Direct evidence for maser emission from the 36.2GHz class I transition of methanol in NGC253

Observations made with the Jansky Very large Array (JVLA) at an angular resolution of ∼0."1 have detected class I methanol maser emission from the 36.2 GHz transition toward the starburst galaxy NGC 253. The methanol emission is detected toward four sites which lie within the regions of extended methanol emission detected in previous lower angular resolution (a few arcseconds) observations. The peak flux densities of the detected compact components are in the range 3-9 mJy beam^-1. Combining the JVLA data with single-dish observations from the Shanghai Tianma Radio Telescope (TMRT) and previous interferometric observations with the Australia Telescope Compact Array (ATCA), we show that the 36.2 GHz class I methanol emission consists of both extended and compact structures, with typical scales of ∼6'' (0.1 kpc) and ∼0."05 (1 pc), respectively. The strongest components have a brightness temperature of >10³ K, much higher than the maximum kinetic temperature (∼100 K) of the thermal methanol emission from NGC 253. Therefore, these observations conclusively demonstrate for the first time the presence of maser emission from a class I methanol transition in an external galaxy.