2014ApJ...786L...4R

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 ∼5x10⁷ 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 and stellar mass of M ∼ 2.0x10⁹ M_☉, which gives a specific star formation rate of sSFR ∼ 0.7 G/yr. 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.