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

We present Spitzer/IRAC observations of nine z'-band dropouts highly magnified (2 less than or similar to mu less than or similar to 12) by the Bullet Cluster. We combine archival imaging with our Exploratory program (SURFS UP), which results in a total integration time of similar to 30 hr per Infrared Array Camera (IRAC) band. We detect (greater than or similar to 3 sigma) in both IRAC bands the brightest of these high-redshift galaxies, with [3.6] = 23.80 +/- 0.28 mag, [4.5] = 23.78 +/- 0.25 mag, and (H - [3.6]) = 1.17 +/- 0.32 mag. The remaining eight galaxies are undetected to [3.6] similar to 26.4 mag and [4.5] similar to 26.0 mag with stellar masses of similar to 5 x 10(7) M-circle dot. The detected galaxy has an estimated magnification of mu = 12 +/- 4, which implies this galaxy has an ultraviolet luminosity of L-1500 similar to 0.3 Lz-7*-the lowest-luminosity individual source detected in IRAC at z greater than or similar to 7. By modeling the broadband photometry, we estimate the galaxy has an intrinsic star formation rate (SFR) of SFR similar to 1.3 M-circle dot yr(-1) and stellar mass of M similar to 2.0 x 10(9) M-circle dot, which gives a specific star formation rate of sSFR similar to 0.7 Gyr(-1). If this galaxy had sustained this SFR since z similar to 20, it could have formed the observed stellar mass (to within a factor of similar to 2). We also discuss alternate star formation histories and argue that the exponentially increasing model is unlikely. Finally, based on the intrinsic SFR, we estimate that this galaxy has a likely [C II] flux of < f([C II])> = 1.6 mJy.