Cloud-scale ISM Structure and Star Formation in M51

Abstract

We compare the structure of molecular gas at 40 pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into 370 pc and 1.1 kpc resolution elements, and within each we estimate the molecular gas depletion time (τ mol Dep ), the star formation efficiency per free fall time (ǫ ff ), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, Σ, line width, σ, andvir , a parameter that traces the boundedness of the gas. We show that the cloud-scale surface density appears to be a reasonable proxy for mean volume density. Applying this, we find a typical star formation efficiency per free-fall time, ǫ ff ( Σ 40pc ) ∼ 0.3−0.36%, lower than adopted in many models and found for local clouds. More, the efficiency per free fall time anti-correlates with both Σ and σ, in some tension with turbulent star formation models. The best predictor of the rate of star formation per unit gas mass in our analysis is b ≡ Σ/σ 2 , tracing the strength of self gravity, with τ mol Dep ∝ b −0.9 . The sense of the correlation is that gas with stronger self-gravity (higher b) forms stars at a higher rate (low τ

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

“…In line with several recent studies, our observations show that current models of turbulence regulated star formation-based on idealized assumptions of cloudscale density structure and turbulence, and assuming a stationary star formation rate-do not fully capture observations across a diverse range of galactic and extragalactic star-forming environments (Lee et al 2016;Barnes et al 2017;Leroy et al 2017a;Ochsendorf et al 2017). The discrepancy may reflect observational limitations in constraining the relevant physical parameters (e.g., uncertain mass-to-light conversion and beam diluted measurements), however, it could also be of physical origin in that cloud-scale turbulent properties insufficiently reflect the dynamic "boundary" conditions of star-forming molecular clouds and that additional largescale processes are relevant (e.g., implying environmental changes in the sonic and Alfénic Mach number and the nature of turbulence driving).…”

“…These studies also found significant scatter in the SFE of individual clouds, far in excess to what the models pre-3 dict (these have been attributed to an accelerating SFR along a cloud's time evolution), and only a weak dependence of the SFE on cloud properties (Murray 2011;Evans et al 2014;Lee et al 2016;Vutisalchavakul et al 2016;Ochsendorf et al 2017). Comparing cloud population averages to the SFR-per-H 2 and ff in M51, Leroy et al (2017a) also found a poor match to the predictions of turbulent models. The sense of the correlations between cloud properties and ff in M51 appears opposite that present in some of the models.…”

The lifecycle of molecular clouds in nearby star-forming disc galaxies

“…Secondly, the strong variation of the cloud lifetime between different galaxies suggests that cloud formation and collapse does not proceed on a universal time-scale but is plausibly governed by environment, such as galactic dynamics, either by directly setting the time-scale or indirectly, by changing the properties of the clouds (e.g. Leroy et al 2017b). We will explore this hypothesis in Section 5.…”

The EDGE-CALIFA survey: exploring the star formation law through variable selection

“…This is generally consistent with expectations given the dynamical state of the gas. Leroy et al (2017) showed that the depletion times for molecular gas in M51 increased if the gravitational "boundedness" (∝ Σ mol /σ CO ) decreased. Our results act in the same sense, but the weak scaling in the star formation law approach (Section 4.1) shows that this effect may manifest from how we are framing the analysis.…”