L53 Spectroscopic Confirmation of an Extreme
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The Astrophysical Journal, 681: L53–L56, 2008 July 10 ᭧ 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A. SPECTROSCOPIC CONFIRMATION OF AN EXTREME STARBURST AT REDSHIFT 4.5471 Peter Capak,2,3 C. L. Carilli,4 N. Lee,4 T. Aldcroft,5 H. Aussel,6 E. Schinnerer,7 G. W. Wilson,8 M. S. Yun,8 A. Blain,3 M. Giavalisco,8 O. Ilbert,9 J. Kartaltepe,9 K.-S. Lee,10 H. McCracken,11 B. Mobasher,12 M. Salvato,3 S. Sasaki,13 K. S. Scott,8 K. Sheth,2,3 Y. Shioya,13 D. Thompson,14 M. Elvis,5 D. B. Sanders,9 N. Z. Scoville,3 and Y. Tanaguchi13 Received 2008 April 4; accepted 2008 June 4; published 2008 June 27 ABSTRACT p We report the spectroscopic confirmation of a submillimeter galaxy (SMG) atz 4.547 with an estimated LIR 13 p (0.5–2.0) # 10 L,. The spectra, mid-IR, and X-ray properties indicate the bolometric luminosity is dominated Ϫ1 by star formation at a rate of 11000 M, yr . Multiple, spatially separated components are visible in the Lya line with an observed velocity difference of up to 380 kmsϪ1 and the object morphology indicates a merger. The best- 10 fit spectral energy distribution and spectral line indicators suggest the object is 2–8 Myr old and contains 110 M, of stellar mass. This object is a likely progenitor for the massive early-type systems seen atz ∼ 2 . Subject headings: galaxies: evolution — galaxies: formation — galaxies: high-redshift — galaxies: interactions — galaxies: starburst — submillimeter 1. INTRODUCTION Pope et al. 2005; Aretxaga et al. 2007). However, the small bandwidth of current millimeter-wave spectrographs makes it The study of galaxies detected at millimeter and submilli- very difficult to measure redshifts directly, and the low angular meter wavelengths is one of the most rapidly developing fields resolution of millimeter single-dish imaging leads to multiple in observational astronomy. It is now known that a large frac- optical candidates for the same source. As a result, millimeter tion of the star formation activity is enshrouded in dust, with surveys have relied on high-resolution radio data to identify the the star formation rate (SFR) being directly proportional to the optical counterparts for subsequent spectroscopic follow-up. This far-infrared (FIR) luminosity of galaxies, modulo possible con- leaves 35%–70% of the population of millimeter-selected gal- tributions from an active galactic nucleus (AGN) (Hughes et axies at milli-Jansky flux levels unidentified and potentially at al. 1998). Surveys performed at millimeter wavelengths di- higher redshift (Wang et al. 2007; Younger et al. 2007). rectly probe the FIR luminosity, and hence the amount of star In this Letter we report the discovery of a millimeter galaxy formation. Furthermore, the shape of the galaxy spectral energy with a spectroscopic redshift ofz p 4.547 that appears to be distributions (SEDs) at rest-frame millimeter wavelengths re- dominated by star formation. This is the highest redshift galaxy ! ! sults in a negative K-correction in the range0.5 z 10 . detected at millimeter wavelengths not associated with an op- Therefore a flux-limited survey is equivalent to an SFR-limited tically bright quasar. The object was independently selected as survey at these redshifts (Blain et al. 2002). a Lyman break galaxy (K.-S. Lee et al. 2008, in preparation) The current redshift distribution of millimeter galaxies peaks for spectroscopic follow-up, a millimeter source, and a radio ∼ 1 atz 2 , with very few galaxies atz 3 (Chapman et al. 2005; source (Carilli et al. 2008). The source reported here is unusual for millimeter sources because it has several nearby optically 1 Based on observations taken at the Keck Observatory, the James Clerk bright counterparts which were selected as V-band dropouts Maxwell Telescope, the Institut de Radioastronomie Millimetrique 30 m tele- and is unusually luminous which allows for a radio detection. scope, the Galaxy Evolution Explorer, the Chandra X-Ray Observatory, the However, the confirmation of this object suggests the popu- Hubble Space Telescope, the Very Large Array, the Subaru Telescope, the lation of sources with similar radio to millimeter flux ratios United Kingdom Infrared Telescope, and the Canada-France-Hawaii Telescope. 2 Spitzer Science Center, 314-6 Caltech, Pasadena, CA 91125. and optical colors may also be at high redshifts. 3 p p p 105-24 Caltech, Pasadena, CA 91125. We assume aH0 70 ,Qv 0.7 ,Qm 0.3 cosmology and 4 National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801. a star formation rate integrated across a Salpeter (1955) IMF 5 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cam- from 0.1 to 100M, throughout this Letter. bridge, MA 02138. 