Investigating the chemical link between H_2CO and CH_3OH within the central molecular zone of NGC 253

Abstract

Context: The formaldehyde (H2CO) and methanol (CH3OH) molecules have served as traditional tracers of the star formation process for decades. Studies of the environments that produce formaldehyde and methanol emission, though, have pointed to significant differences in the physical environments within which each molecule resides.

Aims: In this article we aim to investigate the detailed physical and chemical conditions that give rise to formaldehyde and methanol emission in the nearby starburst galaxy NGC 253.

Methods: We employed high spatial (1.​​′′6 or ∼28 pc) and spectral (∼10 km/s) resolution 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, kinetic temperature, molecular species column density, and source filling factor within eight giant molecular clouds (GMCs). We also measured the relative abundances of the two nuclear spin isomers of CH3OH to investigate its formation history. Results: The physical and chemical conditions derived clearly indicate that these two molecular species originate from distinct physical environments. H2CO traces low volume densities and high kinetic temperatures, while CH3OH traces high volume densities and low kinetic temperatures. The H2CO abundances are constant, though poorly constrained, within the eight NGC 253 GMCs analysed, while the CH3OH abundance shows a radial gradient from low to high values within the NGC 253 CMZ. Conclusions: 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.