X-shooter and ALMA spectroscopy of GRB 161023A

A study of metals and molecules in the line of sight towards a luminous GRB^⋆

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomí, s/n, 18008 Granada, Spain
e-mail: deugarte@iaa.es
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen Ø, Denmark
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁵ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl Street, Rehovot 761000, Israel
⁶ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763 0355, Santiago, Chile
⁷ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
⁸ ASI-Space Science Data Centre, Via del Politecnico snc, 00133 Rome, Italy
⁹ Space Science Group, School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
¹⁰ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹¹ Instituto de Astrofísica y Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
¹² INAF, Istituto Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
¹³ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy
¹⁴ IRAP, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
¹⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁶ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
¹⁷ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
¹⁸ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
¹⁹ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (LC), Italy
²⁰ School of Physics, University of Western Australia, M013, Crawley, WA 6009, Australia
²¹ European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei München, Germany
²² Department of Astronomy and Space Sciences, Istanbul University, 34119 Beyazıt, Istanbul, Turkey
²³ Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
²⁴ Centre for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, Ajdovščina 5270, Slovenia
²⁵ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
²⁶ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
²⁷ Department of Physics, University of the Free State, 9300 Bloemfontein, South Africa
²⁸ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
²⁹ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
³⁰ CAS Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
³¹ Australian Astronomical Observatory, PO Box 915 North Ryde, NSW 1670, Australia

Received: 25 March 2018
Accepted: 21 July 2018

Abstract

Context. Long gamma-ray bursts (GRBs) are produced during the dramatic deaths of massive stars with very short lifetimes, meaning that they explode close to the birth place of their progenitors. Over a short period they become the most luminous objects observable in the Universe, being perfect beacons to study high-redshift star-forming regions.

Aims. We aim to use the afterglow of GRB 161023A at a redshift z = 2.710 as a background source to study the environment of the explosion and the intervening systems along its line of sight.

Methods. For the first time, we complement ultraviolet (UV), optical and near-infrared (NIR) spectroscopy with millimetre spectroscopy using the Atacama Large Millimeter Array (ALMA), which allows us to probe the molecular content of the host galaxy. The X-shooter spectrum shows a plethora of absorption features including fine-structure and metastable transitions of Fe, Ni, Si, C, and O. We present photometry ranging from 43 s to over 500 days after the burst.

Results. We infer a host-galaxy metallicity of [Zn/H] = −1.11 ± 0.07, which, corrected for dust depletion, results in [X/H] = −0.94 ± 0.08. We do not detect molecular features in the ALMA data, but we derive limits on the molecular content of log(N_CO/cm⁻²) < 15.7 and log(N_HCO+/cm^−-12, which are consistent with those that we obtain from the optical spectra, log(N_H2/cm⁻²)< 15.2 and log(N_CO/cm⁻²) < 14.5. Within the host galaxy, we detect three velocity systems through UV, optical and NIR absorption spectroscopy, all with levels that were excited by the GRB afterglow. We determine the distance from these systems to the GRB to be in the range between 0.7 and 1.0 kpc. The sight line to GRB 161023A shows nine independent intervening systems, most of them with multiple components.

Conclusions. Although no molecular absorption was detected for GRB 161023A, we show that GRB millimetre spectroscopy is now feasible and is opening a new window on the study of molecular gas within star-forming galaxies at all redshifts. The most favoured lines of sight for this purpose will be those with high metallicity and dust.