On the Stellar Populations and Evolution of Star-Forming Galaxies at 6.3 < z < 8.6

Understand

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 board the Hubble Space Telescope.

• Accounting for the photometric scatter using 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 Msol for Salpeter initial mass functions to a limiting magnitude of M_1500 ~ -18.