ULTRADEEP INFRARED ARRAY CAMERA OBSERVATIONS OF SUB-L*z∼ 7 ANDz∼ 8 GALAXIES IN THE HUBBLE ULTRA DEEP FIELD: THE CONTRIBUTION OF LOW-LUMINOSITY GALAXIES TO THE STELLAR MASS DENSITY AND REIONIZATION

I. Labbé, V. González, R. J. Bouwens, G. D. Illingworth, P. A. Oesch, P. G. van Dokkum, C. M. Carollo, M. Franx, M. Stiavelli, M. Trenti, D. Magee, M. Kriek
2009 Astrophysical Journal Letters  
We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z=7 z_850-dropout galaxies and 5 z=8 Y_105-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6 micron and 4.5 micron. None of the galaxies are detected to [3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection for the z_850-dropouts and an upper
more » ... ts and an upper limit for the Y_105-dropouts. We construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For the z_850-dropouts, we find z=6.9^+0.1_-0.1, (U-V)_rest=0.4, reddening A_V=0, stellar mass M*=1.2^+0.3_-0.6 x 10^9 M_sun (Salpeter IMF). The best-fit ages 300Myr, M/L_V=0.2, and SSFR=1.7Gyr^-1 are similar to values reported for luminous z=7 galaxies, indicating the galaxies are smaller but not younger. The sub-L* galaxies observed here contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7<z<11. In contrast, the z=8.3^+0.1_-0.2 Y_105-dropouts have stellar masses that are uncertain by 1.5 dex due to the near-complete reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the stellar mass density to M_UV,AB < -18 declines from rho*(z=7)=3.7^+1.0_-1.8 x 10^6 M_sun Mpc^-3 to rho*(z=8) < 8 x 10^5 M_sun Mpc^-3, following (1+z)^-6 over 3<z<8. Lower masses at z=8 would signify more dramatic evolution, which can be established with deeper IRAC observations, long before the arrival of the James Webb Space Telescope.
doi:10.1088/2041-8205/708/1/l26 fatcat:7xveq72jwzarhaalnzyz2uetcm