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

TL;DR

This study analyzes galaxies at redshifts 6.3 to 8.6, revealing their young ages, low dust and metallicity, and their role as progenitors of lower-redshift galaxies, with implications for cosmic reionization.

Contribution

It provides detailed physical characterization of high-redshift galaxies using WFC3 data, highlighting their properties and potential contribution to reionization, with no evidence for top-heavy IMFs.

Findings

Galaxies at z ~ 7 are bluer and younger than local starbursts.

Stellar masses are around 10^8 - 10^9 M_sun, smaller than lower-redshift L* galaxies.

High escape fraction (~50%) of ionizing photons suggests significant role in reionization.

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 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. The masses of ``characteristic'' (L*) z > 7 galaxies are smaller than those of L* 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 progenitors of more evolved galaxies at lower redshifts. We estimate that Lyman alpha emission is able to contribute to the observed WFC3 colors of galaxies at these redshifts, with an estimated typical line flux of ~ 10^-18 erg s^-1 cm^-2, 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, implying that in order to reionize the Universe galaxies at these redshifts have a high ( ~ 50%) escape fraction of Lyman continuum photons, possibly substantiated by the very blue colors of this population.