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

Chevance, M, Kruijssen, JMD, Hygate, APS, Schruba, A, Longmore, SN, Groves, B, Henshaw, JD, Herrera, CN, Hughes, A, Jeffreson, SMR, Lang, P, Leroy, AK, Meidt, SE, Pety, J, Razza, A, Rosolowsky, E, Schinnerer, E, Bigiel, F, Blanc, GA, Emsellem, E , Faesi, CM, Glover, SCO, Haydon, DT, Ho, I-T, Kreckel, K, Lee, JC, Liu, D, Querejeta, M, Saito, T, Sun, J, Usero, A and Utomo, D (2019) The lifecycle of molecular clouds in nearby star-forming disc galaxies. Monthly Notices of the Royal Astronomical Society, 493 (2). pp. 2872-2909. ISSN 0035-8711

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It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially-resolved ($\sim100$ pc) CO-to-H$\alpha$ flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10-30 Myr, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $\Sigma_{\rm H_2}\geqslant8$M$_{\odot}$pc$^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $\Sigma_{\rm H_2}\leqslant8$M$_{\odot}$pc$^{-2}$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H$\alpha$ (75-90% of the cloud lifetime), GMCs disperse within just 1-5 Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4-10% These results show that galactic star formation is governed by cloud-scale, environmentally-dependent, dynamical processes driving rapid evolutionary cycling. GMCs and HII regions are the fundamental units undergoing these lifecycles, with mean separations of 100-300 pc in star-forming discs. Future work should characterise the multi-scale physics and mass flows driving these lifecycles.

Item Type: Article
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords: astro-ph.GA; astro-ph.GA
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
Publisher: Oxford University Press
Related URLs:
Date Deposited: 25 Nov 2020 10:42
Last Modified: 04 Sep 2021 08:22
URI: https://researchonline.ljmu.ac.uk/id/eprint/11811
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