Lyman α Emission from a Luminous z = 8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization

Abstract

We report the discovery of Lyman-alpha emission (Lyα) in the bright galaxy EGSY-2008532660 (hereafter EGSY8p7) using the Multi-Object Spectrometer For Infra-Red Exploration spectrograph at the Keck Observatory. First reported by Roberts-Borsani et al., this galaxy was selected for spectroscopic observations because of its photometric redshift (z_(phot) =(8.57)_(-0.43)^{+0.22)), apparent brightness (H_(160) = 25.26 ± 0.09$), 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 ~4.3 hr, our data reveal an emission line at ≃11776 Å that we argue is likely Lyα at a redshift of z_(spec) = {8.683}_(-0.004)^(+0.001), in good agreement with the photometric estimate. The line was detected independently on two nights using different slit orientations and its detection significance is ~ 7.5σ. 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 × 10^(−17) erg s^(−1) cm^(−2), corresponding to a rest-frame equivalent width of 17–42 Å. EGSY8p7 is the most distant spectroscopically confirmed galaxy to date, and the third luminous source in the EGS field beyond z_(phot) ≳ 7.5 with detectable Lyα 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 insights into the early formation of galaxies.

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