ULTRADEEP INFRARED ARRAY CAMERA OBSERVATIONS OF SUB-L* z {approx} 7 AND z {approx} 8 GALAXIES IN THE HUBBLE ULTRA DEEP FIELD: THE CONTRIBUTION OF LOW-LUMINOSITY GALAXIES TO THE STELLAR MASS DENSITY AND REIONIZATION
- Carnegie Observatories, Pasadena, CA 91101 (United States)
- UCO/Lick Observatory, University of California, Santa Cruz, CA 95064 (United States)
- Institute for Astronomy, ETH Zurich, 8092 Zurich (Switzerland)
- Department of Astronomy, Yale University, New Haven, CT 06520 (United States)
- Leiden Observatory, Leiden University, NL-2300 RA Leiden (Netherlands)
- Space Telescope Science Institute, Baltimore, MD 21218 (United States)
- University of Colorado, Center for Astrophysics and Space Astronomy, 389-UCB, Boulder, CO 80309 (United States)
We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z {approx} 7 z {sub 850}-dropout galaxies and 5z {approx} 8 Y {sub 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 {sub 850}-dropouts and an upper limit for the Y {sub 105}-dropouts. We construct average broadband spectral energy distributions using the stacked Advanced Camera for Surveys (ACS), WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For the z {sub 850}-dropouts, we find z = 6.9{sup +0.1} {sub -0.1}, (U - V){sub rest} {approx} 0.4, reddening A{sub V} = 0, stellar mass (M*) = 1.2{sup +0.3} {sub -0.6} x 10{sup 9} M {sub sun} (Salpeter initial mass function). The best-fit ages {approx}300 Myr, M/L{sub V} {approx} 0.2, and SSFR {approx}1.7 Gyr{sup -1} are similar to values reported for luminous z {approx} 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{sup +0.1} {sub -0.2} Y {sub 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 {sub UV,AB} < -18 declines from {rho}*(z = 7) = 3.7{sup +1.4} {sub -1.8} x 10{sup 6} M {sub sun} Mpc{sup -3} to {rho}*(z = 8) < 8 x 10{sup 5} M {sub sun} Mpc{sup -3}, following {proportional_to}(1 + z){sup -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.
- OSTI ID:
- 21301439
- Journal Information:
- Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 708; ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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