Molecular Gas Dynamics in Star-Forming Galactic Environments

Molecular gas dynamics in star-forming galactic environments encompasses the complex interplay between gravity, turbulence, magnetic fields and radiative processes that govern the assembly, evolution and eventual collapse of cold interstellar clouds into new stars. These environments range from quiescent spiral arms to turbulent merger‐driven starbursts, yet share a common cycle in which diffuse atomic gas accretes onto spiral or bar‐induced flows, cools, and transitions into dense molecular clouds. Within these clouds, supersonic turbulence and shear generate filamentary structures that funnel material into cores, where localised collapse triggers protostellar accretion. Feedback from young massive stars—through ionising radiation, stellar winds and supernovae—both disrupts natal clouds and regulates subsequent star formation efficiency. Observational tracers such as rotational transitions of carbon monoxide and its isotopologues, combined with fine‐structure lines of neutral carbon, provide vital diagnostics of temperature, density, optical depth and kinematics. Advances in high‐resolution interferometry and multi‐tracer surveys have revealed how galactic environment modulates the molecular gas depletion timescale, the CO‐to‐H₂ conversion factor and the spatial clustering of star formation, thereby linking small‐scale physics to global galaxy evolution.

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Technical terms

CO-to-H₂ conversion factor (αCO): The proportionality constant used to convert observed CO line intensity into molecular hydrogen mass.

Isotopologue: A molecular variant differing only in the isotopic composition of its constituent atoms (e.g. ¹²CO vs ¹³CO).

Local thermodynamic equilibrium (LTE): An assumption that molecular level populations are determined solely by the local kinetic temperature, simplifying radiative transfer calculations.

Non-LTE modelling: Radiative transfer analysis that relaxes the LTE assumption, accounting for collisional and radiative processes to derive physical conditions.

Optical depth: A dimensionless measure of transparency, indicating the extent to which radiation is absorbed or scattered as it passes through gas.

Velocity dispersion: The range of velocities present within a gas ensemble, reflecting turbulence, thermal motions and large‐scale flows.

References

1. PHANGS–JWST First Results: Duration of the Early Phase of Massive Star Formation in NGC 628. The Astrophysical Journal Letters (2023).
2. A sensitive APEX and ALMA CO(1–0), CO(2–1), CO(3–2), and [CI](1–0) spectral survey of 40 local (ultra-)luminous infrared galaxies. Astronomy & Astrophysics (2023).
3. CO Isotopologue-derived Molecular Gas Conditions and CO-to-H2 Conversion Factors in M51. The Astronomical Journal (2024).