GRB 980425 host: [C II], [O I], and CO lines reveal recent enhancement of star formation due to atomic gas inflow ^⋆

¹ Scottish Universities Physics Alliance (SUPA), Institute for Astronomy, University of Edinburgh, Royal Observatory, Blakford Hill, Edinburgh, EH9 3HJ, UK
e-mail: mm@roe.ac.uk
² Herschel Science Centre (ESA-ESAC), 28.692 Villanueva de la Cañada, Madrid, Spain
³ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁴ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH13 LE, UK
⁵ Centre for Astronomy, Nicolaus Copernicus University, Faculty of Physics, Astronomy and Informatics, Grudziadzka 5, 87100 Toruń, Poland
⁶ Astronomical Observatory, Adam Mickiewicz University, Słoneczna 36, 60-268, Poznań, Poland
⁷ Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisbon, Portugal
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁰ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
¹¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18. 008 Granada, Spain
¹² Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
¹³ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu – CNRS – Université Paris Diderot, CE-Saclay, pt courrier 131, 91191 Gif-sur-Yvette, France
¹⁴ XMM Science Operation Centre, ESAC/ESA, Villafranca del Castillo, 28692 Madrid, Spain
¹⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁶ Technical University of Denmark, Department of Physics, Fysikvej, building 309, 2800 Kgs. Lyngby, Denmark
¹⁷ Centro de Astrobiología (INTA-CSIC), Ctra. M-108, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁸ INAF-IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
¹⁹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
²⁰ The Oskar Klein Centre, Department of Astronomy, AlbaNova, Stockholm University, 106 91 Stockholm, Sweden
²¹ Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100012, PR China

Received: 30 July 2016
Accepted: 6 September 2016

Abstract

Context. Accretion of gas from the intergalactic medium is required to fuel star formation in galaxies. We have recently suggested that this process can be studied using host galaxies of gamma-ray bursts (GRBs).

Aims. Our aim is to test this possibility by studying in detail the properties of gas in the closest galaxy hosting a GRB (980425).

Methods. We obtained the first ever far-infrared (FIR) line observations of a GRB host, namely Herschel/PACS resolved [C ii] 158 μm and [O i] 63 μm spectroscopy, and an APEX/SHeFI CO(2–1) line detection and ALMA CO(1–0) observations of the GRB 980425 host.

Results. The GRB 980425 host has elevated [C ii]/FIR and [O i]/FIR ratios and higher values of star formation rates (SFR) derived from line ([C ii], [O i], Hα) than from continuum (UV, IR, radio) indicators. [C ii] emission exhibits a normal morphology, peaking at the galaxy centre, whereas [O i] is concentrated close to the GRB position and the nearby Wolf-Rayet region. The high [O i] flux indicates that there is high radiation field and high gas density at these positions, as derived from modelling of photo-dissociation regions. The [C ii]/CO luminosity ratio of the GRB 980425 host is close to the highest values found for local star-forming galaxies. Indeed, its CO-derived molecular gas mass is low given its SFR and metallicity, but the [C ii]-derived molecular gas mass is close to the expected value.

Conclusions. The [O i] and H i concentrations and the high radiation field and density close to the GRB position are consistent with the hypothesis of a very recent (at most a few tens of Myr ago) inflow of atomic gas triggering star formation. In this scenario dust has not had time to build up (explaining high line-to-continuum ratios). Such a recent enhancement of star formation activity would indeed manifest itself in high SFR_line/SFR_continuum ratios because the line indicators are sensitive only to recent (≲10 Myr) activity, whereas the continuum indicators measure the SFR averaged over much longer periods (~100 Myr). Within a sample of 32 other GRB hosts, 20 exhibit SFR_line/SFR_continuum> 1 with a mean ratio of 1.74 ± 0.32. This is consistent with a very recent enhancement of star formation that is common among GRB hosts, so galaxies that have recently experienced inflow of gas may preferentially host stars exploding as GRBs. Therefore GRBs may be used to select a unique sample of galaxies that is suitable for the investigation of recent gas accretion.