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Moss D.,University of Manchester | Stepanov R.,Perm State University | Arshakian T.G.,MPI fur Radioastronomie | Arshakian T.G.,Armenia and Isaac Newton Institute of Chile | And 3 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Magnetic fields in nearby, star-forming galaxies reveal large-scale patterns, as predicted by dynamo models, but also a variety of small-scale structures. In particular, a large-scale field reversal may exist in the Milky Way while no such reversals have been observed so far in external galaxies. Aims. The effects of star-forming regions in galaxies need to be included when modelling the evolution of their magnetic fields, which can then be compared to future radio polarization observations. The conditions leading to large-scale field reversals also need clarification. Methods. Our simplified model of field evolution in isolated disc galaxies includes a standard mean-field dynamo and continuous injection of turbulent fields (the effect of supernova explosions) in discrete star forming regions by implicit small-scale dynamo action. Synthetic maps of radio synchrotron emission and Faraday rotation measures are computed for galaxies at different evolutionary stages. Results.A large-scale dynamo is essential to obtain regular large-scale spiral magnetic fields, as observed in many galaxies. These appear, on kpc scales in near energy equilibrium with the turbulence, after 1-2 Gyr (corresponding to redshift about 4-3). The injection of turbulent fields generates small-scale field structures. Strong injected small-scale fields and a large dynamo number (e.g. rapid rotation) of a galaxy favour the generation of field reversals. Depending on the model parameters, large-scale field reversals may persist over many Gyrs and can survive until the present epoch. Significant polarized radio synchrotron emission from young galaxies is expected at redshift ≤4. Faraday rotation measures (RM) are crucial to detect field reversals. Large-scale RM patterns of rotation measures can be observed at redshift ≤3. Conclusions. Our model can explain the general form of axisymmetric spiral fields with many local distortions, as observed in nearby galaxies. For a slightly different choice of parameters, large-scale field reversals can persist over the lifetime of a galaxy. Comparing our synthetic radio maps with future observations of distant galaxies with the planned Square Kilometre Array (SKA) and its precursors will allow testing and refinement of models of magnetic field evolution. © 2012 ESO. Source

Arshakian T.G.,Max Planck Institute for Radio Astronomy | Arshakian T.G.,Armenia and Isaac Newton Institute of Chile | Beck R.,Max Planck Institute for Radio Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2011

Polarized radio synchrotron emission from interstellar, intracluster and intergalactic magnetic fields is affected by frequency-dependent Faraday depolarization. The maximum polarized intensity depends on the physical properties of the depolarizing medium. New-generation radio telescopes such as Low Frequency Array (LOFAR), the Square Kilometre Array (SKA) and its precursors need a wide range of frequencies to cover the full range of objects. The optimum frequency of maximum polarized intensity (PI) is computed for the cases of depolarization in magneto-ionic media by regular magnetic fields (differential Faraday rotation) or by turbulent magnetic fields (internal or external Faraday dispersion), assuming that the Faraday spectrum of the medium is dominated by one component or that the medium is turbulent. Polarized emission from bright galaxy discs, spiral arms and cores of galaxy clusters are best observed at wavelengths below a few centimetres (at frequencies beyond about 10GHz), haloes of galaxies and clusters around decimetre wavelengths (at frequencies below about 2GHz). Intergalactic filaments need observations at metre wavelengths (frequencies below 300 MHz). Sources with extremely large intrinsic rotation measure | RM | or RM dispersion can be searched with mm-wave telescopes. Measurement of the PI spectrum allows us to derive the average Faraday | RM | or the Faraday dispersion within the source, as demonstrated for the case of the spiral galaxy NGC 6946. Periodic fluctuations in PI at low frequencies are a signature of differential Faraday rotation. Internal and external Faraday dispersion can be distinguished by the different slopes of the PI spectrum at low frequencies. A wide band around the optimum frequency is important to distinguish between varieties of depolarization effects. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Moss D.,University of Manchester | Beck R.,MPI fur Radioastronomie | Sokoloff D.,Moscow State University | Stepanov R.,RAS Institute of Continuous Media Mechanics | And 3 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Observations of polarized radio emission show that large-scale (regular) magnetic fields in spiral galaxies are not fully axisymmetric, but generally stronger in interarm regions. In some nearby galaxies such as NGC 6946 they are organized in narrow magnetic arms situated between the material spiral arms. Aims. The phenomenon of magnetic arms and their relation to the optical spiral arms (the material arms) calls for an explanation in the framework of galactic dynamo theory. Several possibilities have been suggested but are not completely satisfactory; here we attempt a consistent investigation. Methods. We use a 2D mean-field dynamo model in the no-z approximation and add injections of small-scale magnetic field, taken to result from supernova explosions, to represent the effects of dynamo action on smaller scales. This injection of small scale field is situated along the spiral arms, where star-formation mostly occurs. Results. A straightforward explanation of magnetic arms as a result of modulation of the dynamo mechanism by material arms struggles to produce pronounced magnetic arms, at least with realistic parameters, without introducing new effects such as a time lag between Coriolis force and α-effect. In contrast, by taking into account explicitly the small-scale magnetic field that is injected into the arms by the action of the star forming regions that are concentrated there, we can obtain dynamo models with magnetic structures of various forms that can be compared with magnetic arms. These are rather variable entities and their shape changes significantly on timescales of a few 100 Myr. Properties of magnetic arms can be controlled by changing the model parameters. In particular, a lower injection rate of small-scale field makes the magnetic configuration smoother and eliminates distinct magnetic arms. Conclusions. We conclude that magnetic arms can be considered as coherent magnetic structures generated by large-scale dynamo action, and associated with spatially modulated small-scale magnetic fluctuations, caused by enhanced star formation rates within the material arms. © ESO, 2013. Source

