Abstract
Gamma-ray bursts allow us to pinpoint and study star-forming galaxies in the
early universe, thanks to their immense luminosities and association with
deaths of massive stars. We present Hubble Space Telescope Wide Field
Camera 3 detections of three Swift GRBs lying at redshifts $z = 5.913$
(GRB 130606A), $z = 6.295$ (GRB 050904), and $z = 6.327$ (GRB 140515A) in the
F140W (wide-$JH$ band, $łambda_obs\sim1.4\,m$) filter. The hosts
have magnitudes (corrected for Galactic extinction) of
$m_łambda_obs,AB= 26.26^+0.12_-0.14, 27.63^+0.16_-0.18,$ and
$28.23^+0.24_-0.30$ respectively. In all three cases the probability of
chance coincidence of lower redshift galaxies is $łesssim1.5\%$, indicating
that the detected galaxies are most likely the GRB hosts. These are the first
detections of high redshift ($z > 5$) GRB host galaxies in emission. The
galaxies have luminosities in the range $0.1-0.7\,L^*_z=6$ (with
$M_1600^*=-20.95\pm0.12$), along with half-light radii in the range
$0.6-1.2\,kpc$. Both their half-light radii and luminosities are
consistent with existing samples of Lyman-break galaxies at $z\sim6$.
Spectroscopic analysis of the GRB afterglows indicate low metallicities
($M/Hłesssim-1$) and dust extinction ($A_Vłesssim0.1$) along
the line of sight. We consider the implications of these luminosities for their
possible star formation histories and the potential for emission line
metallicity determinations with James Webb Space Telescope spectroscopy.
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