Lyman Break Galaxies in the NGST Era

With SIRTF and NGST in the offing, it is interesting to examine what the stellar populations of z ≈ 3 galaxies models imply for the existence and nature of Lyman-break galaxies at higher redshift. To this end, we “turn back the clock” on the stellar population models that have been fit to optical and infrared data of Lymanbreak galaxies at z ≈ 3 . The generally young ages (typically 108±0.5 yr) of these galaxies imply that their stars were not present much beyond z = 4. For smooth star-formation histories SFR(t) and Salpeter IMFs, the ionizing radiation from early star-formation in these galaxies would be insufficient to reionize the intergalactic medium at z ≈ 6, and the luminosity density at z ≈ 4 would be significantly lower than observed. We examine possible ways to increase the global star-formation rate at higher redshift without violating the stellar-population constraints at z ≈ 3.