Bar-driven Shocks in NGC 1365 Revealed by the Most Luminous Extragalactic Class I Methanol Maser

Abstract We report the first detection of the 36.2 GHz (4 −1 → 3 0 E) Class I methanol (CH 3 OH) maser in the central region of the barred spiral galaxy NGC 1365. This detection using the Australia Telescope Compact Array establishes NGC 1365 as the host of the most luminous extragalactic Class I methanol maser known, with a total isotropic luminosity of 19.3 L ⊙ . The maser emission is unambiguously localized to the southern arm of the circumnuclear starburst ring and exhibits a tight alignment with the bar-driven gas inflow lane. The striking absence of maser emission in the star formation and feedback-dominated northern arm reveals a fundamental bimodality within the ring. Combined with archival multiwavelength data, our results demonstrate that the maser traces low-velocity (∼25−30 km s −1 ), nondissociative molecular shocks in the southern arm, where the gas is cold, dense, and subject to a weak radiation field. This work directly resolves the long-standing ambiguity between bar-driven shocks and stellar feedback, establishing the 36.2 GHz maser as a unique and powerful shock-specific diagnostic for extragalactic studies. It provides a novel tool to spatially pinpoint shock fronts and to disentangle the roles of gravitational dynamics and radiative feedback in shaping the extreme environments of galactic nuclei.