35 research outputs found
3D MHD simulations of magnetic field evolution and radio polarization of barred galaxies
Aims. We study numerically the large-scale gas and magnetic field
evolution of barred galaxies in the gravitational potential of a disk, bulge, halo, and
bar. We solve non-linear MHD equations including the back-reaction of the magnetic field
to the gas. We do not take into account any dynamo process.
Methods. We apply the numerical MHD code to calculate the model of the
galaxy in three dimensions. We construct realistic maps of high-frequency (Faraday
rotation free) polarized radio emission on the basis of the simulated magnetic fields. The
polarization model includes the effects of projection and limited resolution.
Results. The main result is that our modeled polarization maps resemble
the radio polarization structures observed in barred galaxies. The modeled polarization
B-vectors distribution along the bar and between spiral
arms resembles the observed topology of the magnetic field in barred galaxies. Our
calculations for several different rotational velocities and sound speeds give the same
result we got in our previous earlier published model. The reason of this behaviour is the
dynamical evolution of the bar that causes gas to form spiral waves going radially
outward. A gaseous spiral arms in turn generates magnetic ones, which live much longer in
the inter-arm disk space than the gaseous pattern
The effect of supernova rate on the magnetic field evolution in barred galaxies
Context. For the first time, our magnetohydrodynamical numerical calculations provide results for a three-dimensional model of barred galaxies involving a cosmic-ray driven dynamo process that depends on star formation rates. Furthermore, we argue that the cosmic-ray driven dynamo can account for a number of magnetic features in barred galaxies, such as magnetic arms observed along the gaseous arms, magnetic arms in the inter-arm regions, polarized emission that is at the strongest in the central part of the galaxy, where the bar is situated, polarized emission that forms ridges coinciding with the dust lanes along the leading edges of the bar, as well as their very strong total radio intensity. Aims. Our numerical model probes what kind of physical processes could be responsible for the magnetic field topology observed in barred galaxies (modes, etc.). We compare our modelled results directly with observations, constructing models of high-frequency (Faraday rotation-free) polarized radio emission maps out of the simulated magnetic field and cosmic ray pattern in our modeled galaxy. We also take the effects of projection into account as well as the limited resolution. Methods. We applied global 3D numerical calculations of a cosmic-ray driven dynamo in barred galaxies with different physical input parameters such as the supernova (SN) rate. Results. Our simulation results lead to the modelled magnetic field structure similar to the one observed on the radio maps of barred galaxies. Moreover, they cast new light on a number of properties in barred and spiral galaxies, such as fast exponential growth of the total magnetic energy to the present values. The quadrupole modes of magnetic field are often identified in barred galaxies, but the dipole modes (e.g., in NGC 4631) are found very seldom. In our simulations the quadrupole configuration dominates and the dipole configuration only appears once in the case of model S100, apparently as a consequence of the choice of the random number seed. Synthetic radio maps of our models display X-type structure similar to what is observed in real galaxies. Conclusions. We conclude that a cosmic-ray driven dynamo process in barred galaxies can amplify magnetic fields efficiently. The fastest rate of magnetic field increase is 195 yr for a SN frequency of 1/50 yr-1.The obtained strength of magnetic field corresponds to the observational values (a few in spiral arms). The polarization and rotation measure maps also agree with observations. We found the effect of shifting magnetic arms in 4 models (out of the sample of 5)
Cosmic ray driven dynamo in barred and ringed galaxies
