I provide estimates of the ultraviolet and visible light luminosity density at z \sim 6 after accounting for the contribution from faint galaxies below the detection limit of deep Hubble and Spitzer surveys .
I find the rest-frame V - band luminosity density is a factor of \sim 2 - 3 below the ultraviolet luminosity density at z \sim 6 .
This implies that the maximal age of the stellar population at z \sim 6 , for a Salpeter initial mass function , and a single , passively evolving burst , must be \lesssim 100 Myr .
If the stars in z \sim 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 _ { \sun } for the stellar initial mass function ( IMF ; dN/dM \propto M ^ { \alpha } , \alpha = -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 is ruled out .
In order to maintain an ionized intergalactic medium from redshift 9 onwards , the stellar IMF must have a slope of \alpha = -1.65 even if stars as massive as \sim 200 M _ { \sun } are formed .
Correspondingly , if the intergalactic medium was ionized from redshift 11 onwards , the IMF must have \alpha \sim - 1.5 .
The range of stellar mass densities at z \sim 6 straddled by IMFs which result in reionization at z > 7 is 1.3 \pm 0.4 \times 10 ^ { 7 } M _ { \sun } Mpc ^ { -3 } .