Detection of 84 GHz Class I Methanol Maser Emission toward NGC 253

We have investigated the central region of NGC 253 for the presence of 84.5 GHz (5−1 → 40E) methanol emission using the Australia Telescope Compact Array. We present the second detection of 84.5 GHz class I methanol maser emission outside of the Milky Way. This maser emission is offset from the dynamical center of NGC 253, in a region with previously detected emission from class I maser transitions (36.2 GHz 4−1 → 30E and 44.1 GHz 70 → 61A+ methanol lines). The emission features a narrow linewidth (∼12 ${\text{km s}}^{-1}$) with a luminosity approximately that is 5 orders of magnitude higher than typical Galactic sources. We determine an integrated line intensity ratio of 1.2 ± 0.4 between the 36.2 and 84.5 GHz class I methanol maser emission, which is similar to the ratio observed toward 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 also that observations of additional class I methanol transitions will facilitate investigations of the maser pumping regime.