Simulations of the star-forming molecular gas in an interacting M51-like galaxy: cloud population statistics

Abstract

To investigate how molecular clouds react to different environmental conditions at a galactic scale, we present a catalogue of giant molecular clouds resolved down to masses of ∼10 M⊙ from a simulation of the entire disc of an interacting M51-like galaxy and a comparable isolated galaxy. Our model includes time-dependent gas chemistry, sink particles for star formation and supernova feedback, meaning we are not reliant on star formation recipes based on threshold densities and can follow the physics of the cold molecular phase. We extract giant molecular clouds from the simulations and analyse their properties. In the disc of our simulated galaxies, spiral arms seem to act merely as snowplows, gathering gas and clouds without dramatically affecting their properties. In the centre of the galaxy, on the other hand, environmental conditions lead to larger, more massive clouds. While the galaxy interaction has little effect on cloud masses and sizes, it does promote the formation of counter-rotating clouds. We find that the identified clouds seem to be largely gravitationally unbound at first glance, but a closer analysis of the hierarchical structure of the molecular interstellar medium shows that there is a large range of virial parameters with a smooth transition from unbound to mostly bound for the densest structures. The common observation that clouds appear to be virialised entities may therefore be due to CO bright emission highlighting a specific level in this hierarchical binding sequence. The small fraction of gravitationally bound structures found suggests that low galactic star formation efficiencies may be set by the process of cloud formation and initial collapse.

Molecular Cloud Populations in the Context of Their Host Galaxy Environments: A Multiwavelength Perspective

“…Within a galaxy, molecular clouds located closer to the galaxy center appear denser, more massive, and more turbulent (e.g., Oka et al 2001;Colombo et al 2014;Freeman et al 2017;Hirota et al 2018;Miura et al 2018;Brunetti et al 2021, also see Heyer & Dame 2015). Similar trends have been found in galaxy-scale numerical simulations (e.g., Pan et al 2015;Jeffreson et al 2020;Treß et al 2021). Recent observational works also report that more massive and actively star-forming galaxies tend to host clouds with typically larger sizes, masses, surface densities, and velocity dispersions (Hughes et al 2013a; Leroy et al 2015Leroy et al , 2016Schruba et al 2019;Sun et al 2020a, but see Bolatto et al 2008;Fukui & Kawamura 2010;Donovan Meyer et al 2013).…”

“…(2) Both our database and our power-law models can be used to predict molecular cloud properties in other samples of star-forming galaxies with only kpc-resolution data (e.g., Bolatto et al 2017;Sorai et al 2019;Lin et al 2020). (3) Our databases provide a comprehensive set of initial conditions and outcome properties for benchmarking numerical simulations of the cold interstellar gas at high spatial resolution (e.g., Benincasa et al 2013;Kim & Ostriker 2017;Dobbs et al 2019;Jeffreson et al 2020;Li et al 2020;Treß et al 2021). (4) Our measurements allow for crucial tests of analytical star formation theories (e.g., Krumholz & McKee 2005;Hennebelle & Chabrier 2011;Padoan 2011;Federrath & Klessen 2012;Krumholz et al 2018;Burkhart & Mocz 2019;Orr et al 2022), as well as empirical calibrations of "sub-grid star formation recipes" in galaxy evolution models (e.g., Olsen et al 2017;Vallini et al 2018;Popping et al 2019).…”

PHANGS–ALMA: Arcsecond CO(2–1) Imaging of Nearby Star-forming Galaxies

“…These measurements will provide important constraints on GMC formation, destruction, and evolution (e.g., Jeffreson & Kruijssen 2018). This will quantitatively connect GMC studies to models of galaxy evolution (e.g., Somerville & Davé 2015) and provide key benchmarks for numerical simulations aiming to "get the cold gas right" (e.g., Dobbs et al 2019;Jeffreson et al 2020;Tress et al 2021). First work on this topic using PHANGS-ALMA appears in Sun et al (2018Sun et al ( , 2020aSun et al ( , 2020b, Herrera et al (2020), andRosolowsky et al (2021).…”

The formation and early evolution of embedded star clusters in spiral galaxies

“…Simulations focusing on larger scales (e.g. Bending et al 2020;Smilgys & Bonnell 2017;Treß et al 2021) therefore simplify the stellar population into 'sink particles', whose properties are calculated using sub-grid physics. These simulations ignore the dynamical evolution of the stellar population, and don't allow us to study the evolution of the star clusters.…”