Spitzer Imaging of i`-drop Galaxies: Old Stars at z ≈ 6 Laurence P. Eyles 1, Andrew J. Bunker 1, Elizabeth R. Stanway 2, Mark Lacy 3, Richard S. Ellis 4, Michelle Doherty 5 1 University of Exeter, UK 2 University of Wisconsin-Madison, USA, 3 Spitzer Science Center, Caltech, USA, 4 Caltech Astronomy, USA, 5 Institute of Astronomy, Cambridge, We present new evidence for mature stellar populations with ages > 100Myr in massive galaxies (stellar mass > solar masses), seen at a time when the Universe was less than 1Gyr old. We analyse the prominent detections of two z ≈ 6 star-forming galaxies (objects #1 & #3 in Stanway, Bunker & McMahon 2003) in the GOODS-South/CDFS field, made at wavelengths corresponding to the rest-frame optical using the IRAC camera onboard the Spitzer Space Telescope. Fitting a range of population synthesis models (for normal initial mass functions) to the HST/Spitzer photometry (along with supplementary VLT/ISAAC data) yields ages of Myr and implied formation redshifts z f ≈ in presently accepted world models. Remarkably, our sources have best-fit stellar masses of 2-4 x solar masses (95% confidence) assuming a Salpeter IMF (see plot below). This indicates that at least some galaxies with stellar masses > 20% that of a present-day L* galaxy had already assembled within 1Gyr after the Big Bang. By investigating the ratio of a characteristic growth timescale to the Hubble time at the z  6 epoch, we deduce that the SFRs of these galaxies are in a declining state. Therefore we infer that the past average star formation rate must be comparable to, or greater than, the current observed rate (SFR UV ≈ 5 – 30 solar masses / yr) in order to account for the stellar mass, suggesting that there may have been more vigorous episodes of star formation in such systems at higher redshifts. We had previously identified these galaxies in HST/ACS GOODS images of Chandra Deep Field South (example top right) through the “i`- drop” Lyman break technique, and subsequently confirmed spectroscopically with the Keck telescope. The new Spitzer photometry reveals significant Balmer/4000 discontinuities, brightening by a factor of 2 in f ν from the near-infrared (z`, J, Ks) wavebands to the Spitzer/IRAC 3.6μm filter. These breaks are indicative of dominant stellar populations with ages > 100Myr (see plot below). The Spitzer Space Telescope (credit: NASA/Jet Propulsion Laboratory) Best-fitting two-component stellar population model for SBM03#1 (solid line): dominant 450Myr old population (dotted line), with some ongoing star- forming activity (broken line). SBM03#1: Allowed mass range for burst, exponentially-decaying (τ), and continuous SFR models. Shaded regions indicate models that are deemed unrealistic. Contours are 68% confidence (solid line), 95% confidence (dashed line) and 99% confidence (dotted line). SBM03#3 (circled) viewed with HST/ACS. Image is a combination of images taken in red, green and blue filters. Field of view is 10 arcsec across. SBM03#1 seen at several different wavelengths. Each image is 8 arcsec diameter. 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 declining global star formation density and presence of established systems at z ≈ 6 suggests long-lived sources at earlier epochs (z > 7) played a key role in reionizing the Universe.