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{W0}=21+/- 4 Å and is extended with {VFWHM}=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 {109. 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.