The Role of Pressure in the Structure and Stability of GMCs in the Andromeda Galaxy

TL;DR

This study investigates the role of external pressure in the stability of GMCs in M31, introducing a new method to measure internal pressure profiles and confirming pressure's importance for GMC confinement.

Contribution

It presents a novel methodology for measuring radial internal pressure profiles in GMCs and demonstrates the significance of external pressure in GMC stability in M31.

Findings

Most GMCs in M31 require external pressure for confinement.

Internal pressure increases with surface density as p_int ∼ Σ^2.

GMCs exhibit deviations from the power-law at high surface densities.

Abstract

We revisit the role of pressure in the structure, stability and confinement of Giant Molecular Clouds (GMCs) in light of recently published observations and analysis of the GMCs in the Andromeda galaxy (M31). That analysis showed, that in the absence of any external pressure, most GMCs (57\% by number) in M31 would be gravitationally unbound. Here, after a more detailed examination of the global measurements of surface densities and velocity dispersions, we find that GMCs in M31, when they can be traced to their outermost boundaries, require external pressures for confinement that are consistent with estimates for the mid-plane pressure of this galaxy. We introduce and apply a novel methodology to measure the radial profile of internal pressure within any GMC that is spatially resolved by the CO observations. We show that for the best resolved examples in M31 the internal pressures increase steeply with surface density in a power-law fashion with $p_{int} \sim \Sigma^2$. At high surface densities many of these extragalactic GMCs break from the single power-law and exhibit upward curvature. Both these characteristics of the variation of internal pressure with surface density are in agreement with theoretical expectations for hydrostatic equilibrium at each radial surface of a GMC, including the outermost boundary.