We study the Spitzer Infrared Array Camera ( IRAC ) mid-infrared ( rest-frame optical ) fluxes of 14 newly WFC3/IR-detected z \sim 7 z _ { 850 } - dropout galaxies and 5 z \sim 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 \mu m and 4.5 \mu m. None of the galaxies are detected to [ 3.6 ] \approx 26.9 ( AB , 2 \sigma ) , but a stacking analysis reveals a robust detection for the z _ { 850 } -dropouts and an upper limit for the Y _ { 105 } - dropouts .
We construct average broadband spectral energy distributions 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 } \approx 0.4 , reddening A _ { V } = 0 , stellar mass < M ^ { * } > = 1.2 ^ { +0.3 } _ { -0.6 } \times 10 ^ { 9 } ~ { } M _ { \sun } ( Salpeter initial mass function ) .
The best-fit ages \sim 300 ~ { } Myr , M / L _ { V } \approx 0.2 , and SSFR \sim 1.7 Gyr ^ { -1 } are similar to values reported for luminous z \sim 7 galaxies , indicating the galaxies are smaller but not much 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 ^ { +1.4 } _ { -1.8 } \times 10 ^ { 6 } ~ { } M _ { \sun } ~ { } Mpc ^ { -3 } to \rho ^ { * } ( z = 8 ) < 8 \times 10 ^ { 5 } ~ { } M _ { \sun } ~ { } Mpc ^ { -3 } , following \propto ( 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 .