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We present Spitzer/IRAC observations of nine $z'$-band dropouts highlymagnified (2<mu<12) by the Bullet Cluster. We combine archival imaging with ourExploratory program (SURFS UP), which results in a total integration time of~30 hr per IRAC band. We detect (>3sigma) in both IRAC bands the brightest ofthese high-redshift galaxies, with [3.6]=23.80+-0.28 mag, [4.5]=23.78+-0.25mag, and (H-[3.6])=1.17+-0.32 mag. The remaining eight galaxies are undetectedto [3.6]~26.4 mag and [4.5]~26.0 mag with stellar masses of ~5x10^7 M_sol. Thedetected galaxy has an estimated magnification of mu=12+-4, which implies thisgalaxy has an ultraviolet luminosity of L_1500~0.3 L*_{z=7} --- the lowestluminosity individual source detected in IRAC at z>7. By modeling the broadbandphotometry, we estimate the galaxy has an intrinsic star-formation rate ofSFR~1.3 M_sol/yr and stellar mass of M~2x10^9 M_sol, which gives a specificstar-formation rate of sSFR~0.7 Gyr^-1. If this galaxy had sustained thisstar-formation rate since z~20, it could have formed the observed stellar mass(to within a factor of ~2), we also discuss alternate star-formation historiesand argue the exponentially-increasing model is unlikely. Finally, based on theintrinsic star-formation rate, we estimate this galaxy has a likely [C II] fluxof <f_[C II]> = 10^{-17} erg/s/cm2.

Measuring the Stellar Masses of z~7 Galaxies with Spitzer Ultrafaint Survey Program (SURFS UP)