A multi-scale evolutionary study of molecular gas in STARFORGE

I. Synthetic observations of SEDIGISM-like molecular clouds

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
³ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
⁴ Centre for Modern Interdisciplinary Technologies, Nicolaus Copernicus University in Torun, Wileńska 4, 87-100 Toruń, Poland
⁵ National Centre for Nuclear Research, Pasteura 7, 02-093 Warszawa, Poland
⁶ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK
⁷ School of Physics & Astronomy, Cardiff University, Queen’s building, The parade, Cardiff CF24 3AA, UK

^★ Corresponding author: kneralwar@mpifr-bonn.mpg.de

Received: 4 September 2025
Accepted: 8 October 2025

Abstract

Molecular clouds (MCs) are active sites of star formation in galaxies, and their formation and evolution are largely affected by stellar feedback. This includes outflows and winds from newly formed stars, radiation from young clusters, and supernova explosions. High-resolution molecular line observations allow for the identification of individual star-forming regions and the study of their integrated properties. Moreover, state-of-the-art simulations are now capable of accurately replicating the evolution of MCs, including all key stellar feedback processes. We present ¹³CO(2–1) synthetic observations of the STARFORGE simulations produced using the radiative transfer code RADMC-3D, matching the observational setup of the SEDIGISM survey. From these synthetic observations, we identified the population of MCs using hierarchical clustering and analysed them to provide insights into the interpretation of observed MCs as they evolve. The flux distributions of the post-processed synthetic observations and the properties of the MCs, namely, radius, mass, velocity dispersion, virial parameter, and surface density, are consistent with those of SEDIGISM. Both samples of MCs occupy the same regions in the scaling relation plots; however, the average distributions of MCs at different evolutionary stages do not overlap on the plots. This highlights the reliability of our approach in modelling SEDIGISM and suggests that MCs at different evolutionary stages contribute to the scatter in observed scaling relations. We study the trends in MC properties, morphologies, and fragmentation over time to analyse their physical structure as they form, evolve, and are destroyed. MCs appear as small diffuse cloudlets in early stages, and this is followed by their evolution to filamentary structures before being shaped by stellar feedback into 3D bubbles and getting dispersed. These trends in the observable properties of MCs are consistent with other realisations of simulations and provide strong evidence that clouds exhibit distinct morphologies over the course of their evolution.