Connecting Star Formation in the Milky Way and Nearby Galaxies. I. Comparability of Molecular Cloud Physical Properties

TL;DR

This study tackles how to compare molecular clouds across the Milky Way and nearby galaxies by identifying clouds in CO data and evaluating their kinematics at matched spatial and velocity resolutions. Using CO transitions CO(2-1) and CO(1-0), the authors derive cloud properties including mass, radius, density, velocity dispersion, and virial parameter, with radii and other quantities corrected for beam effects through a smoothing framework and a true-radius estimation $r_t$ guided by filling factor $f$ and concentration $c$. They find that, once resolution is matched and beam-affected clouds are filtered, cloud properties and star formation rates are broadly consistent across environments, though some NGC 5236 clouds show environment-driven enhancements in $\,\sigma$; they also reveal strong correlations between cloud mass and internal clump properties, indicating that a cloud’s physical state shapes its clump content and star-forming potential. The work highlights the importance of resolution-matching in extragalactic cloud studies and provides a framework for interpreting large-scale molecular-gas observations in the context of Milky Way cloud physics, ultimately linking cloud-scale dynamics to star formation in galaxies. A key quantitative relation is the virial parameter $\alpha_{ m vir} = \frac{5\sigma^2 r_{ m s}}{G M}$, illustrating how dynamics, size, and mass together govern cloud stability and star-forming efficiency.

Abstract

We used CO (2-1) and CO (1-0) data cubes to identify molecular clouds and study their kinematics and dynamics in three nearby galaxies and the inner Milky Way. When observed at similar spatial and velocity resolutions, molecular clouds in the same mass range across these galaxies show broadly comparable physical properties and similar star formation rates (SFRs). However, this comparability depends on smoothing Milky Way clouds to match the resolution of the extragalactic observations. The beam effect can artificially inflate cloud sizes, leading to inaccurate estimates of radius, density, and virial parameters. By comparing high-resolution and smoothed Milky Way data, we established criteria to exclude beam-affected clouds in the extragalactic sample. After applying this filter, cloud properties remain consistent across galaxies, though some clouds in NGC 5236 show elevated velocity dispersions, likely due to environmental effects. In the inner Milky Way, molecular clouds fall into two groups: those with clumps and those without. Clump-associated clouds are more massive, denser, have higher velocity dispersions, lower virial parameters, and stronger 8~\(μ\)m emission, suggesting more intense feedback. Strong correlations are found between cloud mass and total clump mass, clump number, and the mass of the most massive clump. These results suggest that a cloud's physical conditions regulate its internal clump properties and, in turn, its star-forming potential.

Figures (13)

• Figure 1: Velocity-integrated CO (2-1) intensity maps of three nearby galaxies.
• Figure 2: The Galactic longitude–velocity map of the Milky Way with $300\degr\le l \le 360\degr$ and $-1\degr \le b \le 1\degr$ from the ThrUMMS survey. Black contours show the clouds identified by the dendrogram algorithm with different min_npix values (250, 100, 25 and 5 pixels). Here, both the velocity and spatial axes are expressed in pixel units.
• Figure 3: The first and second rows show the smoothed and original molecular clouds, respectively. In the first row, the three concentric circles represent three different radius estimates, i.e. 26.75 pc, ($r_{\rm max}$+26.75 pc)/2 and $r_{\rm max}$, from innermost to outermost. $f$ is the filling factor. $r_{\rm max}$ is defined in Sec.\ref{['parameters']}.
• Figure 4: Comparison of the physical properties of molecular clouds in the southern and northern inner Milky Way (first row), in three nearby galaxies (second row), and between molecular clouds in the inner Milky Way and those in nearby galaxies (third row). $M_{\rm cloud}$, $\sigma$, $N_{\rm a}$ and $\alpha_{\rm vir}$ represent the cloud mass, velocity dispersion, average column density, and virial parameter, respectively.
• Figure 5: Comparison of star formation rates (SFRs) in molecular clouds within the inner Milky Way and in nearby galaxies.