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Register a new userA Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z=7.730 using Keck/MOSFIRE
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P. A. Oesch
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We present a spectroscopic redshift measurement of a very bright Lyman break galaxy at z=7.7302+-0.0006 using Keck/MOSFIRE. 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 [OIII]+Hbeta line emission. The Lyalpha line is reliably detected at 6.1 sigma and shows an asymmetric profile as expected for a galaxy embedded in a relatively neutral inter-galactic medium near the Planck peak of cosmic reionization. The line has a rest-frame equivalent width of EW0=21+-4 A and is extended with V_FWHM=360+90-70 km/s. The source is perhaps the brightest and most massive z~8 Lyman break galaxy in the full CANDELS and BoRG/HIPPIES surveys, having assembled already 10^(9.9+-0.2) M_sol 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, formation epoch, as well as combined [OIII]+Hbeta line equivalent widths. The redshift confirms that the IRAC [4.5] photometry is very likely dominated by line emission with EW0(OIII+Hbeta)= 720-150+180 A. 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 JWST and, later, ELTs.
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We present the spectroscopic confirmation of a galaxy cluster at $z=2.095$ in the COSMOS field. This galaxy cluster was first reported in the ZFOURGE survey as harboring evolved massive galaxies using photometric redshifts derived with deep near-infrared (NIR) medium-band filters. We obtain medium resolution ($R sim$ 3600) NIR spectroscopy with MOSFIRE on the Keck 1 telescope and secure 180 redshifts in a $12times12$ region. We find a prominent spike of 57 galaxies at $z=2.095$ corresponding to the galaxy cluster. The cluster velocity dispersion is measured to be $sigma_{rm v1D}$ = 552 $pm$ 52 km/s. This is the first study of a galaxy cluster in this redshift range ($z gt 2.0$) with the combination of spectral resolution ($sim$26 km/s) and the number of confirmed members (${>}50$) needed to impose a meaningful constraint on the cluster velocity dispersion and map its members over a large field of view. Our $Lambda$CDM cosmological simulation suggests that this cluster will most likely evolve into a Virgo-like cluster with ${rm M_{vir}}{=}10^{14.4pm0.3} {rm M_odot}$ ($68%$ confidence) at $zsim$ 0. The theoretical expectation of finding such a cluster is $sim$ $4%$. Our results demonstrate the feasibility of studying galaxy clusters at $z > 2$ in the same detailed manner using multi-object NIR spectrographs as has been done in the optical in lower redshift clusters.
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We have observed the dust continuum of ten z=3.1 Lyman Break Galaxies with the Atacama Large Millimeter/Submillimeter Array at ~450 mas resolution in Band 7. We detect and resolve the 870um emission in one of the targets with an integrated flux density of S(870)=(192+/-57) uJy, and measure a stacked 3-sigma signal of S(870)=(67+/-23) uJy for the remaining nine. The total infrared luminosities estimated from full spectral energy distribution fits are L(8-1000um)=(8.4+/-2.3)x10^10 Lsun for the detection and L(8-1000um)=(2.9+/-0.9)x10^10 Lsun for the stack. With HST ACS I-band imaging we map the rest-frame UV emission on the same scale as the dust, effectively resolving the infrared excess (IRX=L_FIR/L_UV) in a normal galaxy at z=3. Integrated over the galaxy we measure IRX=0.56+/-0.15, and the galaxy-averaged UV slope is beta=-1.25+/-0.03. This puts the galaxy a factor of ~10 below the IRX-beta relation for local starburst nuclei of Meurer et al. (1999). However, IRX varies by more than a factor of 3 across the galaxy, and we conclude that the complex relative morphology of the dust relative to UV emission is largely responsible for the scatter in the IRX-beta relation at high-z. A naive application of a Meurer-like dust correction based on the UV slope would dramatically over-estimate the total star formation rate, and our results support growing evidence that when integrated over the galaxy, the typical conditions in high-z star-forming galaxies are not analogous to those in the local starburst nuclei used to establish the Meurer relation.
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