Complex organic molecules towards the central molecular zone of NGC 253

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn, Germany
³ Physics and Astronomy, University College London, UK
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
⁵ Department of Astronomy, University of Virginia, PO Box 400325, 530 McCormick Road, Charlottesville, VA 22904-4325, USA
⁶ Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁷ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁸ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ Department of Astronomy, School of Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, 181-1855 Japan
¹¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala, Sweden
¹² Institute of Astronomy and Astrophysics, Academia Sinica, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 106319, Taiwan

^⋆ Corresponding author: bouvier@strw.leidenuniv.nl

Received: 7 March 2025
Accepted: 25 April 2025

Abstract

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.