How Galactic Environment Affects the Dynamical State of Molecular Clouds and Their Star Formation Efficiency

Abstract

We investigate how the dynamical state of molecular clouds relates to host galaxy environment, and how this impacts the star formation efficiency in the Milky Way and seven nearby galaxies. We compile measurements of molecular cloud and host galaxy properties and determine mass-weighted mean cloud properties for entire galaxies and distinct subregions within. We find molecular clouds to be in ambient pressure-balanced virial equilibrium, where clouds in gas-rich, molecular-dominated, high-pressure regions are close to self-virialization, whereas clouds in gas-poor, atomic-dominated, low-pressure environments achieve a balance between their internal kinetic pressure and external pressure from the ambient medium. The star formation efficiency per free-fall time of molecular clouds is low ∼0.1%−1% and shows systematic variations of 2 dex as a function of the virial parameter and host galactic environment. The trend observed for clouds in low-pressure environments-as the solar neighborhood-is well matched by state-of-the-art turbulence-regulated models of star formation. However, these models substantially overpredict the low observed star formation efficiencies of clouds in high-pressure environments, which suggests the importance of additional physical parameters not yet considered by these models.

Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-forming Galaxies

“…Based on observations from the PdBI Arcsecond Whirlpool Survey (Pety et al 2013;Schinnerer et al 2013), Hughes et al (2013aHughes et al ( , 2013b and Colombo et al (2014) showed strong variations in GMC properties within the inner ∼4 kpc of M51 and among M51, M33, and the Large Magellanic Cloud (LMC). Similar results were found by subsequent works studying GMCs in various types of galaxies in the local universe (e.g., Donovan Meyer et al 2013;Leroy et al 2015;Rebolledo et al 2015;Utomo et al 2015;Freeman et al 2017;Egusa et al 2018;Hirota et al 2018;Schruba et al 2019).…”

“…In this case, a version of dynamical equilibrium is reached-at least in a population-averaged sense-on cloud scales. In a recent observational study, Schruba et al (2019) have shown that GMCs in the Milky Way and seven nearby galaxies indeed appear to achieve this equilibrium state when averaged over the population.…”

“…More recently, following the observations of Heyer et al (2009), Field et al (2011) presented a simple model for how the observed variations in the line width and surface density could be linked to pressure in the ambient medium. The work of Heyer et al (2009) and Field et al (2011) led to widespread appreciation of the potential role of external pressure and adoption of the size-linewidthsurface density parameter space as a crucial diagnostic of the link between molecular clouds and environment (e.g., Hughes et al 2013a;Johnson et al 2015;Leroy et al 2015;Utomo et al 2015; and for recent synthesis works see Colombo et al 2019;Schruba et al 2019). The Field et al (2011) formalism to consider clouds interacting with external pressure has also been revisited by subsequent works (e.g., Meidt 2016;Schruba et al 2019), and further generalized to include the effect of the external, galactic gravitational potential and in-plane gas motions (e.g., Meidt et al 2018Meidt et al , 2020.…”

“…As emphasized above, this paper builds on a number of previous works. Using a larger, more homogeneous sample, we aim to explain the observed variations in cloud-scale gas properties in Sun et al (2018), to explicitly test the hypotheses of Field et al (2011) and Hughes et al (2013a), and to extend the consideration of some of the same topics covered by Schruba et al (2019). Our calculation of the mean ISM pressure adapts from Elmegreen (1989) and many subsequent works (e.g., Wong & Blitz 2002;Blitz & Rosolowsky 2004.…”

Towards a multitracer timeline of star formation in the LMC – I. Deriving the lifetimes of H i clouds

“…However, such a simplistic approach neglects the acceleration due to the molecular phase of the ISM and the stellar disc which is particularly important in low-density environments (e.g. Schruba et al 2019). The consistency between the measured time-scale associated with the atomic clouds and the predicted time-scale for the gravitational collapse of the ISM (the ISM free-fall timescale in Fig.…”