3 research outputs found
Galaxies in box: A simulated view of the interstellar medium
We review progress in the development of physically realistic three
dimensional simulated models of the galaxy.We consider the scales from star
forming molecular clouds to the full spiral disc. Models are computed using
hydrodynamic (HD) or magnetohydrodynamic (MHD) equations and may include cosmic
ray or tracer particles. The range of dynamical scales between the full galaxy
structure and the turbulent scales of supernova (SN) explosions and even cloud
collapse to form stars, make it impossible with current computing tools and
resources to resolve all of these in one model. We therefore consider a
hierarchy of models and how they can be related to enhance our understanding of
the complete galaxy.Comment: Chapter in Large Scale Magnetic Fields in the Univers
Interstellar MHD Turbulence and Star Formation
This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse
Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as
per referee's recommendation
Consequences of Starbursts for the Interstellar and Intergalactic Medium
Star formation in general, and starbursts in particular, drive the evolution of galaxies. To understand the process of galactic matter cycle quantitatively, it is absolutely necessary to follow the evolution of the components of the interstellar medium, such as gas, magnetic fields, cosmic rays in detail over sufficiently long time scales. Due to the non-linearity of the interactions between the various components, and the turbulent nature of the plasma, high resolution numerical simulations offer the best strategy to further our understanding. The results of our numerical studies can be summarized as follows: (i) Supernova explosions are the most important energy input sources in the ISM and lead to a high level of turbulence in the plasma, coupling structures on all scales, (ii) more than half of the disk mass resides in classically thermally unstable temperature regimes, (iii) turbulent mixing is the dominant energy transport process over a wide range of scales, (iv) proportionality between magnetic field and density is generally weak, except for the densest regions, (v) magnetic fields, even if they are parallel to the galactic disk, cannot prevent outflow into the halo, (vi) the ionization structure of the plasma depends on its thermal history, and is in general not in collisional ionization equilibrium, (vii) the cooling function varies in space and time, (viii) X-rays can be emitted even at plasma temperatures of the order of 104K due to delayed recombination, both in the disk and the halo, (ix) cosmic rays can help driving a galactic wind, (x) cosmic rays can be accelerated to high energies beyond 1015eV (the “knee”) in long lived shocks propagating into the galactic halo, because of time-dependent star formation