Complex organic molecules towards the central molecular zone of NGC 253

Journal article, 2025

Context. Interstellar complex organic molecules (iCOMs) could be linked to prebiotic species, which are the key building blocks of life. In Galactic star-forming (SF) regions, spatial variations in iCOM emission could reflect the source’s physical structure or different chemical formation pathways. Thus, investigating iCOMs in extragalactic SF regions can provide crucial information about these regions.
Aims. As an active extragalactic SF region, the central molecular zone (CMZ) of the nearby galaxy NGC 253 provides an ideal template for studying iCOMs under more extreme conditions. We aim to investigate the emission of a few selected iCOMs to understand whether a difference between the iCOMs could shed light on the source’s chemical or physical structure.
Methods. Using high angular resolution (∼27 pc) observations from the ALCHEMI ALMA large programme, we imaged the emission of selected iCOMs and precursors; CH3CHO, C2H5OH, NH2CHO, CH2NH, and CH3NH2. We estimated the gas temperatures and column densities of the iCOMs using a rotational diagram analysis, along with a non-local thermodynamic equilibrium (NLTE) analysis for CH2NH.
Results. The iCOM emission is concentrated mainly towards the inner part of the CMZ of NGC 253 and it can be reproduced with two gas components. Different emission processes can explain iCOM emission towards the CMZ of NGC 253: at giant molecular cloud (GMC) scales (∼27 pc), the iCOMs could trace large-scale shocks; whilst at smaller scales (of only a few parsecs), both shock and heating processes linked with ongoing star formation could be involved. Using trends in the column density correlation and known formation pathways, we find that there might be more than one formation path involved to explain the iCOM emission. Finally, we found chemical differences between the GMCs, such as a decrease in abundance for the N-bearing species towards one of the GMCs and different excitation conditions for NH2CHO and CH3CHO towards two of the GMCs.