The Stellar Mass Density at z~6 from Spitzer Imaging of i-drop Galaxies

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Abstract
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<z7 could have played a major role in reionizing the universe.

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