We study the global evolution of the magnetic field and interstellar medium (ISM) of the barred and ringed galaxies in the presence of non-axisymmetric components of the potential, i.e. the bar and/or the oval perturbations. The magnetohydrodynamical dynamo is driven by cosmic rays (CR), which are continuously supplied to the disk by supernova (SN) remnants. Additionally, weak, dipolar and randomly oriented magnetic field is injected to the galactic disk during SN explosions. To compare our results directly with the observed properties of galaxies we construct realistic maps of high-frequency polarized radio emission. The main result is that CR driven dynamo can amplify weak magnetic fields up to few G within few Gyr in barred and ringed galaxies. What is more, the modelled magnetic field configuration resembles maps of the polarized intensity observed in barred and ringed galaxies
Formation of gaseous arms in barred galaxies with dynamically important magnetic field : 3D MHD simulations
We present results of three-dimensional nonlinear MHD simulations of a
large-scale magnetic field and its evolution inside a barred galaxy with the
back reaction of the magnetic field on the gas. The model does not consider the
dynamo process. To compare our modeling results with observations, we construct
maps of the high-frequency (Faraday-rotation-free) polarized radio emission on
the basis of simulated magnetic fields. The model accounts for the effects of
projection and the limited resolution of real observations. We performed 3D MHD
numerical simulations of barred galaxies and polarization maps. The main result
is that the modeled magnetic field configurations resemble maps of the
polarized intensity observed in barred galaxies. They exhibit polarization
vectors along the bar and arms forming coherent structures similar to the
observed ones. In the paper, we also explain the previously unsolved issue of
discrepancy between the velocity and magnetic field configurations in this type
of galaxies. The dynamical influence of the bar causes gas to form spiral waves
that travel outwards. Each gaseous spiral arm is accompanied by a magnetic
counterpart, which separates and survives in the inter-arm region. Because of a
strong compression, shear of non-axisymmetric bar flows and differential
rotation, the total energy of modeled magnetic field grows constantly, while
the azimuthal flux grows slightly until 0.05\Gyr and then saturates.Comment: 4 pages, 4 figure
Gaseous Structures in Barred Galaxies: Effects of the Bar Strength
Using hydrodynamic simulations, we investigate the physical properties of
gaseous substructures in barred galaxies and their relationships with the bar
strength. The gaseous medium is assumed to be isothermal and unmagnetized. The
bar potential is modeled as a Ferrers prolate with index n. To explore
situations with differing bar strength, we vary the bar mass fbar relative to
the spheroidal component as well as its aspect ratio. We derive expressions as
functions of fbar and the aspect ratio for the bar strength Qb and the radius
r(Qb) where the maximum bar torque occurs. When applied to observations, these
expressions suggest that bars in real galaxies are most likely to have
fbar=0.25-0.5 and n<1. Dust lanes approximately follow one of x1-orbits and
tend to be more straight under a stronger and more elongated bar, but are
insensitive to the presence of self-gravity. A nuclear ring of a conventional
x2 type forms only when the bar is not so massive or elongated. The radius of
an x2-type ring is generally smaller than the inner Lindblad resonance,
decreases systematically with increasing Qb, and slightly larger when
self-gravity is included. This evidences that the ring position is not
determined by the resonance but by the amount of angular momentum loss at
dust-lane shocks. Nuclear spirals exist only when the ring is of the x2-type
and sufficiently large in size. Unlike the other features, nuclear spirals are
transient in that they start out as being tightly-wound and weak, and then due
to the nonlinear effect unwind and become stronger until turning into shocks,
with an unwinding rate higher for larger Qb. The mass inflow rate to the galaxy
center is found to be less than 0.01 Msun/yr for models with Qb<0.2, while
becoming larger than 0.1 Msun/yr when Qb>0.2 and self-gravity is included.Comment: 24 pages, 17 figures, 5 tables; Accepted for publication in the ApJ;
Version with full-resolution figures available at
http://mirzam.snu.ac.kr/~wkim/Bar/barHDn.pd
Two-Dimensional Magnetohydrodynamic Simulations of Barred Galaxies
Barred galaxies are known to possess magnetic fields that may affect the
properties of bar substructures such as dust lanes and nuclear rings. We use
two-dimensional high-resolution magnetohydrodynamic (MHD) simulations to
investigate the effects of magnetic fields on the formation and evolution of
such substructures as well as on the mass inflow rates to the galaxy center.
