An environmental analysis of the fast transient AT2018cow and implications for its progenitor and late-time brightness

Abstract

The nature of the newly discovered fast blue optical transients (FBOTs) is still puzzling astronomers. In this paper we carry out a comprehensive analysis of the molecular gas, ionized gas and stellar populations in the environment of the nearby FBOT AT2018cow based on ALMA, VLT/MUSE and HST/WFC3 observations. A prominent molecular concentration of 6 ($\pm$ 1) $\times$ 10$^6$ $M_\odot$ is found in the vicinity of AT2018cow, which has given rise to two active star-forming complexes with ages of 4 $\pm$ 1 Myr and $\lesssim$2.5 Myr, respectively. Each star-forming complex has a stellar mass of 3 $\times$ 10$^5$ $M_\odot$ and has photoionized a giant H II region with H$α$ luminosity even comparable to that of the 30 Dor mini-starburst region. AT2018cow is spatially coincident with one of the star-forming complexes; however, it is most likely to reside in its foreground since it has a much smaller extinction than the complex. Its progenitor could have been formed in a different star-forming event, and the non-detection of the associated stellar population constrains the progenitor's age to be $\gtrsim$10 Myr and initial mass to be $\lesssim$ 20 $M_\odot$. We further find the late-time brightness of AT2018cow is unlikely to be a stellar object. Its brightness has slightly declined from 2 yr to 4 yr after explosion and is most likely to originate from AT2018cow itself due to some powering mechanism still working at such late times.