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The First Stars and Galaxies: Challenges for the Next Decade Participants
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Talk
Title: Stellar Populations of Galaxies at 6.3 < z < 8.6 Author(s): Steven Finkelstein Abstract: We study the physical characteristics of galaxies at 6.3 < z < 8.6 selected from deep near-infrared imaging with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. Accounting for photometric scatter with simulations, galaxies at z ~ 7 have bluer UV colors compared to typical local starburst galaxies at > 4 sigma confidence. Although these colors necessitate young ages (<100 Myr), low or zero dust attenuation, and low metallicities, these are explicable by normal (albeit unreddened) stellar populations, with no evidence for near-zero metallicities and/or top-heavy initial mass functions. The age of the Universe at these redshifts limits the amount of stellar mass in late-type populations, and the WFC3 photometry implies galaxy stellar masses ~ 10^8 - 10^9 solar masses. The masses of typical z > 7 galaxies are smaller than those of "characteristic" Lyman break galaxies (LBGs) at lower redshifts, and are comparable to less evolved galaxies selected on the basis of their Lyman alpha emission at 3 < z < 6, implying that the 6.3 < z < 8.6 galaxies are the building blocks of more evolved galaxies at lower redshifts. We estimate that Lyman alpha emission can contribute to the observed WFC3 colors of galaxies at these redshifts, with an estimated typical line flux of roughly a factor of four below currently planned surveys. The integrated UV specific luminosity for the detected galaxies at z ~ 7 and z ~ 8 is within factors of a few of that required to reionize the IGM assuming low clumping factors, even with no correction for luminosity incompleteness or (likely minimal) dust extinction. This implies that in order to reionize the Universe, galaxies at these redshifts have a high (~ 50%) escape fraction of Lyman continuum photons, which may be substantiated by the very blue colors of this population.
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