The role of magnetic fields in the interstellar medium

PhD ThesisThe dynamical role of magnetic elds in the interstellar medium (ISM) of galaxies has been widely debated since their discovery approximately seventy years ago. I investigate the possible dynamical role of magnetic elds in the ISM, using numerical simulations of both a region of a spiral galaxy and of individual supernova (SN) explosions. In the galaxy simulations, the magnetic eld evolves from a weak seed- eld via dynamo action, in a manner consistent with the internal gas dynamics of the simulated ISM. I nd that the magnetic eld, and particularly its coherentlystructured large-scale component, evolves spatially in such a way as to avoid the hottest, most violently turbulent regions of the ISM, where SN explosions inject large amounts of energy and produce strong shocks in the surrounding gas. The large-scale magnetic eld is especially sensitive to the hot gas, strongly preferring to reside in the more hospitable warm gas. I also nd that the magnetic eld produces feedback on the local gas dynamics. The main e ect is that large-scale out ows, characterised by the mean vertical velocity of gas which represents gas owing vertically out of the galactic disc, are damped by magnetic pressure gradient close to the galactic midplane, as the magnetic eld becomes dynamically signi cant. This also e ects the vertical distribution of gas density and gas pressure; gas density becomes more homogeneous close to the midplane but decays more strongly, with vertical distance away from the midplane, further away from the midplane. I also nd that while the fractional volume of hot gas decreases by up to an order of magnitude from the early to the late stage of the numerical simulation, the hot gas becomes more dense. Following this, I present results from high-resolution numerical simulations of individual SN remnants to understand possible e ects of the magnetic eld on the main energy injection mechanism for galaxies. I use models that are purely hydrodynamical (HD) as a baseline for comparison with magnetohydrodynamical (MHD) simulations, to ascertain the e ects of magnetic elds. I nd that magnetic elds alter both physical and thermodynamical properties of these simulated SN remnants. The remnant shocks propagate faster perpendicular to the magnetic eld, resulting from magnetic pressure gradient. Inward momentum injection is also enhanced for MHD remnants, resulting in mass loss from the shock inwards towards the remnant core. As a result, the remnant core is magnetically con ned, which reduces the e ciency of heat loss due to adiabatic expansion. Thus, the hot, di use gas in the remnant core remains hotter and becomes more dense throughout the evolution of the remnant. This in agreement with the change to the fractional volume and density of hot gas found in the larger-scale simulations of the ISM presented in this Thesis

Separating the scales in a compressible interstellar medium

We apply Gaussian smoothing to obtain mean density, velocity, magnetic and energy density fields in simulations of the interstellar medium based on three-dimensional magnetohydrodynamic equations in a shearing box $1\times1\times2 \, \rm{kpc}$ in size. Unlike alternative averaging procedures, such as horizontal averaging, Gaussian smoothing retains the three-dimensional structure of the mean fields. Although Gaussian smoothing does not obey the Reynolds rules of averaging, physically meaningful central statistical moments are defined as suggested by Germano (1992). We discuss methods to identify an optimal smoothing scale $\ell$ and the effects of this choice on the results. From spectral analysis of the magnetic, density and velocity fields, we find a suitable smoothing length for all three fields, of $\ell \approx 75 \, \rm{pc}$. We discuss the properties of third-order statistical moments in fluctuations of kinetic energy density in compressible flows and suggest their physical interpretation. The mean magnetic field, amplified by a mean-field dynamo, significantly alters the distribution of kinetic energy in space and between scales, reducing the magnitude of kinetic energy at intermediate scales. This intermediate-scale kinetic energy is a useful diagnostic of the importance of SN-driven outflows