Lyman Break Galaxies, Lyman Alpha Emitters and the Reionization of the Universe: A Simple Physical Model

We explore the onset of star formation in the early Universe, exploiting the high-redshift observations of Lyman-break galaxies (LBGs) and Lyman-alpha emitters (LAEs). Simple physical recipes for the evolution of the star formation rate (SFR) in protogalaxies, coupled with a SFR/metallicity-dependent dust extinction consistent with the available data, are shown to reproduce the UV luminosity functions (LFs) of LBGs and of LAEs at high-z. We argue that LBGs contributing to the observed LFs reside in galactic halos in the mass range 10^10 M_sun <~ M_H <~ 10^12 M_sun, while LAEs are confined within a much narrower range around 10^11 M_sun. The stellar masses of LBGs span the range 10^8 M_sun <~ M_star <~ 10^10 M_sun. In massive galaxies endowed with large SFRs the extinction increases rapidly, so that they shine as UV and Lyman-alpha emitters only for a time <~ 10^8 yr, i.e. for a short fraction of their life; thus such massive galaxies are much more easily detected as sub-millimeter sources. The X-ray luminosity of LBGs is expected to be dominated by high mass X-ray binaries and supernova remnants, although some level of nuclear activity due to accretion onto a central supermassive black hole should be present. Finally, we show that the intergalactic medium can be completely reionized at redshift z~6-7 by massive stars shining in protogalactic spheroids with halo masses from a few 10^10 M_sun to a few 10^11 M_sun, showing up as faint LBGs with magnitude in the range -17 <~ M_1350 <~ -20, without resorting to any special stellar Initial Mass Function