Methanol masers in NGC 253 with ALCHEMI

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.

The Morpho-kinematic Architecture of Super Star Clusters in the Center of NGC 253

“…The nuclear region of NGC 253 is a chemically rich environment (Krieger et al 2020a;Martín et al 2021;Haasler et al 2022) and hosts a number of massive, dense molecular clouds (e.g., Sakamoto et al 2011;Leroy et al 2015a), radio continuum sources (likely H II regions and supernova remnants; Turner & Ho 1985;Watson et al 1996;Ulvestad & Antonucci 1997;Kornei & McCrady 2009), and masers (e.g., Gorski et al 2017Gorski et al , 2019Humire et al 2022). The overwhelming majority of the star formation in the nuclear starburst is concentrated in massive forming "super" star clusters (SSCs; Ando et al 2017;Leroy et al 2018;Rico-Villas et al 2020;Mills et al 2021;Levy et al 2021).…”

ALCHEMI Finds a “Shocking” Carbon Footprint in the Starburst Galaxy NGC 253

“…ALCHEMI is a widefrequency, unbiased spectral scan mosaic toward the CMZ of NGC 253 at a common 1 6 resolution. This survey has discovered high cosmic-ray ionization rates (Holdship et al 2021;Harada et al 2021;Holdship et al 2022), detected a phosphorus-bearing species for the first time in an extragalactic source (Haasler et al 2022), and detected methanol masers (Humire et al 2022) in the CMZ of NGC 253. We utilize the ALCHEMI data to study the multiple transitions of HOCO + .…”

“…As shown in Figure 1, bar orbits (x 1 orbits) and inner orbits (x 2 orbits) intersect at the northeast and southwest parts of the CMZ, near GMCs 1, 8, 9, and 10. Shocks at GMCs 1, 2, 7, 8, 9, and 10 have been suggested by the detection of Class I methanol masers (Humire et al 2022), and some other molecular tracers of shocks (K.-Y. Huang et al 2022, in preparation).…”

“…As already noted by Humire et al (2022) and Behrens et al (2022), the GMC nomenclature was adopted from Leroy et al (2015), but with modified positions. How these modifications are made is explained in Behrens et al (2022).…”

“…These transitions may not be in local thermodynamic equilibrium (LTE), but are "quasi-thermal" and they are not identified as masing(Humire et al 2022). 21 http://cassis.irap.omp.eu/22 The kinematic center of NGC 253 is located near GMC 5(Turner & Ho 1985;Müller-Sánchez et al 2010).…”