A Lyman Break Galaxy in the Epoch of Reionization from Hubble Space Telescope Grism Spectroscopy

The Astrophysical Journal, 773:32 (7pp), 2013 August 10 doi:10.1088/0004-637X/773/1/32 C 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A. A LYMAN BREAK GALAXY IN THE EPOCH OF REIONIZATION FROM HUBBLE SPACE TELESCOPE GRISM SPECTROSCOPY James E. Rhoads1, Sangeeta Malhotra1, Daniel Stern2, Mark Dickinson3, Norbert Pirzkal4, Hyron Spinrad5, Naveen Reddy6, Nimish Hathi7, Norman Grogin4, Anton Koekemoer4, Michael A. Peth4,8, Seth Cohen1, Zhenya Zheng1, Tamas Budavari8, Ignacio Ferreras9, Jonathan P. Gardner10, Caryl Gronwall11, Zoltan Haiman12, Martin Kummel¨ 13, Gerhardt Meurer14, Leonidas Moustakas2, Nino Panagia4,15,16, Anna Pasquali17, Kailash Sahu4, Sperello di Serego Alighieri18, Rachel Somerville19, Amber Straughn10, Jeremy Walsh20, Rogier Windhorst1, Chun Xu21, and Haojing Yan22 1 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA; [email protected] 2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 3 National Optical Astronomy Observatory, Tucson, AZ, USA 4 Space Telescope Science Institute, Baltimore, MD, USA 5 University of California, Berkeley, CA, USA 6 University of California, Riverside, CA, USA 7 Observatories of the Carnegie Institution of Washington, Pasadena, CA, USA 8 Johns Hopkins University, Baltimore, MD, USA 9 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK 10 NASA Goddard Space Flight Center, Astrophysics Science Division, Observational Cosmology Laboratory, Code 665, Greenbelt, MD 20771, USA 11 Department of Astronomy & Astrophysics, and Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA 12 Columbia University, New York, NY, USA 13 Universitats-Sternwarte¨ Munchen,¨ Scheinerstr. 1, D-81679 Munchen,¨ Germany 14 International Centre for Radio Astronomy Research, University of Western Australia, M468, 35 Stirling Highway, Crawley, WA 6009, Australia 15 INAF–Osservatorio Astrofisico di Catania, Italy 16 Supernova Ltd, OYV #131, Northsound Road, Virgin Gorda, VG1150, British Virgin Islands 17 Astronomisches Rechen-Institut, Zentrum fur¨ Astronomie der Universitat¨ Heidelberg, Monchhofstrasse¨ 12–14, D-69120 Heidelberg, Germany 18 INAF–Osservatorio Astrofisico di Arcetri, Italy 19 Rutgers University, Piscataway, NJ, USA 20 European Southern Observatory, Garching, Germany 21 Shanghai Institute of Technical Physics, Shanghai, China 22 Department of Physics and Astronomy, 223 Physics Bldg., University of Missouri, Columbia, MO 65211-7010, USA Received 2013 February 28; accepted 2013 May 22; published 2013 July 22 ABSTRACT We present observations of a luminous galaxy at z = 6.573—the end of the reionization epoch—which has been spectroscopically confirmed twice. The first spectroscopic confirmation comes from slitless Hubble Space Telescope Advanced Camera for Surveys grism spectra from the PEARS survey (Probing Evolution And Reionization Spectroscopically), which show a dramatic continuum break in the spectrum at rest frame 1216 Å. The second confirmation is done with Keck + DEIMOS. The continuum is not clearly detected with ground-based spectra, but high wavelength resolution enables the Lyα emission line profile to be determined. We compare the line profile to composite line profiles at z = 4.5. The Lyα line profile shows no signature of a damping wing attenuation, confirming that the intergalactic gas is ionized at z = 6.57. Spectra of Lyman breaks at yet higher redshifts will be possible using comparably deep observations with IR-sensitive grisms, even at redshifts where Lyα is too attenuated by the neutral intergalactic medium to be detectable using traditional spectroscopy from the ground. Key words: galaxies: evolution – galaxies: formation – galaxies: high-redshift Online-only material: color figures 1. INTRODUCTION range of possible outcomes from these various processes is to take a direct, observational census of galaxies throughout the To properly understand the history of cosmic dawn, we must reionization era—from its end at 6 z 7, back to the earliest be able to reliably identify galaxies observed during the epoch galaxies we can identify. of reionization. Such galaxies are the most likely sources of Much progress has been made recently in this direction, due the radiation that ionized intergalactic hydrogen. They are the primarily to the dramatic increase in near-infrared imaging best places to look for signatures of primordial star formation: sensitivity and survey efficiency afforded by the Wide Field even if the buildup of heavy elements is rapid, the fraction Camera 3 (WFC3) Infrared (IR) channel on the Hubble Space of galaxies forming their first generations of stars should be Telescope (HST). Imaging surveys with WFC3-IR have provided higher if we observe them when the universe itself was young. tens to hundreds of z>7 galaxy candidates, identified by the The pace of their growth depends on incompletely understood Lyα absorption break in their