Abstract

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 similar to 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]+H beta line emission. The Ly alpha line is reliably detected at 6.1 sigma 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 EW0 = 21 +/- 4 angstrom and is extended with V-FWHM = 360(-70)(+90) km s(-1). The source is perhaps the brightest and most massive z similar to 8 Lyman break galaxy in the full CANDELS and BoRG/HIPPIES surveys, having already assembled 10(9.9 +/- 0.2) M-circle dot 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]+H beta line equivalent widths. The redshift confirms that the IRAC [4.5] photometry is very likely dominated by line emission with EW0([O III]+H beta) = 720(-150)(+180) angstrom. 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.