The GRISM LENS-AMPLIFIED SURVEY FROM SPACE (GLASS). III. A CENSUS OF LyαEMISSION AT $Z\gtrsim 7$ FROMHSTSPECTROSCOPY

Draft version November 16, 2015 Preprint typeset using L A TEX style emulateapj v. 5/2/11 THE GRISM LENS-AMPLIFIED SURVEY FROM SPACE (GLASS). III. A CENSUS OF Lyα EMISSION AT z & 7 FROM HST SPECTROSCOPY ˇ 4 , B. Vulcani 5 , K.-H., Huang 4 , A. Hoag 4 , M. Maseda 6 L. Guaita 7 K. B. Schmidt 1,2 , T. Treu 3 , M. Brada c L. Pentericci 7 , G. B. Brammer 8 , M. Dijkstra 9 , A. Dressler 10 A. Fontana 7 , A. L. Henry 11 , T. A. Jones 1 , C. Mason 1 , M. Trenti 12 , X. Wang 1 , 1 Department of Physics, University of California, Santa Barbara, CA, 93106-9530, USA Leibniz-Institut f¨ ur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany 3 Department of Physics and Astronomy, UCLA, Los Angeles, CA, 90095-1547, USA 4 Department of Physics, University of California, Davis, CA, 95616, USA Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, the University of Tokyo, Kashiwa, 277-8582, Japan 6 Max-Planck-Institut f¨ ur Astronomie, K¨ onigstuhl 17, D-69117 Heidelberg, Germany 7 INAF - Osservatorio Astronomico di Roma Via Frascati 33 - 00040 Monte Porzio Catone, I 8 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA 9 Institute of Theoretical Astrophysics, University of Oslo, Postboks 1029, 0858 Oslo, Norway 10 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA 11 Astrophysics Science Division, Goddard Space Flight Center, Code 665, Greenbelt, MD 20771 and 12 School of Physics, The University of Melbourne, VIC, 3010 Australia Draft version November 16, 2015 arXiv:1511.04205v1 [astro-ph.GA] 13 Nov 2015 ABSTRACT We present a census of Lyα emission at z & 7 utilizing deep near infrared HST grism spectroscopy from the first six completed clusters of the Grism Lens-Amplified Survey from Space (GLASS). In 24/159 photometrically selected galaxies we detect emission lines consistent with Lyα in the GLASS spectra. Based on the distribution of signal-to-noise ratios and on simulations we expect the com- pleteness and the purity of the sample to be 40-100% and 60-90%, respectively. For the objects without detected emission lines we show that the observed (not corrected for lensing magnification) 1σ flux limits reaches 5 × 10 −18 erg/s/cm 2 per position angle over the full wavelength range of GLASS (0.8–1.7µm). Based on the conditional probability of Lyα emission measured from the ground at z ∼ 7 we would have expected 12-18 Lyα emitters. This is consistent with the number of detections, within the uncertainties, confirming the drop in Lyα emission with respect to z ∼ 6. Deeper follow-up spectroscopy, here exemplified by Keck spectroscopy, is necessary to improve our estimates of com- pleteness and purity, and to confirm individual candidates as true Lyα emitters. These candidates include a promising source at z = 8.1. The spatial extent of Lyα in a deep stack of the most convincing Lyα emitters with hzi = 7.2 is consistent with that of the rest-frame UV continuum. Extended Lyα emission, if present, has a surface brightness below our detection limit, consistent with the properties of lower redshift comparison samples. From the stack we estimate upper limits on rest-frame UV emission line ratios and find f CIV /f Lyα . 0.32 and f CIII] /f Lyα . 0.23 in good agreement with other values published in the literature. Subject headings: galaxies: high-redshift – techniques: spectroscopic – methods: data analysis 1. INTRODUCTION With the deployment of the wide field camera 3 (WFC3) on the Hubble Space Telescope (HST ) in 2009, the samples of galaxies at the epoch of reionization, the phase-transition from a completely neutral inter-galactic medium (IGM) to a fully ionized IGM at z & 6, have grown dramatically. One of the main results of the WFC3 imaging campaigns has been the accurate deter- mination of the luminosity function of star forming high- redshift (based on their photometry) Lyman break galax- ies (e.g. Bouwens et al. 2015b; Finkelstein et al. 2015b). The UV luminosity functions of Lyman break galaxies have provided key constraints on the physics of reioniza- tion (e.g. Robertson et al. 2013; Schmidt et al. 2014a; Duffy et al. 2014). For example, it is clear that the pop- ulation of galaxies that has been detected so far cannot produce enough hard photons to keep the universe ion- ized. However, the luminosity function is found to have kbschmidt@aip.de a steep faint end slope (approximately φ ∝ L −2 ). Thus, faint galaxies could in principle provide enough ionizing photons (Bouwens et al. 2015a; Robertson et al. 2015; Barone-Nugent et al. 2015; Dressler et al. 2015). even though a contribution from AGN might end up being necessary (Madau & Haardt 2015; Giallongo et al. 2015). Also ground based spectroscopic follow-up of photo- metrically selected high-redshift candidates has been an important part of these studies and has provided addi- tional clues about the reionization epoch. Remarkably, only a handful of sources have been confirmed above red- shift 7 (Vanzella et al. 2011; Schenker et al. 2012, 2014; Ono et al. 2012; Finkelstein et al. 2013; Oesch et al. 2015; Roberts-Borsani et al. 2015; Zitrin et al. 2015b). The low probability of detecting Lyα in Lyman break galaxies, could be interpreted as the result of an increased optical depth in the IGM due to a significant fraction of neutral hydrogen. Thus the decline in detected Lyα is poten- tially a “smoking gun” of reionization (Fontana et al. 2010). The conditional probability of Lyα emission for