6 AIM, Unite´ Mixte de Recherche CEA, CNRS, Universite´ Paris VII, UMR 7158, Centre d’E´ tudes de Saclay, F-91191 Gif sur Yvette Cedex, France. 2. DATA 7 Max-Planck-Institut fu¨r Astronomie, Ko¨nigstuhl 17, Heidelberg D-69117, Germany. p 8 Astronomy Department, University of Massachusetts, Amherst, MA Observations atl 1.1 mm with an average rms noise of 01003. 1.3 mJy were obtained with the AzTEC (Wilson et al. 2008) 9 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, camera at the James Clerk Maxwell Telescope (JCMT) (Scott Honolulu, HI 96822. et al. 2008). Additional observations were obtained by the 10 Department of Astronomy, 260 Whitney Avenue, Yale University, New MAMBO camera on the IRAM 30 m telescope with an rms Haven, CT 06511. ! Љ 11 Institut d’Astrophysique de Paris, UMR 7095 CNRS, Universite´ Pierre et of 0.67 mJy and a positional accuracy of 5 (E. Schinnerer Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France. et al., in preparation). Ground-based optical and near-infrared 12 Department of Physics and Astronomy, University of California, River- imaging in 22 bands, Hubble Space Telescope, Spitzer,and side, CA 92521. Very Large Array images were obtained as part of the COS- 13 Research Center for Space and Cosmic Evolution, Ehime University, 2- 5 Bunkyo-cho, Matsuyama 790-8577, Japan. MOS survey as described in Capak et al. (2007), Scoville et 14 Large Binocular Telescope Observatory, University of Arizona, 933 North al. (2007), Sanders et al. (2007), and Schinnerer et al. (2007), Cherry Avenue, Tucson, AZ 85721. respectively. Additional deep J andK s data were obtained with L53 L54 CAPAK ET AL. Vol. 681 TABLE 1 Object Flux Fluxa Wavelength (mJy) 0.2–8 keV ........... !3 # 10Ϫ16 ergs sϪ1 cmϪ2 150 nm (FUV) ...... !0.2 250 nm (NUV) ...... !0.09 380 nm (u∗ ) ......... !0.01 427 nm .............. !0.02 ! 446 nm (BJ) ......... 0.01 0.04 ע nm .............. 0.04 464 0.02 ע nm (gϩ ) ......... 0.03 478 0.04 ע nm .............. 0.04 484 0.05 ע nm .............. 0.09 505 Fig. 2.—The 2D spectra of the object around the Lya line is shown on the 0.04 ע nm .............. 0.08 527 ϩ ע 548 nm (VJ) ......... 0.15 0.02 right along with the ground-basedz -band image on the left with the slit position -marked in blue, the radio position marked with a magenta cross, and the fore 0.05 ע nm .............. 0.18 574 ground object indicated by a magenta arrow. The red lines are spaced at 0.4Љ 0.06 ע nm .............. 0.25 624 ϩ intervals, which is comparable to the spatial resolution of the spectra, and the 0.03 ע nm (r ) ......... 0.50 630 black bar on the bottom right has a length of 1A˚ in the rest frame. Lya emission 0.06 ע nm .............. 1.24 679 is coming from all portions of the source and there is a clear velocity gradient 0.07 ע nm .............. 1.30 709 along the slit. Note the Lya absorption feature at the physical position of the 0.13 ע nm .............. 1.32 711 .bright continuum emission and the multiple peaks in the Lya line 0.09 ע nm .............. 1.59 738 0.05 ע nm (iϩ ) .......... 1.60 764 -3Љ between exposures to improve backע ered along the slit by 0.09 ע nm .............. 1.59 767 ground subtraction. The data were reduced with a modified 0.09 ע nm .............. 1.60 815 0.09 ע nm .............. 1.48 827 ϩ version of the DEEP2 DEIMOS pipeline (Marinoni et al. 2001). 0.10 ע nm (z ) ......... 1.82 904 In addition to the standard processing this modified pipeline 0.8 ע mm(J) ......... 2.4 1.25 ע 2.15 mm (Ks ) ........ 3.7 0.5 constructs and subtracts a median background and accounts for the shifts when combining the spectra. These additional steps 0.2 ע mm ............... 7.9 3.6 .remove the “ghosting” inherent to the 830 linemmϪ1 grating 0.4 ע mm ............... 5.8 4.5 1.3 ע mm ............... 3.4 5.8 An image of the continuum and Lya emission is shown in 3.6 ע mm ............... 10 8.0 13b Figure 1 and indicates emission from all components of the ע mm ............... 26 24 ,source. The 2D and 1D spectra are shown in Figures 2 and 3 1500 ע mm .............. 4800 1.1 respectively. Lya emission is detected from both the compact 670 ע mm ............. 3410 1.25 ע 20 cm ................ 45 9 and extended portions of the object with a velocity gradient a All limits are 1 j limits. across the slit. Lya emission from the diffuse region is red- b A nearby bright source was modeled and sub- shifted with respect to the compact source, and a deep Lya tracted to make this measurement; the formal upper limit is 70 mJy if the nearby source is not subtracted.