ISM properties in hydrodynamic galaxy simulations: turbulence cascades, cloud formation, role of gravity and feedback

Abstract

We study the properties of interstellar medium (ISM) substructure and turbulence in hydrodynamic [adaptive mesh refinement (AMR)] galaxy simulations with resolutions up to 0.8 pc and 5 × 103 M⊙. We analyse the power spectrum of the density distribution, and various components of the velocity field. We show that the disc thickness is about the average Jeans scalelength, and is mainly regulated by gravitational instabilities. From this scale of energy injection, a turbulence cascade towards small scale is observed, with almost isotropic small‐scale motions. On scales larger than the disc thickness, density waves are observed, but there is also a full range of substructures with chaotic and strongly non‐isotropic gas velocity dispersions. The power spectrum of vorticity in a Large Magellanic Cloud sized model suggests that an inverse cascade of turbulence might be present, although energy input over a wide range of scales in the coupled gaseous+stellar fluid could also explain this quasi‐two‐dimensional regime on scales larger than the disc scaleheight. Similar regimes of gas turbulence are also found in massive high‐redshift discs with high gas fractions. Disc properties and ISM turbulence appear to be mainly regulated by gravitational processes, both on large scales and inside dense clouds. Star formation feedback is however essential to maintain the ISM in a steady state by balancing a systematic gas dissipation into dense and small clumps. Our galaxy simulations employ a thermal model based on a barotropic equation of state aimed at modelling the equilibrium of gas between various heating and cooling processes. Denser gas is typically colder in this approach, which is shown to correctly reproduce the density structures of a star‐forming, turbulent, unstable and cloudy ISM down to scales of a few parsecs.

The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time

“…The relations between these findings is not yet clear. Other suggested mechanisms for generating high dispersions and thick discs are (i) clump-clump gravitational interaction (Dekel et al 2009a;Ceverino et al 2010), (ii) accretion of cold flows (Elmegreen & Burkert 2010;Aumer et al 2010), (iii) disc instabilities and Jeans collapse (Immeli et al 2004;Bournaud et al 2010;Ceverino et al 2010;Aumer et al 2010), and (iv) streams of minor mergers (Bournaud et al 2009). No dominant energetic picture has emerged; rather a consistent theme of these papers is that it is quite likely that there is more than one cause of high dispersion.…”

Studying the spatially resolved Schmidt-Kennicutt law in interacting galaxies: the case of Arp 158

“…The Jeans length which is of the order of 100-200 pc in nearby spirals increases up to 500-1000 pc in mergers because of higher densities and velocity dispersions. In addition this scale also corresponds to the injection scale of turbulence in the ISM (Elmegreen et al 2003;Bournaud et al 2010a). …”

Influence of baryonic physics in galaxy simulations:

“…Schaye & Dalla Vecchia (2008) use an effective equation of state at high densities that causes the Jeans mass to be independent of density, while Hopkins et al (2011) take a density-dependent pressure floor to ensure the Jeans length is resolved. Bournaud et al (2010) uses an equation of state that mimics the effect of cooling at all temperatures and a temperature floor at high densities. We take a temperature floor of 100 K for all densities.…”

“…The main interest here is to determine whether the small scale structure impacts the large-scale transfer of angular momentum, the stability of discs, formation of bulges, concentration of the galaxy, transformation of galaxies from late to early type, and whether feedback can destroy dark matter cusps, etc. (Maio et al 2007;Gnedin et al 2009;Schaye et al 2010;Murante et al 2010;Bournaud et al 2010;Hopkins et al 2011). 2.…”