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The supernova-regulated ISM - I. The multiphase structure

Lookup NU author(s): Fred Gent, Professor Anvar Shukurov, Dr Andrew Fletcher, Dr Graeme Sarson

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Abstract

We simulate the multiphase interstellar medium (ISM) randomly heated and stirred by supernovae(SNe), with gravity, differential rotation and other parameters of the solar neighbourhood.Here we describe in detail both numerical and physical aspects of the model, includinginjection of thermal and kinetic energy by SN explosions, radiative cooling, photoelectric heatingand various transport processes. With a three-dimensional domain extending 1 × 1 kpc2horizontally and 2 kpc vertically (symmetric about the galactic mid-plane), the model routinelyspans gas number densities 10−5–102 cm−3, temperatures 10–108 K and local velocities up to103 km s−1 (with Mach number up to 25). The working numerical resolution of 4 pc has beenselected via simulations of a single expanding SN remnant, where we closely reproduce, atthis resolution, analytical solutions for the adiabatic and snowplough regimes. The feedbackof the halo on the disc cannot be captured in our model where the domain only extends to theheight of 1 kpc above the mid-plane.We argue that to reliably model the disc–halo connectionswould require extending the domain horizontally as well as vertically due to the increasinghorizontal scale of the gas flows with height.The thermal structure of the modelled ISM is classified by inspection of the joint probabilitydensity of the gas number density and temperature. We confirm that most of the complexitycan be captured in terms of just three phases, separated by temperature borderlines at about 103and 5 × 105 K. The probability distribution of gas density within each phase is approximatelylognormal.We clarify the connection between the fractional volume of a phase and its variousproxies, and derive an exact relation between the fractional volume and the filling factorsdefined in terms of the volume and probabilistic averages. These results are discussed inboth observational and computational contexts. The correlation scale of the random flows iscalculated from the velocity autocorrelation function; it is of the order of 100 pc and tendsto grow with distance from the mid-plane. We use two distinct parametrizations of radiativecooling to show that the multiphase structure of the gas is robust, as it does not dependsignificantly on this choice.


Publication metadata

Author(s): Gent FA, Shukurov A, Fletcher A, Sarson GR, Mantere MJ

Publication type: Article

Publication status: Published

Journal: Monthly Notices of the Royal Astronomical Society

Year: 2013

Volume: 432

Issue: 2

Pages: 1396-1423

Print publication date: 28/04/2013

ISSN (print): 0035-8711

ISSN (electronic): 1365-2966

Publisher: Oxford University Press

URL: http://dx.doi.org/10.1093/mnras/stt560

DOI: 10.1093/mnras/stt560


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