Spitzer Imaging of i'-drop Galaxies: Old Stars at z~6

We present new evidence for mature stellar populations with ages >100Myr in massive galaxies (M_stellar>10^10M_sun) seen at a time when the Universe was less than 1Gyr old. We analyse the prominent detections of two z~6 star-forming galaxies (SBM03#1 & #3) made at wavelengths corresponding to the rest-frame optical using the IRAC camera onboard the Spitzer Space Telescope. We had previously identified these galaxies in HST/ACS GOODS images of Chandra Deep Field South through the "i-drop" Lyman break technique, and subsequently confirmed spectroscopically with the Keck telescope. The new Spitzer photometry reveals significant Balmer/4000Ang discontinuities, indicative of dominant stellar populations with ages >100Myr. Fitting a range of population synthesis models (for normal initial mass functions) to the HST/Spitzer photometry yields ages of 250-650Myr and implied formation redshifts z~7.5-13.5 in presently-accepted world models. Remarkably, our sources have best-fit stellar masses of 1.3-3.8x10^10M_sun (95% confidence) assuming a Salpeter initial mass function. This indicates that at least some galaxies with stellar masses >20% of those of a present-day L* galaxy had already assembled within the first Gyr after the Big Bang. We also deduce that the past average star formation rate must be comparable to the current observed rate (SFR_UV~5-30M_sun/yr), suggesting that there may have been more vigorous episodes of star formation in such systems at higher redshifts. Although a small sample, limited primarily by Spitzer's detection efficiency, our result lends support to the hypothesis advocated in our earlier analyses of the Ultra Deep Field and GOODS HST/ACS data. The presence of established systems at z~6 suggests long-lived sources at earlier epochs (z>7) played a key role in reionizing the Universe.