183 research outputs found
Dynamically dominant magnetic fields in the diffuse interstellar medium
Observations show that magnetic fields in the interstellar medium (ISM) often
do not respond to increases in gas density as would be naively expected for a
frozen-in field. This may suggest that the magnetic field in the diffuse gas
becomes detached from dense clouds as they form. We have investigated this
possibility using theoretical estimates, a simple magneto-hydrodynamic model of
a flow without mass conservation and numerical simulations of a thermally
unstable flow. Our results show that significant magnetic flux can be shed from
dense clouds as they form in the diffuse ISM, leaving behind a magnetically
dominated diffuse gas.Comment: 2 pages, 1 figure. In proceedings of IAU Symposium 259: "Cosmic
magnetic fields: from planets to stars and galaxies", eds. K.G. Strassmeier,
A.G. Kosovichev & J.E. Beckman in pres
Magnetic fields and spiral arms in the galaxy M51
(Abridged) We use new multi-wavelength radio observations, made with the VLA
and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51
on scales from 200\pc to several \kpc. Interferometric and single dish data
are combined to obtain new maps at \wwav{3}{6} in total and polarized emission,
and earlier \wav{20} data are re-reduced. We compare the spatial distribution
of the radio emission with observations of the neutral gas, derive radio
spectral index and Faraday depolarization maps, and model the large-scale
variation in Faraday rotation in order to deduce the structure of the regular
magnetic field. We find that the \wav{20} emission from the disc is severely
depolarized and that a dominating fraction of the observed polarized emission
at \wav{6} must be due to anisotropic small-scale magnetic fields. Taking this
into account, we derive two components for the regular magnetic field in this
galaxy: the disc is dominated by a combination of azimuthal modes, , but
in the halo only an mode is required to fit the observations. We disuss
how the observed arm-interarm contrast in radio intensities can be reconciled
with evidence for strong gas compression in the spiral shocks. The average
arm--interam contrast, representative of the radii r>2\kpc where the spiral
arms are broader, is not compatible with straightforward compression: lower
arm--interarm contrasts than expected may be due to resolution effects and
\emph{decompression} of the magnetic field as it leaves the arms. We suggest a
simple method to estimate the turbulent scale in the magneto-ionic medium from
the dependence of the standard deviation of the observed Faraday rotation
measure on resolution. We thus obtain an estimate of 50\pc for the size of
the turbulent eddies.Comment: 21 pages, 18 figures (some at lower resolution than submitted
version), accepted for publication in MNRA
The supernova-regulated ISM. II. The mean magnetic field
The origin and structure of the magnetic fields in the interstellar medium of
spiral galaxies is investigated with 3D, non-ideal, compressible MHD
simulations, including stratification in the galactic gravity field,
differential rotation and radiative cooling. A rectangular domain, 1x1x2
kpc^{3} in size, spans both sides of the galactic mid-plane. Supernova
explosions drive transonic turbulence. A seed magnetic field grows
exponentially to reach a statistically steady state within 1.6 Gyr. Following
Germano (1992) we use volume averaging with a Gaussian kernel to separate
magnetic field into a mean field and fluctuations. Such averaging does not
satisfy all Reynolds rules, yet allows a formulation of mean-field theory. The
mean field thus obtained varies in both space and time. Growth rates differ for
the mean-field and fluctuating field and there is clear scale separation
between the two elements, whose integral scales are about 0.7 kpc and 0.3 kpc,
respectively.Comment: 5 pages, 10 figures, submitted to Monthly Notices Letter
The supernova-regulated ISM. I. The multi-phase structure
We simulate the multi-phase interstellar medium randomly heated and stirred
by supernovae, with gravity, differential rotation and other parameters of the
solar neighbourhood. Here we describe in detail both numerical and physical
aspects of the model, including injection of thermal and kinetic energy by SN
explosions, radiative cooling, photoelectric heating and various transport
processes. With 3D domain extending 1 kpc^2 horizontally and 2 kpc vertically,
the model routinely spans gas number densities 10^-5 - 10^2 cm^-3, temperatures
10-10^8 K, local velocities up to 10^3 km s^-1 (with Mach number up to 25).
The thermal structure of the modelled ISM is classified by inspection of the
joint probability density of the gas number density and temperature. We confirm
that most of the complexity can be captured in terms of just three phases,
separated by temperature borderlines at about 10^3 K and 5x10^5 K. The
probability distribution of gas density within each phase is approximately
lognormal. We clarify the connection between the fractional volume of a phase
and its various proxies, and derive an exact relation between the fractional
volume and the filling factors defined in terms of the volume and probabilistic
averages. These results are discussed in both observational and computational
contexts. The correlation scale of the random flows is calculated from the
velocity autocorrelation function; it is of order 100 pc and tends to grow with
distance from the mid-plane. We use two distinct parameterizations of radiative
cooling to show that the multi-phase structure of the gas is robust, as it does
not depend significantly on this choice.Comment: 28 pages, 22 figures and 8 table
Large-Scale Magnetic-Field Generation by Randomly Forced Shearing Waves
A rigorous theory for the generation of a large-scale magnetic field by
random non-helically forced motions of a conducting fluid combined with a
linear shear is presented in the analytically tractable limit of low Rm and
weak shear. The dynamo is kinematic and due to fluctuations in the net
(volume-averaged) electromotive force. This is a minimal proof-of-concept
quasilinear calculation aiming to put the shear dynamo, a new effect recently
found in numerical experiments, on a firm theoretical footing. Numerically
observed scalings of the wavenumber and growth rate of the fastest growing
mode, previously not understood, are derived analytically. The simplicity of
the model suggests that shear dynamo action may be a generic property of
sheared magnetohydrodynamic turbulence.Comment: Paper substantially rewritten, results changed (relative to v1).
Revised versio
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