We present new evidence for mature stellar populations with ages > 100 Myr in massive galaxies ( M _ { stellar } > 10 ^ { 10 } M _ { \odot } ) seen at a time when the Universe was less than 1 Gyr old .
We analyse the prominent detections of two z \approx 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 ^ { \prime } -drop ” Lyman break technique , and subsequently confirmed spectroscopically with the Keck telescope .
The new Spitzer photometry reveals significant Balmer/4000 Å discontinuities , indicative of dominant stellar populations with ages > 100 Myr .
Fitting a range of population synthesis models ( for normal initial mass functions ) to the HST/Spitzer photometry yields ages of 250 - 650 Myr and implied formation redshifts z _ { f } \approx 7.5 - 13.5 in presently-accepted world models .
Remarkably , our sources have best-fit stellar masses of 1.3 - 3.8 \times 10 ^ { 10 } M _ { \odot } ( 95 % confidence ) assuming a Salpeter IMF .
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 } \sim 5 - 30 M _ { \odot } { yr } ^ { -1 } ) , 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 \approx 6 suggests long-lived sources at earlier epochs ( z > 7 ) played a key role in reionizing the Universe .