32 research outputs found
Magnetic field reversals and galactic dynamos
We argue that global magnetic field reversals similar to those observed in
the Milky Way occur quite frequently in mean-field galactic dynamo models that
have relatively strong, random, seed magnetic fields that are localized in
discrete regions. The number of reversals decreases to zero with reduction of
the seed strength, efficiency of the galactic dynamo and size of the spots of
the seed field. A systematic observational search for magnetic field reversals
in a representative sample of spiral galaxies promises to give valuable
information concerning seed magnetic fields and, in this way, to clarify the
initial stages of galactic magnetic field evolution
Mean magnetic field generation in sheared rotators
A generalized mean magnetic field induction equation for differential
rotators is derived, including a compressibility, and the anisotropy induced on
the turbulent quantities from the mean magnetic field itself and a mean
velocity shear. Derivations of the mean field equations often do not emphasize
that there must be anisotropy and inhomogeneity in the turbulence for mean
field growth. The anisotropy from shear is the source of a term involving the
product of the mean velocity gradient and the cross-helicity correlation of the
isotropic parts of the fluctuating velocity and magnetic field,
\lb{\bfv}\cdot{\bfb}\rb^{(0)}. The full mean field equations are derived to
linear order in mean fields, but it is also shown that the cross-helicity term
survives to all orders in the velocity shear. This cross-helicity term can
obviate the need for a pre-existing seed mean magnetic field for mean field
growth: though a fluctuating seed field is necessary for a non-vanishing
cross-helicity, the term can produce linear (in time) mean field growth of the
toroidal field from zero mean field. After one vertical diffusion time, the
cross-helicity term becomes sub-dominant and dynamo exponential
amplification/sustenance of the mean field can subsequently ensue. The
cross-helicity term should produce odd symmetry in the mean magnetic field, in
contrast to the usually favored even modes of the dynamo amplification in
sheared discs. This may be important for the observed mean field geometries of
spiral galaxies. The strength of the mean seed field provided by the cross-
helicity depends linearly on the magnitude of the cross-helicity.Comment: 15 pages, LaTeX, matches version accepted to ApJ, minor revision
Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields
We study generation of magnetic fields by the Biermann mechanism in the
pair-instability supernovae explosions of first stars. The Biermann mechanism
produces magnetic fields in the shocked region between the bubble and
interstellar medium (ISM), even if magnetic fields are absent initially. We
perform a series of two-dimensional magnetohydrodynamic simulations with the
Biermann term and estimate the amplitude and total energy of the produced
magnetic fields. We find that magnetic fields with amplitude
G are generated inside the bubble, though the amount of
magnetic fields generated depend on specific values of initial conditions. This
corresponds to magnetic fields of erg per each supernova
remnant, which is strong enough to be the seed magnetic field for galactic
and/or interstellar dynamo.Comment: 12 pages, 3 figure
The effects of vertical outflows on disk dynamos
We consider the effect of vertical outflows on the mean-field dynamo in a
thin disk. These outflows could be due to winds or magnetic buoyancy. We
analyse both two-dimensional finite-difference numerical solutions of the
axisymmetric dynamo equations and a free-decay mode expansion using the
thin-disk approximation. Contrary to expectations, a vertical velocity can
enhance dynamo action, provided it is not too strong. In the nonlinear regime
this can lead to super-exponential growth of the magnetic field.Comment: 14 pages, final version after referee comments, accepted in A&
In Situ Origin of Large Scale Galactic Magnetic Fields Without Kinetic Helicity?
The origin and sustenance of large scale galactic magnetic fields has been a
long standing and controversial astrophysical problem. Here an alternative to
the ``standard'' \a-\Omega mean field dynamo and primordial theories is
pursued. The steady supply of supernovae induced turbulence exponentiates the
total field energy, providing a significant seed mean field that can be
linearly stretched by shear. The observed micro-Gauss fields would be produced
primarily within one vertical diffusion time since it is only during this time
that linear stretching can compete with diffusion. This approach does not
invoke exponential mean field dynamo growth from the helicity \a-effect but
does employ turbulent diffusion, which limits the number of large scale
reversals. The approach could be of interest if the helicity effect is
suppressed independently of the turbulent diffusion. This is an important but
presently unresolved issue.Comment: 15 pages TeX, accepted, ApJ
Galactic dynamos with captured magnetic flux and an accretion flow
We examine the behaviour of an axisymmetric galactic dynamo model with a
radial accretion flow in the disc. We also introduce a vertical magnetic flux
through the galactic midplane, to simulate the presence of a large scale
magnetic field trapped by the galaxy when forming. The trapped vertical flux is
conserved and advected towards the disc centre by the radial flow. We confirm
that accretion flows of magnitude several km/s through a significant part of
the galactic disc can markedly inhibit dynamo action. Moreover, advection of
the vertical flux in general results in mixed parity galactic fields. However,
the effect is nonlinear and non-additive -- global magnetic field energies are
usually significantly smaller that the sum of purely dynamo generated and
purely advected field energies. For large inflow speeds, a form of
`semi-dynamo' action may occur.
