2006 (IPP)

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Author: search.filters.author.Shukurov, A.Subject: search.filters.subject.Galaxies

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    Origin and evolution of cluster magnetism
    (2006-04-02) Shukurov, A.; Subramanian, Kandaswamy; Haugen, N. E. L.
    Random motions can occur in the intergalactic gas of galaxy clusters at all stages of their evolution. Depending on the poorly known value of the Reynolds number, these motions can or cannot become turbulent, but in any case they can generate random magnetic fields via dynamo action. We argue that magnetic fields inferred observationally for the intracluster medium require dynamo action, and then estimate parameters of random flows and magnetic fields at various stages of the cluster evolution. Polarization in cluster radio halos predicted by the model would be detectable with the SKA.
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    Galactic dynamo and helicity losses through fountain flow
    (2006-01-19) Shukurov, A.; Sokoloff, Dmitry; Subramanian, Kandaswamy
    Nonlinear behaviour of galactic dynamos is studied, allowing for magnetic helicity removal by the galactic fountain flow. Methods. A suitable advection speed is estimated, and a one-dimensional mean-field dynamo model with dynamic α-effect is explored. Results. It is shown that the galactic fountain flow is efficient in removing magnetic helicity from galactic discs. This alleviates the constraint on the galactic mean-field dynamo resulting from magnetic helicity conservation and thereby allows the mean magnetic field to saturate at a strength comparable to equipartition with the turbulent kinetic energy.
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    Evolving turbulence and magnetic fields in galaxy clusters
    (2006-01-10) Subramanian, Kandaswamy; Shukurov, A.; Haugen, N. E. L.
    We discuss, using simple analytical models and MHD simulations, the origin and parameters of turbulence and magnetic fields in galaxy clusters. Any pre-existing tangled magnetic field must decay in a few hundred million years by generating gas motions even if the electric conductivity of the intracluster gas is high. We argue that tur- bulent motions can be maintained in the intracluster gas and its dynamo action can prevent such a decay and amplify a random seed magnetic field by a net factor typically 10⁴ in 5Gyr. Three physically distinct regimes can be identified in the evolution of turbulence and magnetic field in galaxy clusters. Firstly, the fluctuation dynamo will produce microgauss-strong, random magnetic fields during the epoch of cluster formation and major mergers. At this stage pervasive turbulent flows with r.m.s. velocity of about 300 kms−ᶥ can be maintained at scales 100–200 kpc. The magnetic field is intermittent, has a smaller scale of 20–30 kpc and average strength of 2 G. Secondly, turbulence will decay after the end of the major merger epoch; we discuss the dynamics of the decaying turbulence and the behavior of magnetic field in it. Magnetic field and turbulent speed undergo a power-law decay, decreasing by a factor of two during this stage, whereas their scales increase by about the same factor. Thirdly, smaller-mass subclusters and cluster galaxies will produce turbulent wakes where magnetic fields will be generated as well. Although the wakes plausibly occupy only a small fraction of the cluster volume, we show that their area covering factor can be close to unity, and thus they can produce some of the signatures of turbulence along virtually all lines of sight. The latter could potentially allow one to reconcile the possibility of turbulence with ordered filamentary gas structures, as in the Perseus cluster. The turbulent speeds and magnetic fields in the wakes are estimated to be of order 300 kms−ᶥ and 2 G, respectively, whereas the turbulent scales are of order 200 kpc for wakes behind subclusters of a mass 3 × 10ᶥᶟM⊙ and about 10 kpc in the galactic wakes. Magnetic field in the wakes is intermittent and has the scale of about 30 kpc and 1 kpc in the subcluster and galactic wakes, respectively. Random Faraday rotation measure is estimated to be typically 100–200 radm−², in agreement with observations. We predict detectable polarization of synchrotron emission from cluster radio halos at wavelengths 3–6 cm, if observed at sufficiently high resolution.