The large scale magnetic field structure of the spiral galaxy NGC 5775

In order to better determine the large-scale 3D-structure of magnetic fields in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral galaxy NGC 5775. Deep radio-continuum observations in total power and linear polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg telescope. They were analyzed together with archival 4.86 and 1.49 GHz VLA-data. We thus can derive rotation measures from a comparison of three frequencies and determine the intrinsic magnetic field structure. A very extended halo is detected in NGC 5775, with magnetic field lines forming an X-shaped structure. Close to the galactic disk the magnetic field is plane-parallel. The scaleheights of the radio emission esimated for NGC 5775 are comaprable with other galaxies. The rotation measure distribution varies smoothly on both sides along the major axis from positive to negative values. From the derived distribution of rotation measures and the plane-parallel intrinsic magnetic field orientation along the galactic midplane we conclude that NGC 5775 has an 'even axisymmetric' large-scale magnetic field configuration in the disk as generated by an \alpha \Omega -dynamo which is accompanied by a quadrupolar poloidal field. The magnetic field lines of the plane-parallel component are pointing 'outwards'. The observed X-shaped halo magnetic field, however, cannot be explained by the action of the disk's mean-field dynamo alone. It is probably due to the influence of the galactic wind together with the dynamo action.Comment: 11 pages, 13 figures, A&A accepte

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 $e$-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

A large-scale, regular intergalactic magnetic field associated with Stephan's Quintet?

Regular magnetic fields are frequently found within and in the outskirts of galaxies, but their presence, properties, and origin has not yet been established for galaxy groups. On the basis of broadband radio polarimetric imaging with the Westerbork Synthesis Radio Telescope (WSRT), we made use of Rotation Measure Synthesis to disentangle contributions from magnetic fields on various scales for several polarised radio sources inside, behind, or in the vicinity of the Stephan's Quintet (HCG92, SQ). We recognise the signature of a large-scale, genuinely regular, magnetised screen, seemingly constrained to the Quintet itself. Although we cannot exclude a contribution from the Milky Way, our analysis favours a magnetic structure within the SQ system. If indeed associated with the galaxy group in question, it would span a volume of at least $60\,\times\,40\,\times\,20\,\mathrm{kpc}$ and have a strength at least as high as that previously detected within large spiral galaxies. This field would then surpass the extent of any other known galactic, regular magnetic fields, have a considerable strength of a few microgauss, and would be the first known example of such a structure in a galaxy system other than a galaxy pair. Several other explanations are also presented and evaluated.Comment: 13 pages, 7 figures, 3 table

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

Can the cosmic-ray driven dynamo model explain the observations of the polarized emission of edge-on galaxies ?

In the present paper we construct maps of polarized synchrotron radio emission of a whole galaxy, based on local models of the cosmic ray (CR) driven dynamo. We perform numerical simulations of the dynamo in local Cartesian domains, with shear-periodic boundary conditions, placed at the different galactocentric radii. Those local solutions are concatenated together to construct the synchrotron images of the whole galaxy. The main aim of the paper is to compare the model results with the observed radio continuum emission from nearly edge-on spiral galaxy. On the basis of the modeled evolution of the magnetic field structure, the polarization maps can be calculated at different time-steps and at any orientation of the modeled galaxy. For the first time a self-consistent cosmic-ray electron distribution is used to integrate synchrotron emissivity along the line of sight. Finally, our maps are convolved with the given radiotelescope beam. We show that it is possible to reconstruct the extended magnetic halo structures of the edge-on galaxies (so called X-shaped structures).Comment: 7 pages, 7 figures. ApJ, accepte

Single pulse emission from PSR B0809+74 at 150 MHz using Polish LOFAR station

We report the observations of single pulse emission from the pulsar B0809+74 at 150 MHz using the Polish LOFAR station, PL-611. The three major phenomena of subpulse drifting, nulling, and mode changing associated with single pulse variations are prominently seen in these observations. The pulsar has a single-component conal profile and the single pulses are primarily in the 'normal' drift mode with periodicity (P$_{3}$) 11.1 ± 0.5 P for 96 per cent of the observing duration, while the shorter duration 'slow-drift' mode has P$_{3}$ = 15.7 ± 1.2 P. We were able to measure the phase behaviour associated with drifting from the fluctuation spectral analysis that showed identical linear phase variations across the pulse window for both modes despite their different periodic behaviour. Earlier studies reported that the transitions from the normal state to the slow-drift mode were preceded by the presence of nulling with typical durations of 5 to 10 periods. Our observations however seem to suggest that the transition to nulling follows shortly after the pulsar switches to the slow-drift mode and not at the boundary between the modes, with one instance of complete absence of nulling between mode switching. In addition, we also detected a second type of short-duration nulls not associated with the mode changing that showed quasi-periodic behaviour with periodicity PN ∼ 44 ± 7. The variety of features revealed in the single pulse sequence makes PSR B0809+74 an ideal candidate to understand the physical processes in the Partially Screened Gap dominated by non-dipolar magnetic fields

Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569 : a galactic wind in a ram pressure wind

Using the Effelsberg radio telescope at 4.85 GHz and 8.35 GHz we discovered large symmetric lobes of polarized radio emission around the strongly H

Magnetic fields and hot gas in M 101

Context. Studies of nearby spiral galaxies in radio and X-ray wavelengths reveal the structure and energy balance of the magnetic fields and the hot interstellar medium (ISM). In some spiral galaxies, large-scale ordered magnetic fields have been found between the spiral stellar arms (the so-called magnetic arms). One of the considered explanations of their origin is magnetic reconnection, which according to theoretical studies can efficiently heat the low-density ISM. Aims. We present, for the first time, high-resolution C-band (5 GHz) radio maps of the nearby face-on spiral galaxy M 101 to study the magnetic fields and verify the existence of the magnetic arms. The analysis of the archival XMM-Newton X-ray data is performed to search for signatures of gas heating by magnetic reconnection effects in the disk and the halo of this galaxy. Methods. We combine the Very Large Array (VLA) and Effelsberg radio maps of M 101 to restore the large-scale emission lost in the interferometric observations. From the obtained maps, we derive magnetic field strengths and energy densities, and compare them with the properties of the hot gas found with the spectral analysis of the X-ray data. Results. Most of the X-ray emission likely comes from the hot gas in the halo of M 101. Its temperature is highest above the massive stellar arm and an inter-arm region with enhanced polarised radio emission, as well as in the inter-arm area where neither Hα nor H