The evolution of z=7-8 galaxies from IRAC observations of the deep/wide-area WFC3/IR ERS and ultradeep WFC3/IR HUDF
I. Labbe, V. Gonzalez, R. J. Bouwens, G. D. Illingworth, M. Franx, M. Trenti, P. A. Oesch, P. G. van Dokkum, M. Stiavelli, C. M. Carollo, M. Kriek, D. Magee
Abstract
We investigate the Spitzer/IRAC properties of 36 z~7 z-dropout galaxies and 3 z~8 Y-dropout 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 SEDs to derive mean redshifts, stellar masses, and ages. The z~7 galaxies are best characterized by high ages (>300 Myr) and high M/L. The main trend with decreasing luminosity is that of bluing of the far-UV slope from beta~-2.0 to beta~-3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1~L^*_{z=3}) where low metallicity with smooth SFHs alone 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) or a contribution from emission lines. The stellar mass of our sample of z~7 star forming galaxies correlate with SFR according to log M*= 8.70 (+-0.09) + 1.06 (+-0.10) log SFR. The small scatter ~0.25 dex suggests that the galaxies have similar SFH, on average consistent with CSF since z>10. No galaxies are found with SFRs much higher or lower than the past averaged SFR; strongly rising SFR \propto t^alpha (alpha>1) or exponentially declining tau<t_Hubble SFHs are disfavored. We report the first robust IRAC detection of Y_098-dropout galaxies at z~8. The average rest-frame U-V~0.3 (AB) of the 3 galaxies are similar to faint z~7 galaxies, implying similar M/L and age. The stellar mass density to M_{UV,AB}<-18 is rho*(z=8)=1.8^{+0.7}_{-1.0} x 10^6 M_sun Mpc^{-3}, following log rho^*(z)=11.99-6 log(1+z) [M_sun Mpc^{-3}] over 3<z<8.