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
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
... 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.