arXiv subjects

A. Shukurov

Publications and source records attributed to A. Shukurov.

26 records · Page 2Linked to original sources

arXiv2003

The magnetic field of M31 from multi-wavelength radio polarization observations

The configuration of the regular magnetic field in M31 is deduced from radio polarization observations at the wavelengths 6, 11 and 20 cm. By fitting the observed azimuthal distribution of polarization angles, we find that the regular magnetic field, averaged over scales 1--3 kpc, is almost perfectly axisymmetric in the radial range 8 to 14 kpc, and follows a spiral pattern with pitch angles of $p\simeq -19\degr$ to $p\simeq -8\degr$. In the ring between 6 and 8 kpc a perturbation of the dominant axisymmetric mode may be present, having the azimuthal wave number m=2. A systematic analysis of the observed depolarization allows us to identify the main mechanism for wavelength dependent depolarization -- Faraday rotation measure gradients arising in a magneto-ionic screen above the synchrotron disk. Modelling of the depolarization leads to constraints on the relative scale heights of the thermal and synchrotron emitting layers in M31; the thermal layer is found to be up to three times thicker than the synchrotron disk. The regular magnetic field must be coherent over a vertical scale at least similar to the scale height of the thermal layer, estimated to be $h\therm\simeq 1$ kpc. Faraday effects offer a powerful method to detect thick magneto-ionic disks or halos around spiral galaxies.

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arXiv2002

Magnetic fields in barred galaxies I. The atlas

The total and polarized radio continuum emission of 20 barred galaxies was observed with the Very Large Array (VLA) at 3, 6, 18 and 22 cm and with the Australia Telescope Compact Array (ATCA) at 6 cm and 13 cm. Maps at 30 arcsec angular resolution are presented here. Polarized emission (and therefore a large-scale regular magnetic field) was detected in 17 galaxies. Most galaxies of our sample are similar to non-barred galaxies with respect to the radio/far-infrared flux correlation and equipartition strength of the total magnetic field. Galaxies with highly elongated bars are not always radio-bright. We discuss the correlation of radio properties with the aspect ratio of the bar and other measures of the bar strength. We introduce a new measure of the bar strength, Λ, related to the quadrupole moment of the bar's gravitational potential. The radio surface brightness I of the barred galaxies in our sample is correlated with Λ, I \propto Λ^0.4+/-0.1, and thus is highest in galaxies with a long bar where the velocity field is distorted by the bar over a large fraction of the disc. In these galaxies, the pattern of the regular field is significantly different from that in non-barred galaxies. In particular, field enhancements occur upstream of the dust lanes where the field lines are oriented at large angles to the bar's major axis. Polarized radio emission seems to be a good indicator of large-scale non-axisymmetric motions.

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arXiv2001

Wavelet tomography of the Galactic magnetic field. I. The method

We suggest a two-dimensional wavelet devised to deduce the large-scale structure of a physical field (e.g., the Galactic magnetic field) from its integrals along straight paths from irregularly spaced data points to a fixed interior point (the observer). The method can be applied to the analysis of pulsar rotation and dispersion measures in terms of the large-scale Galactic magnetic field and electron density. The method does not use any 'a priori' assumptions about the physical field and can be considered as an algorithm of wavelet differentiation. We argue that a certain combination of the wavelet transformation with model fitting would be most efficient in the interpretation of the available pulsar RM data.

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arXiv2001

Magnetic fields in barred galaxies. II. Dynamo models

We study the generation and maintenance of large-scale magnetic fields in barred galaxies. We take a velocity field (with strong noncircular components) from a published gas dynamical simulation of Athanassoula (1992), and use this as input to a galactic dynamo calculation. Our work is largely motivated by recent high quality VLA radio observations of the barred galaxy NGC 1097, and we compare our results in detail with the regular magnetic fields deduced from these observations. We are able to reproduce most of the conspicuous large-scale features of the observed regular field, including the field structure in the central regions, by using a simple mean-field dynamo model in which the intensity of interstellar turbulence (more precisely, the turbulent diffusivity) is enhanced by a factor of 2-6 in the dust lanes and near the circumnuclear ring. We argue that magnetic fields can be dynamically important, and therefore should be included in models of gas flow in barred galaxies.

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arXiv2001

Structures in the RM sky

Coherent structures in the distribution of the Faraday rotation measure of extragalactic radio sources are isolated using wavelet transformation technique. Signatures of the magnetic fields in the local (Orion) spiral arm, the Sagittarius and Carina arms, the synchrotron Loop I and, possibly, the Perseus arm have been revealed using the RM catalogues of Simard-Normandin et al. (1981, 551 source)and Broten et al. (1988, 663 sources). Unlike earlier analyses of the RM sky, our approach has allowed us to assess the stability of the results with respect to modifications of the data sample. We estimate the mean magnetic field strength in the Orion arm as 1.4\pm0.3 mkGauss and find clear indication of a magnetic field reversal at a distance of 0.6-1 kpc towards the Galactic centre. Our analysis has revealed for the first time the extension of the reversal in the Carina arm. The direction of magnetic field in the Perseus arm is reversed with respect to that in the Orion arm. The average pitch angle of magnetic field in the nearby spiral arms is 15^\circ, and the mean field strength in the Sagittarius-Carina and Perseus arms is 1.7\pm0.3 mkGauss and 1.4\pm1.2 mkGauss, respectively. The line-of-sight magnetic field in Loop I is estimated as 0.9\pm0.3 mkGauss. We find firm evidence of a dominant even symmetry of the local mean field with respect to the galactic equator.

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arXiv2001

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.

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arXiv2000

Two-dimensional disk dynamos with vertical outflows into a halo

We study the effects of vertical outflows on mean-field dynamos in disks. These outflows could be due to thermal winds or magnetic buoyancy. We analyse numerical solutions of the nonlinear mean-field dynamo equations using a two-dimensional finite-difference model. Contrary to expectations, a modest vertical velocity can enhance dynamo action. This can lead to super-exponential growth of the magnetic field and to higher magnetic energies at saturation in the nonlinear regime.

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arXiv1996

Magnetic fields in the disk and halo of M51

We discuss the regular magnetic field in the galaxy M51 on the basis of a new interpretation of polarization angles observed at the wavelengths 2.8, 6.2, 18.0 and 20.5 cm. We found a magneto-ionic halo in M51 with a radial extent of about 10 kpc. The regular magnetic fields in the disk and the halo have different structures. The regular magnetic field in the halo is axisymmetric and horizontal. Its field lines are spirals pointing inwards and generally opposite to those in the disk. The azimuthal structure of the magnetic field in the disk is neither axisymme- tric nor bisymmetric but can be represented by a superposition of these two basic harmonics with about equal weights. Magnetic lines of the regular field in the disk are spirals generally directed outwards. They are well aligned with the optical spiral arms as traced by the dust lanes. The regular magnetic field strength averaged in 3 kpc wide rings is about 5-10 muG in the disk and 3muG in the radial range 3--6 kpc in the halo. These values are in good agreement with independent estimates from the synchrotron emission. The general features of the magnetic patterns revealed in the disk and the halo seem to be in agreement with predictions of dynamo theory. Our results are also based on an analysis of the magneto-ionic medium in M51. For a range of radii in the galactic disk, we estimated the scale heights of synchrotron and thermal disks, electron densities and filling factors as well as the amount of depolarization.

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