Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-Forming Galaxies (2002.08964v1)

Abstract: We compare the observed turbulent pressure in molecular gas, $P_\mathrm{turb}$, to the required pressure for the interstellar gas to stay in equilibrium in the gravitational potential of a galaxy, $P_\mathrm{DE}$. To do this, we combine arcsecond resolution CO data from PHANGS-ALMA with multi-wavelength data that traces the atomic gas, stellar structure, and star formation rate (SFR) for 28 nearby star-forming galaxies. We find that $P_\mathrm{turb}$ correlates with, but almost always exceeds the estimated $P_\mathrm{DE}$ on kiloparsec scales. This indicates that the molecular gas is over-pressurized relative to the large-scale environment. We show that this over-pressurization can be explained by the clumpy nature of molecular gas; a revised estimate of $P_\mathrm{DE}$ on cloud scales, which accounts for molecular gas self-gravity, external gravity, and ambient pressure, agrees well with the observed $P_\mathrm{turb}$ in galaxy disks. We also find that molecular gas with cloud-scale ${P_\mathrm{turb}}\approx{P_\mathrm{DE}}\gtrsim{10^5\,k_\mathrm{B}\,\mathrm{K\,cm^{-3}}}$ in our sample is more likely to be self-gravitating, whereas gas at lower pressure appears more influenced by ambient pressure and/or external gravity. Furthermore, we show that the ratio between $P_\mathrm{turb}$ and the observed SFR surface density, $\Sigma_\mathrm{SFR}$, is compatible with stellar feedback-driven momentum injection in most cases, while a subset of the regions may show evidence of turbulence driven by additional sources. The correlation between $\Sigma_\mathrm{SFR}$ and kpc-scale $P_\mathrm{DE}$ in galaxy disks is consistent with the expectation from self-regulated star formation models. Finally, we confirm the empirical correlation between molecular-to-atomic gas ratio and kpc-scale $P_\mathrm{DE}$ reported in previous works.

Overview of "Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-Forming Galaxies"

This paper presents a comprehensive paper of the turbulent pressure within molecular gas in 28 nearby star-forming galaxies, using high-resolution CO data from the PHANGS-ALMA survey. The authors compare this observed turbulent pressure, $P_\mathrm{turb}$, with the dynamical equilibrium pressure required for the interstellar medium (ISM) to remain stable in the host galaxy's gravitational potential, $P_\mathrm{DE}$. The paper provides crucial insights into the equilibrium state of the molecular ISM, a key determinant of star formation processes and molecular cloud stability.

Turbulent and Dynamical Equilibrium Pressure

The research shows that the turbulent pressure in molecular gas generally correlates with, but is consistently larger than, the estimated dynamical equilibrium pressure on kiloparsec scales. This observation that molecular gas is over-pressurized indicates a complex interrelation between internal cloud dynamics and external forces. The degree of over-pressurization ranges from factors of 1.3 to 6.3 at different spatial scales and environments, suggesting the influence of local gas clumpiness on pressure dynamics.

Cloud-Scale Dynamics

The authors propose that this over-pressurization can be explained by the clumpy nature of molecular gas, which results in enhanced self-gravity not captured in large-scale smooth disk models. By recalculating $P_\mathrm{DE}$ on smaller, cloud scales taking into account molecular gas self-gravity, ambient pressures, and external gravitational influence, the paper finds that these revised equilibrium pressures align more closely with observed turbulent pressures. Specifically, regions where $P_\mathrm{turb}\approx{P_\mathrm{DE}}$ are prevalent in areas with pressures surpassing $10^5\,k_\mathrm{B}\,\mathrm{K\,cm^{-3}}$, suggesting self-gravity's dominance in dense molecular clouds, particularly in galactic disk regions.

Star Formation Rate Correlation

An intriguing aspect of the paper is the identified correlation between the ratio of $P_\mathrm{turb}$ to the star formation rate surface density, $\Sigma_\mathrm{SFR}$, and the potential momentum injection from stellar feedback. The correlation confirms that, in most cases, ISM turbulence might primarily be driven by stellar processes, though a subset of regions indicate possible alternative turbulence sources. This finding is essential in understanding feedback mechanisms that regulate star formation in disk galaxies.

Implications and Future Prospects

The confirmation of the dynamical equilibrium model in various galactic environments implies significant avenues for future research. Understanding cloud-scale equilibrium might improve theoretical models of star formation and elucidate the role of feedback in disk galaxy evolution. Furthermore, speculating on broader implications, if dynamics seen in these local galaxies apply universally, upcoming high-resolution observations of more distant systems could reveal whether similar equilibrium mechanisms operate across cosmic epochs.

This work cements the integral role of gravitational forces and turbulence in shaping the structure and stability of molecular clouds within galaxies. By thoroughly addressing the balance of forces on different scales, the paper provides a solid foundation for advancing both observational and theoretical studies of ISM dynamics, which are pivotal to galaxy evolution models. The paper showcases how high-resolution data combined with careful multi-wavelength analysis supremely enhances our understanding of star-forming processes in the universe.