34 research outputs found
3D model of magnetic fields evolution in dwarf irregular galaxies
Radio observations show that magnetic fields are present in dwarf irregular
galaxies (dIrr) and its strength is comparable to that found in spiral
galaxies. Slow rotation, weak shear and shallow gravitational potential are the
main features of a typical dIrr galaxy. These conditions of the interstellar
medium in a dIrr galaxy seem to unfavourable for amplification of the magnetic
field through the dynamo process. Cosmic-ray driven dynamo is one of the
galactic dynamo model, which has been successfully tested in case of the spiral
galaxies. We investigate this dynamo model in the ISM of a dIrr galaxy. We
study its efficiency under the influence of slow rotation, weak shear and
shallow gravitational potential. Additionally, the exploding supernovae are
parametrised by the frequency of star formation and its modulation, to
reproduce bursts and quiescent phases. We found that even slow galactic
rotation with a low shearing rate amplifies the magnetic field, and that rapid
rotation with a low value of the shear enhances the efficiency of the dynamo.
Our simulations have shown that a high amount of magnetic energy leaves the
simulation box becoming an efficient source of intergalactic magnetic fields.Comment: 4 pages, 4 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche
Magnetic field evolution in dwarf and Magellanic-type galaxies
Low-mass galaxies radio observations show in many cases surprisingly high
levels of magnetic field. The mass and kinematics of such objects do not favour
the development of effective large-scale dynamo action. We attempted to check
if the cosmic-ray-driven dynamo can be responsible for measured magnetization
in this class of poorly investigated objects. We investigated how starburst
events on the whole, as well as when part of the galactic disk, influence the
magnetic field evolution. We created a model of a dwarf/Magellanic-type galaxy
described by gravitational potential constituted from two components: the stars
and the dark-matter halo. The model is evolved by solving a three-dimensional
(3D) magnetohydrodynamic equation with an additional cosmic-ray component,
which is approximated as a fluid. The turbulence is generated in the system via
supernova explosions manifested by the injection of cosmic-rays.The
cosmic-ray-driven dynamo works efficiently enough to amplify the magnetic field
even in low-mass dwarf/Magellanic-type galaxies. The -folding times of
magnetic energy growth are 0.50 and 0.25 Gyr for the slow (50 km/s) and fast
(100 km/s) rotators, respectively. The amplification is being suppressed as the
system reaches the equipartition level between kinetic, magnetic, and
cosmic-ray energies. An episode of star formation burst amplifies the magnetic
field but only for a short time while increased star formation activity holds.
We find that a substantial amount of gas is expelled from the galactic disk,
and that the starburst events increase the efficiency of this process.Comment: 8 pages and 5 figures, accepted for publication in Astronomy and
Astrophysic
Cosmic-ray driven dynamo in the medium of irregular galaxy
We investigate the cosmic ray driven dynamo in the interstellar medium of
irregular galaxy. The observations (Chyzy et al. 2000, 2003) show that the
magnetic field in irregular galaxies is present and its value reaches the same
level as in spiral galaxies. However the conditions in the medium of irregular
galaxy are very unfavorable for amplification the magnetic field due to slow
rotation and low shearing rate.
In this work we present numerical model of the interstellar medium in
irregular galaxies. The model includes magnetohydrodynamical dynamo driven by
cosmic rays in the interstellar medium provided by random supernova explosions.
We describe models characterized by different shear and rotation. We find that
even slow galactic rotation with low shearing rate gives amplification of the
magnetic field. Simulations have shown that high amount of the magnetic energy
flow out off the simulation region becoming an efficient source of
intergalactic magnetic fields.Comment: 2 pages, 2 figures, To be published in "Cosmic Magnetic Fields: From
Planets, to Stars and Galaxies", K.G. Strassmeier, A.G. Kosovichev & J.E.
Beckman, eds., Proc. IAU Symp. 259, CU
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)
Gammapy: A Python package for gamma-ray astronomy
In this article, we present Gammapy, an open-source Python package for the
analysis of astronomical -ray data, and illustrate the functionalities
of its first long-term-support release, version 1.0. Built on the modern Python
scientific ecosystem, Gammapy provides a uniform platform for reducing and
modeling data from different -ray instruments for many analysis
scenarios. Gammapy complies with several well-established data conventions in
high-energy astrophysics, providing serialized data products that are
interoperable with other software packages. Starting from event lists and
instrument response functions, Gammapy provides functionalities to reduce these
data by binning them in energy and sky coordinates. Several techniques for
background estimation are implemented in the package to handle the residual
hadronic background affecting -ray instruments. After the data are
binned, the flux and morphology of one or more -ray sources can be
estimated using Poisson maximum likelihood fitting and assuming a variety of
spectral, temporal, and spatial models. Estimation of flux points, likelihood
profiles, and light curves is also supported. After describing the structure of
the package, we show, using publicly available -ray data, the
capabilities of Gammapy in multiple traditional and novel -ray analysis
scenarios, such as spectral and spectro-morphological modeling and estimations
of a spectral energy distribution and a light curve. Its flexibility and power
are displayed in a final multi-instrument example, where datasets from
different instruments, at different stages of data reduction, are
simultaneously fitted with an astrophysical flux model.Comment: 26 pages, 16 figure