2008ApJ...680...32C

I provide estimates of the ultraviolet and visible light luminosity density at z∼6 after accounting for the contribution from faint galaxies below the detection limit of deep HST and Spitzer surveys. I find that the rest-frame V-band luminosity density is a factor of ∼2-3 below the ultraviolet luminosity density at z∼6. This implies that the maximal age of the stellar population at z∼6, for a Salpeter initial mass function (IMF) and a single, passively evolving burst, must be ≲100 Myr. If the stars in z∼6 galaxies are remnants of the star formation that was responsible for ionizing the intergalactic medium, reionization must have been a brief process that was completed at z<7. This assumes the most current estimates of the clumping factor and escape fraction and a Salpeter slope extending up to 200 M_☉ for the stellar IMF (dN/dM∝M^α, α=-2.3). Unless the ratio of the clumping factor to escape fraction is less than 60, a Salpeter slope for the stellar IMF and reionization redshift higher than 7 are ruled out. In order to maintain an ionized intergalactic medium from redshift 9 onward, the stellar IMF must have a slope of α=-1.65 even if stars as massive as ∼200 M_☉ are formed. Correspondingly, if the intergalactic medium was ionized from redshift 11 onward, the IMF must have α~-1.5. The range of stellar mass densities at z∼6 straddled by IMFs which result in reionization at z>7 is 1.3±0.4x10⁷ M_☉/Mpc3.