broad band colors (e.g., Bouwens physical processes—both the onset of star-formation in low- et al. 2010; Yan et al. 2010, 2012; Finkelstein et al. 2012). (We metallicity conditions, and the potential disruption of later star- will refer to these as “Lyman break galaxies (LBGs),” while formation by the ionizing radiation and/or supernovae produced noting that selection by a strong continuum break can identify by the first stellar generation. The best way to constrain the either the 912 Å break due to Lyman continuum absorption, or 1 The Astrophysical Journal, 773:32 (7pp), 2013 August 10 Rhoads et al. the 1216 Å break due to Lyα absorption. Since the Lyα forest is 2. PEARS GRISM OBSERVATIONS optically thick for z 5, surveys for z>5 galaxies use the Lyα absorption break, while those at z 3 primarily identify the PEARS is the most extensive systematic survey conducted 912 Å break.) These broad band HST searches have broken new with the G800L grism on the HST’s ACS Wide Field Camera ground, primarily because the NIR sky is orders of magnitude (ACS-WFC). PEARS is an HST Treasury program led by darker in space. Alternative, ground-based search methods can S. Malhotra (program ID HST-GO-10530). It covers a total find Lyα emitting galaxies efficiently at selected redshifts (z = of nine fields, including one deep pointing in the Hubble Ultra 6.5, 6.9, 7.3, 7.7, 8.8) where the line falls in dark windows in Deep Field (HUDF), and eight wide-field pointings (four each in the night sky spectrum, using either narrow bandpass imaging the GOODS-North and GOODS-South regions). Each pointing (e.g., Hu et al. 2002, 2010; Rhoads et al. 2004, 2012; Iye et al. was observed at three or four distinct roll angles to mitigate the 2006; Willis et al. 2008; Ouchi et al. 2010; Hibon et al. 2010; impact of overlap between spectra of nearby objects. Tilvi et al. 2010; Kashikawa et al. 2011;Clement´ et al. 2012; The HST slitless spectra were reduced using the aXe pack- Shibuya et al. 2012;Krugetal.2012), or direct spectroscopic age (Kummeletal.¨ 2009), closely following the procedure used searches (e.g., Kurk et al. 2004; Martin & Sawicki 2004; for the earlier GRism ACS Program for Extragalactic Science van Breukelen et al. 2005; Martin et al. 2008; Dressler et al. survey (Pirzkal et al. 2004). For each roll angle, the relative 2011). offsets of all exposures were determined using zero-order im- However, issues remain. Ground-based near-IR spectroscopy ages and narrow emission lines. The data for each roll angle can only confirm these objects easily when they have strong were ultimately combined into two-dimensional (2D) spectro- Lyα lines in clean regions of the night sky spectrum. Thus, while scopic stacks and extracted one-dimensional (1D) spectra for dozens have been confirmed up to z = 6.5 (Hu et al. 2010; Ouchi each source and each observed position angle. et al. 2010; Kashikawa et al. 2011), only a handful are confirmed To identify and spectroscopically confirm the highest redshift at higher redshifts (Iye et al. 2006; Rhoads et al. 2012; Shibuya LBGs in the survey, we followed a procedure based on Malhotra et al. 2012; Pentericci et al. 2011; Ono et al. 2012; Schenker et al. et al. (2005). We started with the GOODS v1.9 images and 2012). The crucial Lyα line may be rare and/or weak at redshifts performed our own SExtractor photometry. We then applied where the intergalactic medium (IGM) was mostly neutral a “liberal” i-dropout criterion to generate a list of candidate (and hence able to scatter Lyα photons). Meanwhile, sample LBGs. Since the GOODS data do not include observations contamination by foreground galaxies becomes an increasing redder than z-band (and our candidate selection was done worry at higher redshifts, where the volume available for such prior to the installation of WFC3), this ultimately amounts to using i775 − z850 > 0.9 mag. For each of these objects, we contaminants becomes large. Finally, the candidate lists from N the highest redshift galaxy surveys can be disturbingly unstable, calculated the net significance (“netsig”) parameter (Pirzkal showing little overlap when different groups examine the same et al. 2004) to determine which spectra might have sufficient data, or even when the same group re-observes the same field information for a redshift measurement. (“Netsig” is defined (e.g., Yan et al. 2012; Oesch et al. 2012). by first sorting all pixels in a spectrum in descending order Slitless spectroscopy with the HST offers a solution to many of signal-to-noise ratio; calculating the signal-to-noise ratio Sn of these issues. Space telescopes avoid the crippling effects of obtained by combining flux from the brightest n pixels, for all Earth’s atmosphere on the near-IR sky. HST’s spatial resolution n between 1 and the total number of pixels in the spectrum; and finally taking N = max{Sn}.) is well matched to the sizes of high redshift galaxies.

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