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

摘要

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 × 10~7M_⊙. 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 ~ 1.3M_⊙ yr~(-1) and stellar mass of M ~ 2.0×1~09M_⊙, 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]) = 1.6mJy.