A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z = 7.730 Using Keck/MOSFIRE

We present a spectroscopic redshift measurement of a very bright Lyman break galaxy at z=7.7302+/- 0.0006 using the Keck/Multi-Object Spectrometer for Infra-Red Exploration. The source was pre-selected photometrically in the EGS field as a robust z ∼ 8 candidate with H = 25.0 mag based on optical non-detections and a very red Spitzer/IRAC [3.6]-[4.5] broad-band color driven by high equivalent width [O iii]+Hβ line emission. The Lyα line is reliably detected at 6.1σ and shows an asymmetric profile as expected for a galaxy embedded in a relatively neutral intergalactic medium near the Planck peak of cosmic reionization. The line has a rest-frame equivalent width of E{{W}₀}=21+/- 4 Å and is extended with {{V}_FWHM}=360_-70⁺⁹⁰ km s^-1. The source is perhaps the brightest and most massive z ∼ 8 Lyman break galaxy in the full CANDELS and BoRG/HIPPIES surveys, having already assembled {{10}^{9.9+/- 0.2}} {{M}_⊙ } of stars at only 650 Myr after the Big Bang. The spectroscopic redshift measurement sets a new redshift record for galaxies. This enables reliable constraints on the stellar mass, star formation rate, and formation epoch, as well as combined [O iii]+Hβ line equivalent widths. The redshift confirms that the IRAC [4.5] photometry is very likely dominated by line emission with EW₀([O iii]+Hβ) = 720_-150⁺¹⁸⁰ Å. This detection thus adds to the evidence that extreme rest-frame optical emission lines are a ubiquitous feature of early galaxies promising very efficient spectroscopic follow-up in the future with infrared spectroscopy using the James Webb Space Telescope and, later, ELTs.