Nature of the unusual transient AT 2018cow from HI observations of its host galaxy^⋆

¹ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
e-mail: mj.michalowski@gmail.com
² Astronomisches Institut der Ruhr-Universität Bochum (AIRUB), Universitätsstrasse 150, 44801 Bochum, Germany
³ DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
⁴ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, s/n, 18008 Granada, Spain
⁵ PITT PACC, Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
⁶ The Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
⁷ ARIES, Manora Peak, Nainital 263002, India
⁸ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹⁰ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹¹ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu – CNRS – Université Paris Diderot, CE-Saclay, pt courrier 131, 91191 Gif-sur-Yvette, France
¹² Department of Physics, University of California, 1 Shields Ave, Davis, CA 95616-5270, USA
¹³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁴ INAF-OAS Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁵ Technical University of Denmark, Department of Physics, Fysikvej, building 309, 2800 Kgs. Lyngby, Denmark
¹⁶ Indian Institute of Space Science & Technology, Thiruvananthapuram 695547, India
¹⁷ Physics Department, University of Calabria, 87036 Arcavacata di Rende, Italy
¹⁸ Department of Physics, University of Bath, Bath BA2 7AY, UK
¹⁹ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel
²⁰ South African Radio Astronomy Observatory, 2 Fir Street, Black Rivetexr Park, Observatory, Cape Town, South Africa
²¹ Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
²² Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
²³ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy

Received: 26 February 2019
Accepted: 7 June 2019

Abstract

Context. Unusual stellar explosions represent an opportunity to learn about both stellar and galaxy evolution. Mapping the atomic gas in host galaxies of such transients can lead to an understanding of the conditions that trigger them.

Aims. We provide resolved atomic gas observations of the host galaxy, CGCG137-068, of the unusual and poorly understood transient AT 2018cow, which we obtained in searching for clues to understand its nature. We test whether it is consistent with a recent inflow of atomic gas from the intergalactic medium, as suggested for host galaxies of gamma-ray bursts (GRBs) and some supernovae (SNe).

Methods. We observed the HI hyperfine structure line of the AT 2018cow host with the Giant Metrewave Radio Telescope.

Results. There is no unusual atomic gas concentration near the position of AT 2018cow. The gas distribution is much more regular than the distributions of GRB/SN hosts. The AT 2018cow host has an atomic gas mass lower by 0.24 dex than predicted from its star formation rate (SFR) and is at the lower edge of the galaxy main sequence. In the continuum we detected the emission of AT 2018cow and of a star-forming region in the north-eastern part of the bar (away from AT 2018cow). This region hosts a third of the galaxy’s SFR.

Conclusions. The absence of atomic gas concentration close to AT 2018cow, along with a normal SFR and regular HI velocity field, sets CGCG137-068 apart from GRB/SN hosts studied in HI. The environment of AT 2018cow therefore suggests that its progenitor may not have been a massive star. Our findings are consistent with an origin of the transient that does not require a connection between its progenitor and gas concentration or inflow: an exploding low-mass star, a tidal disruption event, a merger of white dwarfs, or a merger between a neutron star and a giant star. We interpret the recently reported atomic gas ring in CGCG 137-068 as a result of internal processes connected with gravitational resonances caused by the bar.