Variability in extragalactic class I methanol masers: New maser components toward NGC4945 and NGC253

TL;DR

This study reports new detections and variability of extragalactic class I methanol masers in NGC 4945 and NGC 253, along with upper limits on emission in other galaxies, enhancing understanding of maser phenomena beyond the Milky Way.

Contribution

First evidence of variability in extragalactic class I methanol masers and new maser components detected in NGC 4945 and NGC 253.

Findings

Detected new maser components in NGC 4945 and NGC 253.

Provided the first clear evidence of variability in extragalactic class I methanol masers.

Established upper limits on methanol maser emissions in 12 other galaxies.

Abstract

We have used the Australia Telescope Compact Array (ATCA) to make new observations of the 36.2-GHz ($4_{-1}\rightarrow3_0$E) methanol transition toward NGC 4945 and NGC 253. These observations have revealed the presence of new maser components toward these galaxies, and have provided the first clear evidence for variability in extragalactic class~I methanol masers. Alongside the new observations of NGC 4945 and NGC 253, we present the results of recent 36.2-GHz methanol maser searches toward 12 galaxies, placing upper limits on the emission from the 36.2-GHz class~I transition and the 37.7-GHz ($7_{2}\rightarrow8_1$E) class~II maser line toward these sources. Flux density values for the 7-mm continuum emission toward these sources are also reported where applicable. A re-analysis of the published 36.2-GHz methanol observations of Arp 220 undertaken as part of the search revealed some issues with previous imaging procedures. The re-analysis, combined with non-detections in independent follow-up observations suggest that there is no 36.2-GHz methanol emission toward Arp 220 stronger than 3.5 mJy in a 10 km s$^{-1}$ channel (5$\sigma$ upper limit).