Humire, PK; Henkel, C; Hernández-Gómez, A; Martin, S; Mangum, J; Harada, N; Muller, S; ... Emig, KL; + view all Humire, PK; Henkel, C; Hernández-Gómez, A; Martin, S; Mangum, J; Harada, N; Muller, S; Sakamoto, K; Tanaka, K; Yoshimura, Y; Nakanishi, K; Mühle, S; Herrero-Illana, R; Meier, DS; Caux, E; Aladro, R; Mauersberger, R; Viti, S; Colzi, L; Rivilla, VM; Gorski, M; Menten, KM; Huang, K-Y; Aalto, S; van der Werf, PP; Emig, KL; - view fewer (2022) Methanol masers in NGC 253 with ALCHEMI. Astronomy & Astrophysics , 663 , Article A33. 10.1051/0004-6361/202243384.

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Abstract

Context. Methanol masers of Class I (collisionally pumped) and Class II (radiatively pumped) have been studied in great detail in our Galaxy in a variety of astrophysical environments such as shocks and star-forming regions and are they are helpful to analyze the properties of the dense interstellar medium. However, the study of methanol masers in external galaxies is still in its infancy. / Aims. Our main goal is to search for methanol masers in the central molecular zone (CMZ; inner 500 pc) of the nearby starburst galaxy NGC 253. / Methods. Covering a frequency range between 84 and 373 GHz (λ = 3.6–0.8 mm) at high angular (1.″6 ∼ 27 pc) and spectral (∼8–9 km s−1) resolution with ALCHEMI (ALMA Comprehensive High-resolution Extragalactic Molecular Inventory), we have probed different regions across the CMZ of NGC 253. In order to look for methanol maser candidates, we employed the rotation diagram method and a set of radiative transfer models. / Results. We detect for the first time masers above 84 GHz in NGC 253, covering an ample portion of the J−1 → (J − 1)0 − E line series (at 84, 132, 229, and 278 GHz) and the J0 → (J − 1)1 − A series (at 95, 146, and 198 GHz). This confirms the presence of the Class I maser line at 84 GHz, which was already reported, but now being detected in more than one location. For the J−1 → (J− 1)0 − E line series, we observe a lack of Class I maser candidates in the central star-forming disk. / Conclusions. The physical conditions for maser excitation in the J−1 → (J − 1)0 − E line series can be weak shocks and cloud-cloud collisions as suggested by shock tracers (SiO and HNCO) in bi-symmetric shock regions located in the outskirts of the CMZ. On the other hand, the presence of photodissociation regions due to a high star-formation rate would be needed to explain the lack of Class I masers in the very central regions.

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