A SPECTROSCOPIC REDSHIFT MEASUREMENT FOR A LUMINOUS LYMAN BREAK GALAXY AT <i>z</i> = 7.730 USING KECK/MOSFIRE

Abstract

We present a spectroscopic redshift measurement of a very bright Lyman break\ngalaxy at z=7.7302+-0.0006 using Keck/MOSFIRE. The source was pre-selected\nphotometrically in the EGS field as a robust z~8 candidate with H=25.0 mag\nbased on optical non-detections and a very red Spitzer/IRAC [3.6]-[4.5]\nbroad-band color driven by high equivalent width [OIII]+Hbeta line emission.\nThe Lyalpha line is reliably detected at 6.1 sigma and shows an asymmetric\nprofile as expected for a galaxy embedded in a relatively neutral\ninter-galactic medium near the Planck peak of cosmic reionization. The line has\na rest-frame equivalent width of EW0=21+-4 A and is extended with\nV_FWHM=360+90-70 km/s. The source is perhaps the brightest and most massive z~8\nLyman break galaxy in the full CANDELS and BoRG/HIPPIES surveys, having\nassembled already 10^(9.9+-0.2) M_sol of stars at only 650 Myr after the Big\nBang. The spectroscopic redshift measurement sets a new redshift record for\ngalaxies. This enables reliable constraints on the stellar mass, star-formation\nrate, formation epoch, as well as combined [OIII]+Hbeta line equivalent widths.\nThe redshift confirms that the IRAC [4.5] photometry is very likely dominated\nby line emission with EW0(OIII+Hbeta)= 720-150+180 A. This detection thus adds\nto the evidence that extreme rest-frame optical emission lines are a ubiquitous\nfeature of early galaxies promising very efficient spectroscopic follow-up in\nthe future with infrared spectroscopy using JWST and, later, ELTs.\n