Measuring the Stellar Masses of z ~ 7 Galaxies with the Spitzer UltRaFaint SUrvey Program (SURFS UP)
Schrabback, T.; Bradač, M.; Huang, K. -H.; Treu, T.; von der Linden, A.; Zaritsky, D.; Stiavelli, M.; Gladders, M.; Gonzalez, A. H.; Lemaux, B. C.; Casertano, S.; Ryan, R. E., Jr.; Lubin, L.; Allen, S.; Hinz, J.; Cain, B.; Hall, N.; Hildebradt, H.
United States, France, Germany, Denmark
Abstract
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 (gsim 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 L_{1500}\sim 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 langf [C II]rang = 1.6 mJy.
Observations were carried out using the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research is also based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555 and NNX08AD79G. These observations are associated with programs Spitzer 3550, 60034, 90009, HST GO 10200, GO 10863, 11099, and 11591, and ESO Large Program 181.A-0485.