MEASURING THE STELLAR MASSES OF z ∼ 7 GALAXIES WITH THE SPITZER ULTRAFAINT SURVEY PROGRAM (SURFS UP)

We present Spitzer/IRAC observations of nine z'-band dropouts highly magnified (2 ≲ μ ≲ 12) by the Bullet Cluster. We combine archival imaging with our Exploratory program (SURFS UP), which results in a total integration time of ∼30 hr per Infrared Array Camera (IRAC) band. We detect (≳ 3σ) 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] ∼ 26.4 mag and [4.5] ∼ 26.0 mag with stellar masses of ∼5 × 10⁷ M _☉. The detected galaxy has an estimated magnification of μ = 12 ± 4, which implies this galaxy has an ultraviolet luminosity of L₁₅₀₀∼0.3 L_z=7^∗—the lowest-luminosity individual source detected in IRAC at z ≳ 7. By modeling the broadband photometry, we estimate the galaxy has an intrinsic star formation rate (SFR) of SFR ∼ 1.3 M _☉ yr^–1 and stellar mass of M ∼ 2.0 × 10⁹ M _☉, which gives a specific star formation rate of sSFR ∼ 0.7 Gyr^–1. If this galaxy had sustained this SFR since z ∼ 20, it could have formed the observed stellar mass (to within a factor of ∼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