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

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 triggering them. Aims. We provide resolved atomic gas observations of the host galaxy, CGCG137-068, of the unusual, poorly-understood transient AT 2018cow 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 those 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.

Studying the environment of AT 2018cow with MUSE

“…Environmental arguments have been made to suggest AT 2018cow was not a massive star based solely on the comparison to GRB-SNe environments by observing the form of Hi gas distributions of the respective host galaxies Micha lowski et al (2019). However, GRB-SNe production is a relatively unknown process that occurs in only a few percent of all CCSNe (e.g.…”

“…The host galaxy CGCG 137-068 has been studied at radio wavelengths, in Hi 21cm emission and continuum, by Roychowdhury et al (2019) and Micha lowski et al (2019). Both studies find a ring-like morphology of the atomic Hi gas distribution in CGCG 137-068, but infer a different origin for this structure (interaction with an external galaxy, or due to resonance from the bar, respecively for the two studies.)…”

“…Roychowdhury et al (2019) suggest the detection of Hi indicates the presence of compact star forming regions, giving a viable massive stellar progenitor route for AT 2018cow. Micha lowski et al (2019) conversely use an argument that the Hi distribution is not as asymmetric or as concentrated at the explosion site as seen in a handful of GRB-SNe hosts to argue that AT 2018cow may not have been formed by a massive star. Using ALMA data, Morokuma-Matsui et al 2019found the explosion site of AT 2018cow to share similarities with Type I CCSNe and the host galaxy CGCG 137-068 to be a typical star-forming dwarf in the local Universe.…”

AT 2018cow VLBI: no long-lived relativistic outflow

“…Finally, Perley et al (2019), Michałowski et al (2019), and Kuin et al (2019) all suggest the possibility that AT 2018cow might not be a core-collapse SN, but rather a tidal disruption event (TDE), where a star is disrupted by an intermediate-mass black hole that resides in the outskirts of CGCG 137−68. Some TDEs can produce radio-bright relativistic jets, for example, Swift 164449.3+573451 (Zauderer et al 2011;Berger et al 2012), although Swift 164449.3+573451 was much more radio luminous than AT 2018cow.…”

Connection of supernovae 2002ap, 2003gd, 2013ej, and 2019krl in M 74 with atomic gas accretion and spiral structure

“…The only possible counter-example of a potential explosion of a massive star without an associated gas concentration is the enigmatic transient AT 2018cow, whose host galaxy does not show such off-centre asymmetric Hi features (Michałowski et al 2019) and possibly only a gas ring (Roychowdhury et al 2019). Such an Hi ring would also be apparent for M 74 if it was further away so the sensitivity and resolution were poorer, because the central part is devoid of atomic gas, likely due to conversion to the molecular phase (Fig.…”