Cloud-Scale Molecular Gas Properties in 15 Nearby Galaxies

Abstract: We measure the velocity dispersion, $\sigma$, and surface density, $\Sigma$, of the molecular gas in nearby galaxies from CO spectral line cubes with spatial resolution $45$-$120$ pc, matched to the size of individual giant molecular clouds. Combining $11$ galaxies from the PHANGS-ALMA survey with $4$ targets from the literature, we characterize ${\sim}30,000$ independent sightlines where CO is detected at good significance. $\Sigma$ and $\sigma$ show a strong positive correlation, with the best-fit power law slope close to the expected value for resolved, self-gravitating clouds. This indicates only weak variation in the virial parameter $\alpha_\mathrm{vir}\propto\sigma^2/\Sigma$, which is ${\sim}1.5$-$3.0$ for most galaxies. We do, however, observe enormous variation in the internal turbulent pressure $P_\mathrm{turb}\propto\Sigma\,\sigma^2$, which spans ${\sim}5\rm\;dex$ across our sample. We find $\Sigma$, $\sigma$, and $P_\mathrm{turb}$ to be systematically larger in more massive galaxies. The same quantities appear enhanced in the central kpc of strongly barred galaxies relative to their disks. Based on sensitive maps of M31 and M33, the slope of the $\sigma$-$\Sigma$ relation flattens at $\Sigma\lesssim10\rm\;M_\odot\,pc^{-2}$, leading to high $\sigma$ for a given $\Sigma$ and high apparent $\alpha_\mathrm{vir}$. This echoes results found in the Milky Way, and likely originates from a combination of lower beam filling factors and a stronger influence of local environment on the dynamical state of molecular gas in the low density regime.

Overview of Cloud-Scale Molecular Gas Properties in Nearby Galaxies

This paper presents a detailed analysis of molecular gas on a cloud scale across 15 nearby galaxies. The focus is on the line width and surface density of molecular gas, using CO spectral line cubes from the PHANGS-ALMA survey and additional targets. The study characterizes approximately 30,000 independent sightlines with CO detections, providing an unprecedented sample to understand cloud-scale molecular gas properties in diverse galactic environments.

Data and Methods

By analyzing CO spectral line data with spatial resolutions of 45-120 pc, the study aims to capture the properties of individual giant molecular clouds (GMCs). Using the PHANGS-ALMA survey data complemented by observations from M31, M33, M51, and the Antennae, the paper evaluates molecular gas properties, emphasizing the velocity dispersion ($\sigma$) and surface density ($\Sigma$) scaling relations. The study utilises a fixed-spatial-scale measurement approach, allowing a consistent comparison of observations across different galaxies while accounting for variations in instrument sensitivity and galaxy-specific conditions.

Key Findings

1. Surface Density and Velocity Dispersion Distributions: The distribution of molecular gas mass with respect to surface density and line width varies significantly between galaxies, generally showing a positive correlation. The median surface densities vary from 30 to 200 M⊙ pc⁻², while line width medians span from 3 to 10 km s⁻¹, with observable galaxy-to-galaxy variations.

2. Scaling Relation: The paper identifies a strong correlation between $\sigma$ and $\Sigma$, reminiscent of the $$^2/R \propto $$ scaling relation often observed in individual GMC studies. The slope of this relation, typically around 0.5, suggests a nearly constant virial parameter across various environments, implying the kinetic and gravitational energy balance of the clouds is largely uniform.

3. Virial Parameter and Turbulent Pressure: The analysis of the virial parameter indicates most molecular gas structures are close to energy equipartition, with slight kinetic energy superiority. Internal turbulent pressures vary significantly, spanning several orders of magnitude, and correlate with environmental factors such as galaxy mass and central density enhancements.

4. Environmental Influence: Central regions of strongly barred galaxies exhibit higher $\sigma$ and $\Sigma$, suggesting a dynamic environment that affects gas properties. This region-specific enhancement implies that while the mean state of molecular clouds may be similar across galaxies, the local environment has a significant impact.

5. High and Low Surface Density Extremes: By including galaxies like the Antennae and Local Group members M31 and M33, the study extends observable trends to extreme conditions. For the Antennae, the scaling relation aligns with expectations but shifted to higher pressures due to merger dynamics. Conversely, M31 and M33 show higher $\sigma$ at low $\Sigma$, indicating external pressures from the ambient medium and potential underestimation of mass.

Implications and Future Directions

The paper's findings underscore the importance of both universal properties of molecular clouds and local environmental effects. The observed universality of the virial parameter suggests homogeneous cloud conditions at large; however, substantial variations in turbulent pressure point to significant environmental regulation on smaller scales. Future studies will benefit from linking cloud-scale properties to galactic-scale processes, possibly integrating stellar and gas dynamics models to map these relations more accurately.

Enhancing our understanding of molecular gas dynamics promises progress in star formation theories, given the role of molecular clouds as star formation sites. The continuation of the PHANGS-ALMA project, including forthcoming data on more galaxies, will further elucidate these processes, allowing a refined understanding of star-forming galaxies in the local universe and across cosmic time.