Magnetic fields near the peripheries of galactic discs

Михайлов, Е. А.; Kasparova, Anastasia V.; Moss, D.; Beck, Rainer; Sokoloff, D.; Засов, А. В.

doi:10.1051/0004-6361/201323341

Cited by 10 publications

(14 citation statements)

References 66 publications

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“…Similar phenomena were qualitatively studied in [5], and some estimates were obtained in [6]. The existence of these waves for par ticular parameters of the interstellar environment of the Milky Way has been confirmed by simulation results [7]. Nevertheless, many details of this process remain unclear at large distances from the center of the galaxy, where the parameter values are at the bor der or even below the generation threshold of the mag netic field.…”

Section: Introductionsupporting

confidence: 68%

Problems with a small parameter and propagation of fronts in the galactic dynamo theory

Михайлов

2015

Moscow Univ. Phys.

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Propagation of the magnetic field to the periphery of the galactic disc and the possibility of its transfer using a dynamo wave are studied. For this purpose, a system of two equations with a small parameter of high order derivatives is considered. Its important feature is that for a certain value of the independent vari able the stability of the roots of the degenerate equation changes. For this problem the numerical velocity val ues of transition layers are obtained. The problem of their possible penetration beyond the bounds of the point of stability change is studied. A similar scalar equation is studied, for which asymptotic estimates of the front propagation velocity can be obtained. These estimates are in good agreement with the numerical simulation results.Keywords: magnetic fields of galaxies, dynamo wave, problems with a small parameter.

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Section: Introductionsupporting

confidence: 68%

Problems with a small parameter and propagation of fronts in the galactic dynamo theory

Михайлов

2015

Moscow Univ. Phys.

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“…3) even increases radially. This field may be assumed to be mostly regular so that we can invoke the α−Ω dynamo that also operates in the outer disk of galaxies (Mikhailov et al 2014). Although the star-formation activity is low in the outer disk, the magneto-rotational instability (MRI) may serve as the source of turbulence required for α − Ω dynamo action (Sellwood and Balbus 1999;Gressel et al 2013).…”

Section: Magnetic Energy Densitiesmentioning

confidence: 99%

Magnetic fields in spiral galaxies

Beck¹

2015

Astron Astrophys Rev

368

269

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Radio synchrotron emission, its polarization and Faraday rotation of the polarization angle are powerful tools to study the strength and structure of magnetic fields in galaxies. Unpolarized synchrotron emission traces isotropic turbulent fields which are strongest in spiral arms and bars (20-30 μG) and in central starburst regions (50-100 μG). Such fields are dynamically important; they affect gas flows and drive gas inflows in central regions. Polarized emission traces ordered fields, which can be regular or anisotropic turbulent, where the latter originates from isotropic turbulent fields by the action of compression or shear. The strongest ordered fields (10-15 μG) are generally found in interarm regions. In galaxies with strong density waves, ordered fields are also observed at the inner edges of spiral arms. Ordered fields with spiral patterns exist in grand-design, barred and flocculent galaxies and in central regions. Ordered fields in interacting galaxies have asymmetric distributions and are a tracer of past interactions between galaxies or with the intergalactic medium.-Faraday rotation measures of the diffuse polarized radio emission from galaxy disks reveal large-scale spiral patterns that can be described by the superposition of azimuthal modes; these are signatures of regular fields generated by mean-field dynamos. "Magnetic arms" between gaseous spiral arms may also be products of dynamo action, but need a stable spiral pattern to develop. Helically twisted field loops winding around spiral arms were found in two galaxies so far. Large-scale field reversals, like the one found in the Milky Way, could not yet be detected in external galaxies. In radio halos around edgeon galaxies, ordered magnetic fields with X-shaped patterns are observed. The origin and evolution of cosmic magnetic fields, in particular their first occurrence in young

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“…The mean-field dynamo can also operate in the outer disk of galaxies (Mikhailov et al 2014). The dynamo efficiency in the thin-disk approximation can be estimated by a few parameters, rotational shear S at radius r given by S = r (∂Ω/∂r), turbulent velocity v tur , and scale height h of the ionized gas (Arshakian et al 2009).…”

Section: Dynamo Action In the Outer Diskmentioning

confidence: 99%

“…With a propagation speed of a few kpc per Gyr (Mikhailov et al 2014), this process is slow and needs continuous dynamo action without disturbing merger events.…”

Section: Dynamo Action In the Outer Diskmentioning

confidence: 99%

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

Beck

2015

A&A

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Context. Magnetic fields play an important role in the formation and stabilization of spiral structures in galaxies, but the interaction between interstellar gas and magnetic fields has not yet been understood. In particular, the phenomenon of "magnetic arms" located between material arms is a mystery. Aims. The strength and structure of interstellar magnetic fields and their relation to spiral arms in gas and dust are investigated in the nearby and almost face-on spiral galaxy IC 342. Methods. The total and polarized radio continuum emission of IC 342 was observed with high spatial resolution in four wavelength bands with the Effelsberg and VLA telescopes. At λ6.2 cm the data from both telescopes were combined. I separated thermal and nonthermal (synchrotron) emission components with the help of the spectral index distribution and derived maps of the magnetic field strength, degree of magnetic field order, magnetic pitch angle, Faraday rotation measure, and Faraday depolarization. Results. IC 342 hosts a diffuse radio disk with an intensity that decreases exponentially with increasing radius. The frequency dependence of the scalelength of synchrotron emission indicates energy-dependent propagation of the cosmic-ray electrons, probably via the streaming instability. The equipartition strength of the total field in the main spiral arms is typically 15 μG, that of the ordered field about 5 μG. The total radio emission, observed with the VLA's high resolution, closely follows the dust emission in the infrared at 8 μm (Spitzer telescope) and 22 μm (WISE telescope). The polarized emission is not diffuse, but concentrated in spiral arms of various types: (1) a narrow arm of about 300 pc width, displaced inwards with respect to the eastern arm by about 200 pc, indicating magnetic fields compressed by a density wave; (2) a broad arm of 300-500 pc width around the northern arm with systematic variations in polarized emission, polarization angles, and Faraday rotation measures on a scale of about 2 kpc, indicative of a helically twisted flux tube generated by the Parker instability; (3) a rudimentary magnetic arm in an interarm region in the north-west; (4) several broad spiral arms in the outer galaxy, related to spiral arms in the total neutral gas; (5) short features in the outer south-western galaxy, probably distorted by tidal interaction. Faraday rotation of the polarization angles reveals an underlying regular field of only 0.5 μG strength with a large-scale axisymmetric spiral pattern, probably a signature of a mean-field α − Ω dynamo, and an about 10× stronger field that fluctuates on scales of a few 100 pc. The magnetic field around the bar in the central region of IC 342 resembles that of large barred galaxies; it has a regular spiral pattern with a large pitch angle, is directed outwards, and is opposite to the large-scale regular field in the disk. Polarized emission at λ20.1 cm is strongly affected by Faraday depolarization in the western and northern parts of the galaxy. Helical fields extending fro...

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Magnetic fields near the peripheries of galactic discs

Cited by 10 publications

References 66 publications

Problems with a small parameter and propagation of fronts in the galactic dynamo theory

Problems with a small parameter and propagation of fronts in the galactic dynamo theory

Magnetic fields in spiral galaxies

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

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