Investigating The Chemical Link Between H2CO And CH3OH Within The CMZ Of NGC 253

Formaldehyde (H₂CO) and methanol (CH₃OH) have served as traditional tracers of the star formation process for decades. Studies of the environments which produce these species, though, have pointed to significant differences in the physical environments within which each molecule resides.

In this paper we investigate the physical and chemical conditions which give rise to formaldehyde and methanol emission in the nearby starburst galaxy NGC 253. We employ high spatial (1.″6 or ∼28 pc) and spectral (∼10 km/s) imaging of the NGC 253 central molecular zone (CMZ) from the ALCHEMI Large Program to constrain radiative transfer models of the dense gas volume density, and temperature, molecular species column density, and source filling factor within eight giant molecular clouds (GMCs).

We also measure the relative abundances of the two nuclear spin isomers of CH₃OH to investigate its formation history. The physical and chemical conditions derived clearly indicate that H₂CO and CH₃OH originate from distinct physical environments. H2CO traces low volume density and high kinetic temperatures, while CH₃OH traces high volume density and low kinetic temperatures. The H₂CO abundances are constant, though poorly constrained, within the eight NGC 253 GMCs analyzed, while the CH₃OH abundance shows a radial gradient from low to high values within the NGC 253 CMZ.

Our findings highlight the complex chemical and physical differentiation of CH₃OH and H₂CO in the starburst environment of NGC 253. Methanol formation appears to be influenced by warm, dynamic processes rather than cold cloud chemistry, while formaldehyde primarily forms via gas-phase reactions. These results challenge the assumption of a direct chemical link between CH₃OH and H₂CO and underscores the impact of starburst-driven shocks, turbulence, and cosmic rays on molecular gas chemistry.

K.-Y. Huang, E. Behrens, M. Bouvier, S. Viti, J. G. Mangum, C. Eibensteiner

Comments: 19 pages, 11 figures, accepted in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2505.16255 [astro-ph.GA] (or arXiv:2505.16255v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2505.16255
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Submission history
From: Mathilde Bouvier
[v1] Thu, 22 May 2025 05:45:39 UTC (23,642 KB)
https://arxiv.org/abs/2505.16255
Astrobiology, Astrochemistry,