We apply our results to the accumulation and redistribution, by a radial
inflow, of a vertical magnetic flux captured by the Galactic disc. Taking
representative values, it appears difficult to obtain mean vertical fields near
the centre of the Milky Way that are much in excess of 10 microgauss, largely
because the galactic dynamo and turbulent magnetic diffusion modify the
external magnetic field before it can reach the disc centre.Comment: 18 pages, 12 figures, LaTE
Magnetic fields in barred galaxies. IV. NGC 1097 and NGC 1365
We present 3.5cm and 6.2cm radio continuum maps in total and polarized
intensity of the barred galaxies NGC 1097 and NGC 1365. Both galaxies exhibit
radio ridges roughly overlapping with the massive dust lanes in the bar region.
The contrast in total intensity across the radio ridges is compatible with
compression and shear of an isotropic random magnetic field. The contrast in
polarized intensity is significantly smaller than that expected from
compression and shearing of the regular magnetic field; this could be the
result of decoupling of the regular field from the dense molecular clouds. The
regular field in the ridge is probably strong enough to reduce significantly
shear in the diffuse gas (to which it is coupled) and hence to reduce magnetic
field amplification by shearing. This contributes to the misalignment of the
observed field orientation with respect to the velocity vectors of the dense
gas. Our observations, for the first time, indicate that magnetic forces can
control the flow of the diffuse interstellar gas at kiloparsec scales. The
total radio intensity reaches its maximum in the circumnuclear starburst
regions, where the equipartition field strength is about 60\mu G, amongst the
strongest fields detected in spiral galaxies so far. The regular field in the
inner region has a spiral shape with large pitch angle, indicating the action
of a dynamo. Magnetic stress leads to mass inflow towards the centre,
sufficient to feed the active nucleus in NGC 1097. We detected diffuse X-ray
emission, possibly forming a halo of hot gas around NGC 1097.Comment: 32 pages with 45 PostScript figures. Accepted for publication in A&A;
Typos corrected 12/10/200
Constraining the Magnetic Effects on HI Rotation Curves and the Need for Dark Halos
The density profiles of dark halos are usually inferred from the rotation
curves of disk galaxies based on the assumption that the gas is a good tracer
of the gravitational potential of the galaxies. Some authors have suggested
that magnetic pinching forces could alter significantly the rotation curves of
spiral galaxies. In contrast to other studies which have concentrated in the
vertical structure of the disk, here we focus on the problem of magnetic
confinement in the radial direction to bound the magnetic effects on the HI
rotation curves. It is shown that azimuthal magnetic fields can hardly speed up
the HI disk of galaxies as a whole. In fact, based on virial constraints we
show that the contribution of galactic magnetic fields to the rotation curves
cannot be larger than ~10 km/s at the outermost point of HI detection, if the
galaxies did not contain dark matter at all, and up to 20 km/s in the
conventional dark halo scenario. The procedure to estimate the maximum effect
of magnetic fields is general and applicable to any particular galaxy disk. The
inclusion of the surface terms, namely the intergalactic (thermal, magnetic or
ram) pressure, does not change our conclusions. Other problems related with the
magnetic alternative to dark halos are highlighted. The relevance of magnetic
fields in the cuspy problem of dark halos is also discussed.Comment: 12 pages, 1 figure, accepted for publication in The Astrophysical
Journa
Constraining the regular Galactic Magnetic Field with the 5-year WMAP polarization measurements at 22 GHz
[ABRIDGED] The knowledge of the regular component of the Galactic magnetic
field gives important information about the structure and dynamics of the Milky
Way, as well as constitutes a basic tool to determine cosmic rays trajectories.
It can also provide clear windows where primordial magnetic fields could be
detected. We want to obtain the regular (large scale) pattern of the magnetic
field distribution of the Milky Way that better fits the polarized synchrotron
emission as seen by the 5-year WMAP data at 22 GHz. We have done a systematic
study of a number of Galactic magnetic field models: axisymmetric, bisymmetric,
logarithmic spiral arms, concentric circular rings with reversals and
bi-toroidal. We have explored the parameter space defining each of these models
using a grid-based approach. In total, more than one million models are
computed. The model selection is done using a Bayesian approach. For each
model, the posterior distributions are obtained and marginalised over the
unwanted parameters to obtain the marginal 1-D probability distribution
functions. In general, axisymmetric models provide a better description of the
halo component, although attending to their goodness-of-fit, the rest of the
models cannot be rejected. In the case of disk component, the analysis is not
very sensitive for obtaining the disk large scale structure, because of the
effective available area (less than 8% of the whole map and less than 40% of
the disk). Nevertheless, within a given family of models, the best-fit
parameters are compatible with those found in the literature. The family of
models that better describes the polarized synchrotron halo emission is the
axisymmetric one, with magnetic spiral arms with a pitch angle of ~24 degrees,
and a strong vertical field of 1 microG at z ~ 1 kpc. When a radial variation
is fitted, models require fast variations.Comment: 14 pages, 9 figures. Accepted for publication in A&