An ALCHEMI Inspection of Sulphur-bearing Species Towards the Central Molecular Zone of NGC 253

By Keith Cowing
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May 15, 2024
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An ALCHEMI Inspection of Sulphur-bearing Species Towards the Central Molecular Zone of NGC 253
Schematic (not to scale) showing the proposed origin of emission of the S-bearing species towards the CMZ of NGC 253 (Sec. 6.3) and summarising the main results for the physical properties (Tgas, ngas) of the gas emitting the S-bearing species (Sec 5.2). We illustrate two scenarios, for a giant molecular cloud in the outer (left) CMZ and inner (right) CMZ. The timescale for the shocks induced by star-formation are from Huang et al. (2023). The temperatures and densities reported are averaged over the regions and variations could be present in some of the regions. — astro-ph.GA

Sulphur-bearing species are detected in various environments within Galactic star-forming regions and are particularly abundant in the gas phase of outflows and shocks, and photo-dissociation regions.

In this work, we aim to investigate the nature of the emission from the most common sulphur-bearing species observable at millimetre wavelengths towards the nuclear starburst of the galaxy NGC 253. We intend to understand which type of regions are probed by sulphur-bearing species and which process(es) dominate(s) the release of sulphur into the gas phase.

We used the high-angular resolution (1.6″ or 27 pc) observations from the ALCHEMI ALMA Large Program to image several sulphur-bearing species towards the central molecular zone (CMZ) of NGC 253. We performed local thermodynamic equilibrium (LTE) and non-LTE large velocity gradient (LVG) analyses to derive the physical conditions of the gas in which S-bearing species are emitted, and their abundance ratios across the CMZ. Finally, we compared our results with previous ALCHEMI studies and a few selected Galactic environments.

We found that not all sulphur-bearing species trace the same type of gas: strong evidence indicates that H2S and part of the emission of OCS, H2CS, and SO, are tracing shocks whilst part of SO and CS emission rather trace the dense molecular gas. For some species, such as CCS and SO2, we could not firmly conclude on their origin of emission. The present analysis indicates that the emission from most sulphur-bearing species throughout the CMZ is likely dominated by shocks associated with ongoing star formation.

In the inner part of the CMZ where the presence of super star clusters was previously indicated, we could not distinguish between shocks or thermal evaporation as the main process releasing the S-bearing species.

NGC 253 also goes by the name of “Sculptor Galaxy” after its parent constellation, or by the name “Silver Dollar Galaxy”, presumably after its visual appearance in the eyepiece. NGC 253 is classified as a starburst galaxy, but interestingly, only 1 supernova has been found within it. It is a bright galaxy, easily visible in binoculars, about 10 million light years from us. — NOIRlab

M. Bouvier, S. Viti, E. Behrens, J. Butterworth, K.-Y. Huang, J. G. Mangum, N. Harada, S. Martín, V. M. Rivilla, S. Muller, K. Sakamoto, Y. Yoshimura, K. Tanaka, K. Nakanishi, R. Herrero-Illana, L. Colzi, M. D. Gorski, C. Henkel, P. K. Humire, D. S. Meier, P. P. van der Werf, Y. T. Yan

Comments: 44 pages, 20 figures, Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2405.08408 [astro-ph.GA] (or arXiv:2405.08408v1 [astro-ph.GA] for this version)
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Submission history
From: Mathilde Bouvier
[v1] Tue, 14 May 2024 07:59:09 UTC (11,157 KB)
Astrobiology, Astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