The Gas–Star Formation Cycle in Nearby Star-forming Galaxies. I. Assessment of Multi-scale Variations

Abstract

The processes regulating star formation in galaxies are thought to act across a hierarchy of spatial scales. To connect extragalactic star formation relations from global and kpc-scale measurements to recent cloud-scale resolution studies, we have developed a simple, robust method that quantifies the scale dependence of the relative spatial distributions of molecular gas and recent star formation. In this paper, we apply this method to eight galaxies with ∼ 1 resolution molecular gas imaging from arXiv:1910.10520v1 [astro-ph.GA] 23 Oct 2019 Schinnerer, Hughes, Leroy, Groves & PHANGS the PHANGS-ALMA and PAWS surveys that have matched resolution, high quality narrowband Hα imaging. At a common scale of 140 pc, our massive (log(M [M ]) = 9.3 − 10.7), normally starforming (SFR[M yr −1 ] = 0.3 − 5.9) galaxies exhibit a significant reservoir of quiescent molecular gas not associated with star formation as traced by Hα emission. Galactic structures act as backbones for both molecular and H II region distributions. As we degrade the spatial resolution, the quiescent molecular gas disappears, with the most rapid changes occurring for resolutions up to ∼ 0.5 kpc. As the resolution becomes poorer, the morphological features become indistinct for spatial scales larger than ∼ 1 kpc. The method is a promising tool to search for relationships between the quiescent or star-forming molecular reservoir and galaxy properties, but requires a larger sample size to identify robust correlations between the star-forming molecular gas fraction and global galaxy parameters.

The Molecular Cloud Lifecycle

“…This latter observation is an essential complement to CO data, because it provides an absolute 'reference timescale' that enables translating the relative lifetimes of regions bright in CO and star formation rate tracers to absolute timescales (see Haydon et al 2018). High-resolution observations of gas and star formation in nearby galaxies now show that CO and Hα emission rarely coincide on the cloud scale (Kreckel et al 2018;Kruijssen et al 2019b;Schinnerer et al 2019a). Kruijssen et al (2019b) and Chevance et al (2020) used this empirical result to constrain the GMC lifecycle in the nearby flocculent spiral galaxy NGC300 and to nine nearby star-forming spiral galaxies observed as part of the PHANGS-ALMA survey (Leroy et al in prep.…”

“…However, the feedback from young stellar populations in this simulation is insufficient to disperse GMCs, causing CO and Hα emission to be correlated down to the cloud scale, in strong disagreement with observations. The observed cloudscale decorrelation between tracers of molecular gas and massive star formation (Schruba et al 2010;Kreckel et al 2018;Kruijssen et al 2019b;Schinnerer et al 2019a;Chevance et al 2020;Hygate et al 2019) thus provides a fundamental test of how well numerical simulations reproduce the evolutionary lifecycle of GMCs in the real Universe, because it probes the GMC lifecycle more directly than the demographics of the GMC population (Fujimoto et al 2019).…”

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

“…The result of this localized collapse is the star formation feedback that pressurizes the rest of the ISM, providing internal support. Although this is likely a violent process with alternating episodes of collapse and expansion (see, e.g., Kruijssen et al 2019;Rahner et al 2019;Schinnerer et al 2019;Chevance et al 2020), dynamical equilibrium is expected when considering the entire ISM across large spatial scales (? typical size of GMCs and starforming regions) and long timescales (?…”

“…Unlike á ñ P turb,120 pc , which is estimated on 120pc scales and then averaged over the kpc-scale aperture, our S SFR,1 kpc measurements are derived directly on kpc scale. Just like the molecular gas, we expect star formation to cluster on sub-kpc scales (e.g., Grasha et al 2018Grasha et al , 2019Schinnerer et al 2019;Chevance et al 2020). This will cause the S SFR,1 kpc values in Figure 8 to appear lower due to the inclusion of area without star formation, and thus it underestimates the actual SFR surface density relevant to feedback momentum injection.…”

“…, and its 16th-84th percentile range is 0.73dex.A direct estimation of C fb requires high-resolution data tracing SFR on matched∼10-100 pc scales. This is currently not available for our entire sample (c.f.,Kreckel et al 2018;Schinnerer et al 2019;Chevance et al 2020). However, we can obtain a rough estimate of C fb based on our high-resolution CO data, if we assume that the clustering of star formation matches that of the molecular gas.…”