studied samples ofgalaxies as a function of redshift out to z = 10 for various best-fit andlimiting cases. Taken at face value, the generally young ages (typically 10^8 +- 0.5 yr) ofthe z = 3 Lyman break galaxies imply that their stars were not present muchbeyond z=4. By z = 6 the cosmic star-formation rate from the progenitors ofthese galaxies is less than 10% of star-formation rate at z=3 +- 0.5, even formaximally-old models, provided the derivative of the star-formation rate SFR(t)is monotonic. The escaping Lyman-continuum radiation from such galaxies wouldbe insufficient to reionize the IGM. Thus other sources of ionizing photons(e.g. very massive population III stars) may be needed, and the more normalLyman-break galaxies may be a phenomenon confined to redshifts z <~ 4. Thisconclusion changes if SFR(t) was episodic, and we examine the parameters ofsuch bursty star-formation that might be consistent with both the z=2-4luminosity functions and the z~3 spectral energy distributions.Comment: Accepted for publication in ApJ, Letters. 12 pages, 2 figure