Long-duration Gamma-Ray Bursts ( GRBs ) allow us to pinpoint and study star-forming galaxies in the early universe , thanks to their orders of magnitude brighter peak luminosities compared to other astrophysical sources , and their association with deaths of massive stars .
We present Hubble Space Telescope Wide Field Camera 3 detections of three Swift GRB host galaxies 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 , \lambda _ { obs } \sim 1.4 \micron ) filter .
The hosts have magnitudes ( corrected for Galactic extinction ) of m _ { { \lambda _ { obs } ,AB } } = 26.34 ^ { +0.14 } _ { -0.16 } , 27.56 ^ { +0.18 } _ { -0.22 } , and 28.30 ^ { +0.25 } _ { -0.33 } respectively .
In all three cases the probability of chance coincidence of lower redshift galaxies is \lesssim 2 \% , 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 - 0.6 L ^ { * } _ { z = 6 } ( with M _ { 1600 } ^ { * } = -20.95 \pm 0.12 ) , and half-light radii in the range 0.6 - 0.9 kpc .
Both their half-light radii and luminosities are consistent with existing samples of Lyman-break galaxies at z \sim 6 .
Spectroscopic analysis of the GRB afterglows indicate low metallicities ( [ M / H ] \lesssim - 1 ) and low dust extinction ( A _ { V } \lesssim 0.1 ) along the line of sight .
Using stellar population synthesis models , we explore the implications of each galaxy ’ s luminosity for its possible star formation history , and consider the potential for emission-line metallicity determination with the upcoming James Webb Space Telescope .