Leon-Tavares J.,University of Turku | Leon-Tavares J.,Aalto University | Chavushyan V.,National Institute of Astrophysics, Optics and Electronics | Patino-Alvarez V.,National Institute of Astrophysics, Optics and Electronics | And 12 more authors.
Astrophysical Journal Letters | Year: 2013

We report the detection of a statistically significant flare-like event in the Mg II λ2800 emission line of 3C 454.3 during the outburst of autumn 2010. The highest levels of emission line flux recorded over the monitoring period (2008-2011) coincide with a superluminal jet component traversing through the radio core. This finding crucially links the broad emission line fluctuations to the non-thermal continuum emission produced by relativistically moving material in the jet and hence to the presence of broad-line region clouds surrounding the radio core. If the radio core were located at several parsecs from the central black hole, then our results would suggest the presence of broad-line region material outside the inner parsec where the canonical broad-line region is envisaged to be located. We briefly discuss the implications of broad emission line material ionized by non-thermal continuum in the context of virial black hole mass estimates and gamma-ray production mechanisms. © 2013. The American Astronomical Society. All rights reserved. Source

Cohen M.H.,California Institute of Technology | Meier D.L.,California Institute of Technology | Meier D.L.,Jet Propulsion Laboratory | Arshakian T.G.,University of Cologne | And 13 more authors.
Astrophysical Journal | Year: 2015

We study the kinematics of ridge lines on the parsec-scale jet of the active galactic nucleus BL Lacertae. We show that the ridge lines display transverse patterns that move superluminally downstream, and that the moving patterns are analogous to waves on a whip. Their apparent speeds βapp (units of c) range from 3.9 to 13.5, corresponding to in the galaxy frame. We show that the magnetic field in the jet is well ordered with a strong transverse component, and assume that it is helical and that the transverse patterns are Alfvn waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic (). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behavior. The Alfvn waves appear to be excited by changes in the position angle of the recollimation shock, in analogy to exciting a wave on a whip by shaking the handle. A simple model of the system with plasma sound speed βs=0.3 and apparent speed of a slow MHD wave βapp,S=4 yields Lorentz factor of the beam Γbeam ∼ 4.5, pitch angle of the helix (in the beam frame) α ∼ 67°, Alfvn speed βA ∼ 0.64, and magnetosonic Mach number Mms ∼ 4.7. This describes a plasma in which the magnetic field is dominant and in a rather tight helix, and Alfvn waves are responsible for the moving transverse patterns. © 2015. The American Astronomical Society. All rights reserved.. Source

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