GRB hosts through cosmic time

VLT/X-Shooter emission-line spectroscopy of 96 γ-ray-burst-selected galaxies at 0.1 <z < 3.6^{⋆,⋆⋆,⋆⋆⋆}

¹ European Southern Observatory, 3107 Alonso de Córdova, Vitacura, Casilla 19001 Santiago 19 Chile
e-mail: t.kruehler@gmail.com
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark
³ Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁴ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland
⁵ Department of Astronomy, California Institute of Technology, MC 249-17, 1200 East California Blvd, Pasadena, CA 91125, USA
⁶ INAF-IASF Bologna, Area della Ricerca CNR, via Gobetti 101, 40129 Bologna, Italy
⁷ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁸ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁰ Millennium Institute of Astrophysics, Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹¹ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
¹² Laboratoire Galaxies Étoiles Physique et Instrumentation, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹³ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
¹⁴ American River College, Physics and Astronomy Dpt., 4700 College Oak Drive, Sacramento, CA 95841, USA
¹⁵ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
¹⁶ ASI-Science Data Centre, via Galileo Galilei, 00044 Frascati, Italy
¹⁷ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁸ Institute of Experimental andApplied Physics, Czech Technical University in Prague, Horska 3a/22, 128 00 Prague 2, Czech Republic
¹⁹ APC, Univ. Paris Diderot, CNRS/IN2P3, CEA/Irfu, Obs. de Paris, Sorbonne Paris Cité, 75205 Paris Cedex 13, France
²⁰ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, 1000 Ljubljana, Slovenia
²¹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
²² Department of Particle Physics & Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
²³ INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
²⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
²⁵ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
²⁶ Unidad Asociada Grupo Ciencias Planetarias (UPV/EHU, IAA-CSIC), Departamento de Física Aplicada I, E.T.S. Ingeniería, Universidad del País Vasco (UPV/EHU), Alameda de Urquijo s/n, 48013 Bilbao, Spain
²⁷ Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, 48008 Bilbao, Spain
²⁸ Physics Department, University of Calabria, via P. Bucci, 87036 Arcavacata di Rende, Italy

Received: 20 December 2014
Accepted: 15 June 2015

Abstract

We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 galaxies selected by long γ-ray bursts (GRBs) at 0.1 <z< 3.6, the largest sample of GRB host spectra available to date. Most of our GRBs were detected by Swift and 76% are at 0.5 <z< 2.5 with a median z_med ~ 1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFR), visual attenuations (A_V), oxygen abundances (12 + log (O/H)), and emission-line widths (σ). We study GRB hosts up to z ~ 3.5 and find a strong change in their typical physical properties with redshift. The median SFR of our GRB hosts increases from SFR_med ~ 0.6 M_⊙ yr^-1 at z ~ 0.6 up to SFR_med ~ 15 M_⊙ yr^-1 at z ~ 2. A higher ratio of [O iii]/[O ii]  at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the Baldwin-Phillips-Terlevich diagram. There is weak evidence for a redshift evolution in A_V and σ, with the highest values seen at z ~ 1.5 (A_V) or z ~ 2 (σ). Oxygen abundances of the galaxies are distributed between 12 + log (O/H) = 7.9 and 12 + log (O/H) = 9.0 with a median 12 + log (O/H)_med ~ 8.5. The fraction of GRB-selected galaxies with super-solar metallicities is ~20% at z< 1 in the adopted metallicity scale. This is significantly less than the fraction of total star formation in similar galaxies, illustrating that GRBs are scarce in high metallicity environments. At z ~ 3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z ≲ 0.5 Z_⊙. Together with a high incidence of Z ~ 0.5 Z_⊙ galaxies at z ~ 1.5, this indicates that a metallicity dependence at low redshift will not be dominant at z ~ 3. Significant correlations exist between the hosts’ physical properties. Oxygen abundance, for example, relates to A_V (12 + log (O/H) ∝ 0.17·A_V), line width (12 + log (O/H) ∝ σ^0.6), and SFR (12 + log (O/H) ∝ SFR^0.2). In the last two cases, the normalization of the relations shift to lower metallicities at z> 2 by ~0.4 dex. These properties of GRB hosts and their evolution with redshift can be understood in a cosmological context of star-forming galaxies and a picture in which the hosts’ properties at low redshift are influenced by the tendency of GRBs to avoid the most metal-rich environments.