Low-Metallicity Star Formation in High-Redshift Galaxies at z~8

Abstract

Based on the recent very deep near-infrared imaging of the Hubble Ultra Deep Field with WFC3 on the Hubble Space Telescope, five groups published most probable samples of galaxies at z~8, selected by the so-called dropout method or photometric redshift; e.g., Y_105-dropouts (Y_105-J_125 > 0.8). These studies are highly useful for investigating both the early star formation history of galaxies and the sources of cosmic re-ionization. In order to better understand these issues, we carefully examine if there are low-$z$ interlopers in the samples of z~8 galaxy candidates. We focus on the strong emission-line galaxies at z~2 in this paper. Such galaxies may be selected as Y_105-dropouts since the [OIII] lambda 5007 emission line is redshifted into the J_125-band. We have found that the contamination from such low-$z$ interlopers is negligibly small. Therefore, all objects found by the five groups are free from this type of contamination. However, it remains difficult to extract real z~8 galaxies because all the sources are very faint and the different groups have found different candidates. With this in mind, we construct a robust sample of eight galaxies at z~8 from the objects found by the five groups: each of these eight objects has been selected by at least two groups. Using this sample, we discuss their UV continuum slope. We also discuss the escape fraction of ionizing photons adopting various metallicities. Our analysis suggests that massive stars forming in low-metallicity gas (Z~5 \times 10^-4 Z_sun) can be responsible for the completion of cosmic re-ionization if the escape fraction of ionizing continuum from galaxies is as large as 0.5, and this is consistent with the observed blue UV continua.