The gaseous medium is assumed to be infinitesimally-thin, isothermal,
non-self-gravitating, and threaded by initially uniform, azimuthal magnetic
fields. We find that there exists an outermost x1-orbit relative to which
gaseous responses to an imposed stellar bar potential are completely different
between inside and outside. Inside this orbit, gas is shocked into dust lanes
and infalls to form a nuclear ring. Magnetic fields are compressed in dust
lanes, reducing their peak density. Magnetic stress removes further angular
momentum of the gas at the shocks, temporarily causing the dust lanes to bend
into an 'L' shape and eventually leading to a smaller and more centrally
distributed ring than in unmagnetized models. The mass inflow rates in
magnetized models correspondingly become larger, by more than two orders of
magnitude when the initial fields have an equipartition value with thermal
energy, than in the unmagnetized counterparts. Outside the outermost x1-orbit,
on the other hand, an MHD dynamo due to the combined action of the bar
potential and background shear operates near the corotation and bar-end
regions, efficiently amplifying magnetic fields. The amplified fields shape
into trailing magnetic arms with strong fields and low density. The base of the
magnetic arms has a thin layer in which magnetic fields with opposite polarity
reconnect via a tearing-mode instability. This produces numerous magnetic
islands with large density which propagate along the arms to turn the outer
disk into a highly chaotic state.Comment: 22 pages, 19 figures, 3 tables; Accepted for publication in the ApJ;
Version with full-resolution figures available at
http://mirzam.snu.ac.kr/~wkim/Bar/mhdbar.pd
Fast Magnetic Reconnection and Energetic Particle Acceleration
Our numerical simulations show that the reconnection of magnetic field
becomes fast in the presence of weak turbulence in the way consistent with the
Lazarian and Vishniac (1999) model of fast reconnection. We trace particles
within our numerical simulations and show that the particles can be efficiently
accelerated via the first order Fermi acceleration. We discuss the acceleration
arising from reconnection as a possible origin of the anomalous cosmic rays
measured by Voyagers.Comment: 11 pages, 9 figures, submitted to Planetary and Space Scienc
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
Theory and Applications of Non-Relativistic and Relativistic Turbulent Reconnection
Realistic astrophysical environments are turbulent due to the extremely high
Reynolds numbers. Therefore, the theories of reconnection intended for
describing astrophysical reconnection should not ignore the effects of
turbulence on magnetic reconnection. Turbulence is known to change the nature
of many physical processes dramatically and in this review we claim that
magnetic reconnection is not an exception. We stress that not only
astrophysical turbulence is ubiquitous, but also magnetic reconnection itself
induces turbulence. Thus turbulence must be accounted for in any realistic
astrophysical reconnection setup. We argue that due to the similarities of MHD
turbulence in relativistic and non-relativistic cases the theory of magnetic
reconnection developed for the non-relativistic case can be extended to the
relativistic case and we provide numerical simulations that support this
conjecture. We also provide quantitative comparisons of the theoretical
predictions and results of numerical experiments, including the situations when
turbulent reconnection is self-driven, i.e. the turbulence in the system is
generated by the reconnection process itself. We show how turbulent
reconnection entails the violation of magnetic flux freezing, the conclusion
that has really far reaching consequences for many realistically turbulent
astrophysical environments. In addition, we consider observational testing of
turbulent reconnection as well as numerous implications of the theory. The
former includes the Sun and solar wind reconnection, while the latter include
the process of reconnection diffusion induced by turbulent reconnection, the
acceleration of energetic particles, bursts of turbulent reconnection related
to black hole sources as well as gamma ray bursts. Finally, we explain why
turbulent reconnection cannot be explained by turbulent resistivity or derived
through the mean field approach.Comment: 66 pages, 24 figures, a chapter of the book "Magnetic Reconnection -
Concepts and Applications", editors W. Gonzalez, E. N. Parke