Lyman-alpha Emission from a Luminous z=8.68 Galaxy: Implications for
  Galaxies as Tracers of Cosmic Reionization
  We report the discovery of Lyman-alpha emission (Ly$\alpha$) in the bright
galaxy EGSY-2008532660 (hereafter EGSY8p7) using the MOSFIRE spectrograph at
the Keck Observatory. First reported by Roberts-Borsani et al. (2015), it was
selected for spectroscopic observations because of its photometric redshift
($z_{phot}=8.57^{+0.22}_{-0.43}$), apparent brightness (H$_{160}=25.26\pm0.09$)
and red Spitzer/IRAC [3.6]-[4.5] color indicative of contamination by strong
oxygen emission in the [4.5] band. With a total integration of $\sim$4.3 hours,
our data reveal an emission line at $\simeq$11776 {\AA} which we argue is
likely Ly$\alpha$ at a redshift $z_{spec}=8.683^{+0.001}_{-0.004}$, in good
agreement with the photometric estimate. The line was detected independently on
two nights using different slit orientations and its detection significance is
$\sim7.5\sigma$. An overlapping skyline contributes significantly to the
uncertainty on the total line flux although the significance of the detected
line is robust to a variety of skyline-masking procedures. By direct addition
and a Gaussian fit, we estimate a 95\% confidence range of
1.0--2.5$\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$, corresponding to a rest-frame
equivalent width of 17--42 {\AA}. EGSY8p7 is the most distant galaxy confirmed
spectroscopically to date, and the third luminous source in the EGS field
beyond $z_{phot}\gtrsim7.5$ with detectable Ly$\alpha$ emission viewed at a
time when the intergalactic medium is believed to be fairly neutral. Although
the reionization process was probably patchy, we discuss whether luminous
sources with prominent IRAC color excesses may harbor harder ionizing spectra
than the dominant fainter population thereby creating earlier ionized bubbles.
Further spectroscopic follow-up of such bright sources promises important
insight into the early formation of galaxies.
