%O Copyright © P. K. Humire et al. 2022. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
%T Methanol masers in NGC 253 with ALCHEMI
%I EDP SCIENCES S A
%J Astronomy & Astrophysics
%A PK Humire
%A C Henkel
%A A Hernández-Gómez
%A S Martin
%A J Mangum
%A N Harada
%A S Muller
%A K Sakamoto
%A K Tanaka
%A Y Yoshimura
%A K Nakanishi
%A S Mühle
%A R Herrero-Illana
%A DS Meier
%A E Caux
%A R Aladro
%A R Mauersberger
%A S Viti
%A L Colzi
%A VM Rivilla
%A M Gorski
%A KM Menten
%A K-Y Huang
%A S Aalto
%A PP van der Werf
%A KL Emig
%X 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.
%L discovery10152832
%V 663
%D 2022
%K galaxies: spiral; galaxies: starburst; masers; submillimeter: galaxies; radio lines: galaxies