CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy

V. Chemical composition gradients as a function of the galactocentric radius

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
² Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany
⁴ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, 28850, Torrejón de Ardoz, Madrid, Spain
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn, Bonn, Germany
⁸ Department of Physics and Astronomy, University College London, Gower Street, London, UK
⁹ Institut de Ciències de l’Espai (ICE), CSIC, Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra (Barcelona), Spain
¹⁰ Institut d’Estudis Espacials de Catalunya (IEEC), 08860, Castelldefels (Barcelona), Spain

^★ Corresponding author: diegogigli.dg@gmail.com

Received: 15 June 2025
Accepted: 16 September 2025

Abstract

Context. The outer Galaxy is characterized by lower metallicity compared to regions near the Sun, which suggests that the formation and survival of molecules in star-forming regions within the inner and outer Galaxy are likely to be different.

Aims. To understand how chemistry evolves across the Milky Way, deriving molecular abundances in star-forming regions in the outer Galaxy is essential to refining chemical models designed for environments with subsolar metallicity.

Methods. We analyzed IRAM 30 m observations in several spectral windows at 3 and 2 mm, toward a sample of 35 sources located at galactocentric distances of ~9–24 kpc in the context of the project CHEMical complexity in star-forming regions of the outer Galaxy (CHEMOUT).

Results. We focused on the species that have the highest detection rate (i.e., HCN, HCO⁺, c-C₃H₂, H¹³CO⁺, HCO, and SO), and searched for possible trends in column densities, abundances, and line widths with the galactocentric distance. We also updated the abundances for H₂CO and CH₃OH, presented in a previous work, using H₂ column densities from new NIKA2 dust continuum maps. The fractional abundances with respect to H₂ of most of the species (i.e., HCN, HCO⁺, c-C₃H₂, HCO, H₂CO, and CH₃OH) scale at most as the elemental fractional abundance of carbon ([C/H]) up to the investigated galactocentric distance of ~24 kpc. For the abundances of SO, we find a steeper gradient than that of sulfur elemental abundance ([S/H]). In contrast, the abundances of H¹³CO⁺ exhibit a shallower gradient relative to that of [¹³C/H]. Interestingly, we find that gas turbulence, as derived from the full width at half maximum of the lines, decreases with galactocentric distance for all the species investigated, suggesting a more quiescent environment in the outer Galaxy.

Conclusions. These results suggest that, in the outer Galaxy, the efficiency in the formation of most of the molecules studied, scaling with the availability of the parent element, is at least as high as in the local Galaxy, or perhaps even higher (e.g., for H¹³CO⁺). Yet, other species, such as SO, are characterized by a lower formation efficiency. These outcomes have important implications for the chemistry occurring in the outermost star-forming regions of the Galaxy and can help to constrain models adapted to lower metallicity environments.