A new hot core in the outer Galaxy: Impact of metallicity on the formation of complex organic molecules

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 November 2025
Accepted: 20 April 2026

Abstract

Context. Many complex organic molecules (COMs) in star-forming regions are believed to form on dust grains. We thus expect both the reduced metallicity and dust-to-gas ratio in the outer Galaxy to have an impact on the chemical composition of these regions.

Aims. We investigate if certain COMs are more sensitive than others to metallicity by measuring the chemical composition of hot cores in the outer Galaxy.

Methods. We used the interferometer NOEMA to perform an imaging spectral line survey of G135.27+2.79, a hot core candidate located at a galactocentric distance of 13.1 kpc. We derived the rotational temperatures and column densities of the detected molecules while assuming local thermodynamic equilibrium and compared the chemical composition of G135.27+2.79 to other sources and to the predictions of the three-phase astrochemical code MAGICKAL.

Results. G135.27+2.79 hosts three continuum cores, labeled MM1, MM2, and MM3. Toward MM1, we detected 28 molecules, including 12 COMs, and several of their less abundant isotopologs. Most species trace a hot, compact region, confirming MM1 as a hot core (the third one identified in the outer Galaxy). MM1 drives a bipolar CO outflow. COMs show a velocity gradient along the outflow axis but opposite to that of CO, which may be related to a wide-angle disk wind. The chemical composition of MM1 correlates rather well with that of the inner and outer Galaxy hot cores G31.41+0.31 and WB89-789 SMM1, but its molecular abundances relative to methanol lie in between, which may reflect the influence of metallicity on COM formation. The model results agree reasonably well, though with a few notable exceptions, with the COM abundances of MM1 relative to methanol and with the abundance ratios between MM1 and G31.41+0.31. Sensitivity to the reduced metallicity and dust-to-gas ratio varies between molecules, with carbon chains and nitriles most negatively affected. The lower dust-to-gas ratio leads to slower adsorption under low-metallicity conditions so that more carbon is locked up into CO in the gas. Slow adsorption means that CO is hydrogenated more efficiently on grains, enhancing CO-related COM abundances above expectations.

Conclusions. These results demonstrate that metallicity has a significant impact on the formation of COMs. A larger source sample is needed to investigate the robustness of the deviations noted between the model and the observations for a few species.