The Stellar Mass Density at z~6 from Spitzer Imaging of i-drop Galaxies
We measure the ages, stellar masses, and star formation histories of z~6 galaxies, observed within 1Gyr of the Big Bang. We use imaging from HST and Spitzer from the public "Great Observatories Origins Deep Survey", coupled with ground-based near-infrared imaging, to measure the spectral energy distributions from 0.8-5microns, spanning the rest-frame UV and optical. From our sample of ~50 i-drop Lyman-break star-forming galaxies in GOODS-South with z'(AB)<27mag, we focus on ~30 with reliable photometric or spectroscopic redshifts. Half of these are confused with foreground sources at Spitzer resolution, but from the 16 with clean photometry we find that a surprisingly large fraction (40%) have evidence for substantial Balmer-breaks. This indicates the presence of old underlying stellar populations that dominate the stellar masses. For these objects, we find ages of 200-700Myr, implying formation redshifts of 7<z<14, and large stellar masses in the range 1-3x10^10M_sun. Analysis of 7 i-drops that are undetected at 3.6microns indicates that these are younger, considerably less massive systems. We calculate that line contamination should not severely affect our photometry or derived results. Using data out to 8microns, we find little evidence for substantial intrinsic dust reddening. Correcting for incompleteness in our sample, we find a lower limit on the comoving stellar mass density at z~6 to be 2.5x10^6M_sun/Mpc^3. We are able to explore the star formation histories of our selected galaxies, and we infer that the past global star formation rate may have been much higher than that observed at z~6. The associated UV flux we infer at z>7 could have played a major role in reionizing the universe.