Detection of a methanol megamaser in a major-merger galaxy

TL;DR

This paper reports the first detection of methanol megamasers in a major-merger galaxy, Arp 220, revealing their association with starburst activity and galactic outflows, and highlighting their potential as probes of feedback processes.

Contribution

It presents the first detection of 36.2 and 37.7 GHz methanol megamasers in Arp 220, expanding knowledge of extragalactic methanol masers and their relation to galaxy feedback.

Findings

Detected methanol megamasers in Arp 220 at 36.2 and 37.7 GHz.

Methanol megamasers are offset from the nucleus and linked to starburst regions.

Strong correlation with X-ray outflows suggests shocks and cosmic rays drive maser production.

Abstract

We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and 7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of the closest ultra-luminous infrared galaxy Arp 220. The emission in both the methanol transitions show narrow spectral features and have luminosities approximately 8 orders of magnitude stronger than that observed from typical class I methanol masers observed in Galactic star formation regions. The emission is also orders of magnitude stronger than the expected intensity of thermal emission from these transitions and based on these findings we suggest that the emission from the two transitions are masers. These observations provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz transitions and represents only the second detection of a methanol megamaser, following the recent report of an 84 GHz methanol megamaser in NGC1068. We find the methanol megamasers are significantly offset from the nuclear region and arise towards regions where there is Ha emission, suggesting that it is associated with starburst activity. The high degree of correlation between the spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests that the production of strong extragalactic class I methanol masers is related to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH and H2O megamasers which originate close to the nucleus, methanol megamasers provide a new probe of feedback (e.g. outflows) processes on larger-scales and of star formation beyond the circumnuclear starburst regions of active galaxies.