Detection of a methanol megamaser in a major-merger galaxy
Chen, X and Ellingsen, SP and Baan, WA and Qiao, H-H and Li, J and An, T and Breen, SL, Detection of a methanol megamaser in a major-merger galaxy, Astrophysical Journal Letters, 800, (1) Article L2. ISSN 2041-8205 (2015) [Refereed Article]
We have detected emission from both the 4_1 → 30 E (36.2 GHz) class I and 7_2 → 8_1 E (37.7 GHz) class II methanol transitions toward the center of the closest ultra-luminous infrared galaxy Arp 220. The emission in both methanol transitions shows narrow spectral features and has luminosities approximately 8 orders of magnitude stronger than those observed from typical class I methanol masers observed in Galactic star formation regions. The emission is also orders of magnitude stronger than the expected intensity of thermal emission from these transitions and based on these findings we suggest that the emission from the two transitions are masers. These observations provide the first detection of a methanol megamaser in the 36.2 and 37.7 GHz transitions and represent only the second detection of a methanol megamaser, following the recent report of an 84 GHz methanol megamaser in NGC 1068. We find that the methanol megamasers are significantly offset from the nuclear region and arise toward regions where there is Hα emission, suggesting that they are associated with starburst activity. The high degree of correlation between the spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests that the production of strong extragalactic class I methanol masers is related to galactic-outflow-driven shocks and perhaps cosmic rays. In contrast to OH and H2O megamasers which originate close to the nucleus, methanol megamasers provide a new probe of feedback (e.g., outflows) processes on larger scales and of star formation beyond the circumnuclear starburst regions of active galaxies.