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

IV. ALMA observations of organic species at a galactocentric radius of ~23 kpc

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

^★ Corresponding author; francesco.fontani@inaf.it

Received: 14 July 2024
Accepted: 30 September 2024

Abstract

Context. Single-dish observations suggest that the abundances of organic species in star-forming regions of the outer Galaxy, which are characterised by sub-solar metallicities, are comparable to those found in the local Galaxy.

Aims. To understand this counter-intuitive result and avoid a misleading interpretation due to beam dilution effects at these large distances, spatially resolved molecular emission maps are needed to correctly link the measured abundances and local physical properties.

Methods. We observed several organic molecules with the Atacama Large Millimeter Array towards WB89-671, the source with the largest galactocentric distance (23.4 kpc) of the project CHEMical complexity in star-forming regions of the OUTer Galaxy (CHEMOUT) at a resolution of ~15 000 au. We compared the observed molecular abundances with chemical model predictions.

Results. We detected emission of c-C₃H₂, C₄H, CH₃OH, H₂CO, HCO, H¹³CO⁺, HCS⁺, CS, HN¹³C, and SO. The emission morphology is complex, extended, and different in each tracer. In particular, the most intense emission in H¹³CO⁺, H₂CO and c-C₃H₂ arises from two millimeter-continuum infrared-bright cores. The most intense CH₃OH and SO emission predominantly arises from the part of the filament that lacks continuum sources. The narrow line widths across the filament indicate quiescent gas in spite of the two embedded protostars. The derived molecular column densities are comparable with those in local star-forming regions, and they suggest an anti-correlation between hydrocarbons, ions, HCO, and H₂CO on the one hand, and CH₃OH and SO on the other.

Conclusions. The static chemical models that match the observed column densities best favour low-energy conditions that are expected at large galactocentric radii, but they also favour carbon elemental abundances that exceed those derived by extrapolating the [C/H] galactocentric gradient at 23 kpc by three times. This would indicate a flatter [C/H] trend at large galactocentric radii, which is in line with a flat abundance of organics. However, to properly reproduce the chemical composition of each region, models should include dynamical evolution.