Quantifying the energetics of molecular superbubbles in PHANGS galaxies

Abstract

Context. Star formation and stellar feedback are interlinked processes that redistribute energy, turbulence, and material throughout galaxies. Because young and massive stars form in spatially clustered environments, they create pockets of expanding gas termed superbubbles, which retain information about the physical processes that drive them. As these processes play a critical role in shaping galaxy discs and regulating the baryon cycle, measuring the properties of superbubbles provides important input for galaxy evolution models. Aims. With the wide coverage and high angular resolution (∼50–150 pc) of the PHANGS–ALMA 12CO (J = 2−1) survey, we can now resolve, identify and characterise a statistically representative number of superbubbles using molecular gas in nearby galaxies. Methods. We identify superbubbles by requiring spatial correspondence between shells in CO with stellar populations identified in PHANGS–HST. Then, by combining the properties of the stellar populations with the CO, we quantify the energetics of the stars and constrain feedback models. We visually find 325 cavities across 18 PHANGS–ALMA galaxies, 88 of which have clear superbubble signatures (unbroken shells, central clusters, kinematic signatures of expansion). We measure their radii and expansion velocities using CO (2–1) to dynamically derive their ages and the mechanical power driving the bubbles, which we use to compute the expected properties of the parent stellar populations driving the bubbles. Results. We find consistency between the predicted and derived stellar ages and masses of the stellar populations if we use a supernova (SN) model that injects energy with a coupling efficiency of ∼10%. Not only does this confirm that molecular gas accurately traces superbubble properties, but it also provides key observational constraints for superbubble models. We also find evidence that the bubbles are sweeping up gas as they expand, and speculate that these sites have the potential to host new generations of stars. Conclusions. This work demonstrates that molecular superbubbles provide novel quantitative constraints on SNe feedback efficiencies and gas clearing times, and represent a promising environment to search for the propagation of star formation, all of which are needed to understand what sets the observed star formation rates in galaxies.

Metallicity Dependence of Molecular Cloud Hierarchical Structure at Early Evolutionary Stages

“…The stronger inhomogeneity in the inflow density/velocity induces stronger turbulence (e.g., the systematic survey by Kobayashi et al 2020), but the turbulent speed is expected to be on the order of the WNM sound speed (see Appendix C). Expansion of multiple supernova remnants also induces cloud compressions as often observed in galactic-scale numerical simulations (Girichidis et al 2016;Kim et al 2023a) and may explain the bubble features in nearby galaxies observed in JWST (Watkins et al 2023). Each compression event occurs at diverse angles at various times, and the generation of solenoidal mode turbulence locally occurs due to the deformed shock fronts.…”

Investigating the Drivers of Electron Temperature Variations in H ii Regions with Keck-KCWI and VLT-MUSE

“…The CO velocity dispersion can be enhanced by low-velocity shocks originating from the interaction of molecular gas with late-time expansion of supernovae remnants (see Koo et al 2001;Zhou et al 2023), as well as from interaction with low-velocity shocks from H II region expansion driven by radiation pressure (Hill & Hollenbach 1978;Kothes & Kerton 2002;Watkins et al 2023). These low-velocity shocks are also predicted to enhance T e,[O III] .…”

“…It has also been shown that shocks driven by the radiation pressure from H II region expansion, as well as supernova remnants, impact the surrounding cold molecular and ionized gas. If the velocity of the expansion is greater than the sound speed of the ionized gas, ∼10 km s −1 , a layer of shocked H gas will form in between the expanding ionization front and a surrounding molecular gas (Hill & Hollenbach 1978;Kothes & Kerton 2002;Tremblin et al 2014;Watkins et al 2023). The impact of shock interaction with molecular gas has been studied in 18 galaxies observed as part of PHANGS-ALMA (Leroy et al 2021a(Leroy et al , 2021b, where Watkins et al (2023) identified hundreds of superbubbles (i.e., pockets of expanding gas arising as byproducts of feedback).…”

“…If the velocity of the expansion is greater than the sound speed of the ionized gas, ∼10 km s −1 , a layer of shocked H gas will form in between the expanding ionization front and a surrounding molecular gas (Hill & Hollenbach 1978;Kothes & Kerton 2002;Tremblin et al 2014;Watkins et al 2023). The impact of shock interaction with molecular gas has been studied in 18 galaxies observed as part of PHANGS-ALMA (Leroy et al 2021a(Leroy et al , 2021b, where Watkins et al (2023) identified hundreds of superbubbles (i.e., pockets of expanding gas arising as byproducts of feedback). The superbubbles were identified using spatial correspondence between CO shells and stellar populations contained in PHANGS-HST catalogs (Lee et al 2022;Thilker et al 2022;Larson et al 2023).…”

“…The superbubbles were identified using spatial correspondence between CO shells and stellar populations contained in PHANGS-HST catalogs (Lee et al 2022;Thilker et al 2022;Larson et al 2023). Due to the ALMA spatial resolution (50-150 pc), Watkins et al (2023) measured the expansion velocity only for the largest superbubbles. Assuming the CO expansion velocity is equal to the shock velocity, Watkins et al (2023) measure the velocity of the approaching/receding CO shells and determine an average line-of-sight expansion velocity of v exp = 9.8 ± 4.3 km s −1 .…”