Low-metallicity star formation in high-redshift galaxies at z ∼ 8

Using the very deep near-infrared imaging of the Hubble Ultra Deep Field with WFC3 on the Hubble Space Telescope, five groups published the most probable samples of galaxies at z ∼ 8, selected by the so-called dropout method or photometric redshift. 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 whether there are low-z interlopers in the samples of z ∼ 8 galaxy candidates. We focus on the strong emissionline galaxies at z ∼ 2. Such galaxies may be selected as Y₁₀₅-dropouts since the [O III] λ 5007 emission line is redshifted into the J₁₂₅ 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 is still 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 and the escape fraction of ionizing photons for various metallicities. Our analysis suggests that massive stars forming in low-metallicity gas (⁠$Z∼5×10−4Z⊙$⁠) can be responsible for the completion of cosmic re-ionization if the escape fraction of the ionizing continuum from galaxies is as large as 0.5, and this is consistent with the observed blue UV continua.