Galaxy evolution: modelling the role of non-thermal pressure in the interstellar medium

Abstract

Galaxy evolution depends strongly on the physics of the interstellar medium (ISM). Motivated by the need to incorporate the properties of the ISM in cosmological simulations we construct a simple method to include the contribution of non-thermal components in the calculation of pressure of interstellar gas. In our method we treat three non-thermal components -turbulence, magnetic fields and cosmic rays -and effectively parametrize their amplitude. We assume that the three components settle into a quasi-steady-state that is governed by the star formation rate, and calibrate their magnitude and density dependence by the observed Radio-FIR correlation, relating synchrotron radiation to star formation rates of galaxies. We implement our model in single cell numerical simulation of a parcel of gas with constant pressure boundary conditions and demonstrate its effect and potential. Then, the non-thermal pressure model is incorporated into RAMSES and hydrodynamic simulations of isolated galaxies with and without the non-thermal pressure model are presented and studied. Specifically, we demonstrate that the inclusion of realistic non-thermal pressure reduces the star formation rate by an order of magnitude and increases the gas depletion time by as much. We conclude that the non-thermal pressure can prolong the star formation epoch and achieve consistency with observations without invoking artificially strong stellar feedback.

Magnetic fields in spiral galaxies

“…The dynamical importance of galactic fields, with typical strengths of 15 μG, is now widely appreciated in many fields of astrophysics. Magnetic and cosmic-ray pressures control the overall star-formation rate (Birnboim et al 2015). Magnetic fields are important for the gas dynamics of molecular clouds (Planck Collaboration et al 2015a).…”

IMAGINE: a comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays

“…The hammurabi code 11 [285] is an astrophysical simulator based on 3D models of the components of the magnetised ISM such as magnetic fields, thermal electrons, relativistic electrons, and dust grains. It performs an efficient line-of-sight integration through the simulated Galaxy model using a HEALPix 12 -based nested grid to produce observables such as Faraday rotation measure and diffuse synchrotron and thermal dust emission in full Stokes I, Q and U, while taking into account beam and depth depolarisation as well as Faraday effects.…”

Galactic magnetic fields and hierarchical galaxy formation

“…Half of the total interstellar pressure is thus due to non-thermal contributions, and, hence, magnetic fields directly affect the mean gas density. In turn, this affects significantly the star formation rate (SFR); it is perhaps surprising that the role of magnetic fields and cosmic rays in galaxy evolution has avoided attention for so long (Birnboim, Balberg & Teyssier 2015). Magnetic fields also regulate star formation locally by controlling the collapse and fragmentation of molecular clouds Peters et al 2011;Crutcher 2012).…”