Detection of 84-GHz class I methanol maser emission towards NGC 253

TL;DR

This study reports the second detection of 84.5-GHz class I methanol maser emission outside the Milky Way, revealing insights into extragalactic maser phenomena and physical conditions in NGC 253.

Contribution

It presents the first detailed analysis of 84.5-GHz methanol maser emission in NGC 253, expanding knowledge of extragalactic masers and their physical environments.

Findings

Detected 84.5-GHz methanol maser emission in NGC 253

Line intensity ratio similar to Galactic sources

Different distributions of methanol transitions indicate varying physical conditions

Abstract

We have investigated the central region of NGC 253 for the presence of 84.5-GHz ($5_{-1}\rightarrow4_0$E) methanol emission using the Australia Telescope Compact Array. We present the second detection of 84.5-GHz class~I methanol maser emission outside the Milky Way. This maser emission is offset from dynamical centre of NGC 253, in a region with previously detected emission from class~I maser transitions (36.2-GHz $4_{-1}\rightarrow3_0$E and 44.1-GHz $7_{0}\rightarrow6_1$A$^{+}$ methanol lines) . The emission features a narrow linewidth ($\sim$12 km s$^{-1}$) with a luminosity approximately 5 orders of magnitude higher than typical Galactic sources. We determine an integrated line intensity ratio of $1.2\pm0.4$ between the 36.2 GHz and 84.5-GHz class I methanol maser emission, which is similar to the ratio observed towards Galactic sources. The three methanol maser transitions observed toward NGC 253 each show a different distribution, suggesting differing physical conditions between the maser sites and that observations of additional class~I methanol transitions will facilitate investigations of the maser pumping regime.