Star Formation Rates and Stellar Masses of z = 7-8 Galaxies from IRAC Observations of the WFC3/IR Early Release Science and the HUDF Fields

Abstract

We investigate the Spitzer/IRAC properties of 36 z ~ 7 z ₈₅₀-dropout galaxies and three z ~ 8 Y ₀₉₈ galaxies derived from deep/wide-area WFC3/IR data of the Early Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit stellar population synthesis models to the spectral energy distributions to derive mean redshifts, stellar masses, and ages. The z ~ 7 galaxies are best characterized by substantial ages (>100 Myr) and M/L_V ≈ 0.2. The main trend with decreasing luminosity is that of bluing of the far-UV slope from β ~ -2.0 to β ~ -3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1L* _{z = 3}), where low metallicity and smooth star formation histories (SFHs) fail to match the blue far-UV and moderately red H - [3.6] color. Such colors may require episodic SFHs with short periods of activity and quiescence ("on-off" cycles) and/or a contribution from emission lines. The stellar mass of our sample of z ~ 7 star-forming galaxies correlates with star formation rate (SFR) according to log M* = 8.70(±0.09) + 1.06(±0.10)log SFR, implying that star formation may have commenced at z > 10. No galaxies are found with SFRs much higher or lower than the past averaged SFR suggesting that the typical star formation timescales are probably a substantial fraction of the Hubble time. We report the first IRAC detection of Y ₀₉₈-dropout galaxies at z ~ 8. The average rest-frame U - V ≈ 0.3 (AB) of the three galaxies are similar to faint z ~ 7 galaxies, implying similar M/L. The stellar mass density to M _UV,AB < -18 is ρ*(z = 8) = 1.8^+0.7 _-1.0 × 10⁶ M _sun Mpc^-3, following log ρ*(z) = 10.6(±0.6) - 4.4(±0.7) log(1 + z) [M _sun Mpc^-3] over 3 < z < 8.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 11563, 9797. Based on observations with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through contract 125790 issued by JPL/Caltech. Based on service mode observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 073.A-0764A). Based on data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.