A subresolution multiphase interstellar medium model of star formation and supernova energy feedback

Abstract

We present a new multiphase subresolution model for star formation and feedback in smoothed particle hydrodynamics (SPH) numerical simulations of galaxy formation. Our model, called MUPPI (MUlti-Phase Particle Integrator), describes each gas particle as a multiphase system, with cold and hot gas phases, coexisting in pressure equilibrium and a stellar component. Cooling of the hot tenuous gas phase feeds the cold gas phase. We compute the cold gas molecular fraction using the phenomenological relation of Blitz & Rosolowsky between this fraction and the external disc pressure, which we identify with the SPH pressure. Stars are formed out of molecular gas with a given efficiency, which scales with the dynamical time of the cold phase. Our prescription for star formation is not based on imposing the Schmidt-Kennicutt relation, which is instead naturally produced by MUPPI. Energy from supernova explosions is deposited partly into the hot phase of the gas particles, and partly to that of neighbouring particles. Mass and energy flows among the different phases of each particle are described by a set of ordinary differential equations which we explicitly integrate for each gas particle, instead of relying on equilibrium solutions. This system of equations also includes the response of the multiphase structure to energy changes associated to the thermodynamics of the gas. Our model has an intrinsically runaway behaviour: energy from supernovae increases gas pressure which increases in turn the star formation rate through the molecular fraction. This runaway is stabilized in simulations by the hydrodynamic response of the gas: when it receives enough energy, it expands thereby decreasing its pressure. We apply our model to two isolated disc galaxy simulations and two spherical cooling flows. MUPPI is able to reproduce the Schmidt-Kennicutt relation for disc galaxies. It also reproduces the basic properties of the interstellar medium in disc galaxies, the surface densities of cold and molecular gas, of stars and of star formation rate, the vertical velocity dispersion of cold clouds and the flows connected to the galactic fountains. Quite remarkably, MUPPI also provides efficient stellar feedback without the need to include a scheme of kinetic energy feedback.

A fluid-dynamical subgrid scale model for highly compressible astrophysical turbulence

“…In particular, it will be useful to attempt a further generalization of the SGS model to multiphase turbulence. A very simple ansatz has recently been presented by Murante et al (2010). A complete SGS model that treats a warm and a cold gas phase is presently under development (Braun & Schmidt, in prep.…”

The resolution bias: low-resolution feedback simulations are better at destroying galaxies

“…Such processes have previously been missed in analytical models and cosmological simulations mainly because the multiphase nature of the ISM is not resolved and has to be implemented as a subresolution model. For example Springel & Hernquist (2003) include a sub-grid multiphase model for star formation while Murante et al (2010) include a non-equilibrium model that includes the three ISM phases for each SPH particle. Unfortunately such methods mean that the intricate structure one expects is washed out due to low resolution.…”

Galactic outflow and diffuse gas properties at z ≥ 1 using different baryonic feedback models

“…Here star formation depends on local pressure through the molecular fraction of gas (Eq. 5 in Murante et al 2010), and the gas pressure is expected to depend on halo mass. The star forming gas undergoes kinetic feedback and produces galactic outflows.…”

“…The original MUPPI sub-resolution algorithm is described in Murante et al (2010), and its latest features (chemical evolution, metal cooling, SN kinetic feedback) in Murante et al (2014 to represent a multiphase ISM composed of hot and cold phases, in pressure equilibrium. A fraction of the cold phase (the molecular gas fraction) provides the reservoir for SF.…”