A multiscale evolutionary study of molecular gas in STARFORGE. I. Synthetic observations of SEDIGISM-like molecular clouds

TL;DR

This work bridges state-of-the-art simulations and Galactic MC observations by generating 13CO(2-1) synthetic PPV cubes from STARFORGE and processing them through a pipeline analogous to the SEDIGISM survey. Using RADMC-3D radiative transfer, two-way projections, and dendrogram-based MC extraction, the authors show that the synthetic MCs reproduce SEDIGISM flux distributions and occupy the same parameter space in key scaling relations. They further demonstrate that MC properties, morphology, and substructure evolve systematically under stellar feedback, with distinct evolutionary stages occupying different regions in scaling diagrams and driving observed scatter. The results illuminate how feedback shapes molecular clouds—from initial diffuse clumps to filamentary structures and finally to 3D bubbles—and provide a framework for interpreting current and future cloud surveys, while outlining paths to isolate specific feedback processes with additional tracers.

Abstract

Molecular clouds 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, 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, 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 over time to analyse their physical structure as they evolve. MCs appear as small, diffuse cloudlets in early stages, 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 provide strong evidence that clouds exhibit distinct morphologies over the course of their evolution.

Figures (21)

• Figure 1: Left: A synthetic integrated $^{13}$CO(2-1) emission map created with RADMC-3D. Right: The same map after convolution and with noise of sigma = $0.2 \pm 0.02$ K.
• Figure 2: Comparison between the flux values for the noisy pixels from all the STARFORGE snapshots along all three projections and the complete SEDIGISM survey ($^{13}$CO(2-1) emission).
• Figure 3: Comparison of MC properties from STARFORGE (left side violins) and SEDIGISM (right side violins). The properties are radius (R), velocity dispersion ($\sigma_v$), luminosity (L), virial parameter ($\alpha_{vir}$) and surface density ($\Sigma$). The horizontal dashed lines represent the quartiles of the distributions.
• Figure 4: $^{13}$CO(2-1) moment 0 maps at different evolutionary time. The background greyscale represents H$_2$ gas density with $^{13}$CO(2-1) emission overlaid as viridis maps (molecular gas complex), and coloured contours represent different MCs (dendrogram trunks), with red contours representing the largest MCs ($R$) in the cube. The $^{13}$CO(2-1) maps for multiple snapshots along different projections are presented in App. \ref{['app: GMC and MC imshow images']}.
• Figure 5: Normalised medians of MC properties (log scales) as a function of time: radius (R), velocity dispersion ($\sigma_v$), molecular gas mass (M), virial parameter ($\alpha_{vir}$), surface mass density ($\Sigma$), and luminosity (L). Count refer to the total number of clouds in a time bin. The normalisation includes subtracting the initial value (first bin) of the property from it, followed by a min-max standardisation. The symbols on the top represent the times at which outflows, photoionisation radiation, stellar winds and supernovae begin in the simulation.