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