* Astronomy

Title: Lyman-alpha Emission from a Luminous z=8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization
Author: Adi Zitrin, Ivo Labbe, Sirio Belli, Rychard Bouwens, Richard S. Ellis, Guido Roberts-Borsani, Daniel P. Stark, Pascal A. Oesch, Renske Smit

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±0.09) and red Spitzer/IRAC [3.6]-[4.5] colour indicative of contamination by strong oxygen emission in the [4.5] band. With a total integration of ~4.3 hours, our data reveal an emission line at \simeq 11776 {\AA} which we argue is likely Ly at a redshift z_spec=8.68, in good agreement with the photometric estimate. The line was detected independently on two nights using different slit orientations and its detection significance is ~7.5 sigma. An overlapping sky line contributes significantly to the uncertainty on the total line flux but not the overall significance. By direct addition and a Gaussian fit, we estimate a 95% confidence range of 1.0 - 2.5 x 10^-17 ergs cm^-2 sec^-1, 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 \gtrsim 7.5 with detectable Ly alpha emission viewed at a time when the intergalactic medium is expected to be fairly neutral. Although the reionisation process was probably patchy, we discuss whether luminous sources with prominent IRAC colour excesses may harbour 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.

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