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تسجيل مستخدم جديدThe Magnetized Disk-Halo Transition Region of M51
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An excellent laboratory for studying large scale magnetic fields is the grand de- sign face-on spiral galaxy M51. Due to wavelength-dependent Faraday depolarization, linearly polarized synchrotron emission at different radio frequencies gives a picture of the galaxy at different depths: Observations at L-band (1-2 GHz) probe the halo region while at C- and X- band (4-8 GHz) the linearly polarized emission probe the disk region of M51. We present new observations of M51 using the Karl G. Jansky Very Large Array (VLA) at S-band (2-4 GHz), where previously no polarization observations existed, to shed new light on the transition region between the disk and the halo. We discuss a model of the depolarization of synchrotron radiation in a multilayer magneto-ionic medium and compare the model predictions to the multi-frequency polarization data of M51 between 1-8GHz. The new S-band data are essential to distinguish between different models. Our study shows that the initial model parameters, i.e. the total reg- ular and turbulent magnetic field strengths in the disk and halo of M51, need to be adjusted to successfully fit the models to the data.
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The grand-design face-on spiral galaxy M51 is an excellent laboratory for studying magnetic fields in galaxies. We present new observations of M51 using the VLA at the frequency range of S-band (2-4GHz), to shed new light on the transition region bet
ween the disk and halo. We present images of the distributions of the total intensity, polarized intensity, degree of polarization, and rotation measure (RM). The RM distribution in S-band shows a fluctuating pattern without any apparent large-scale structure. We discuss a model of the depolarization of synchrotron radiation in a multi-layer magneto-ionic medium and compare the model predictions to the polarization data of M51 between 1-8GHz. Since the model predictions strongly differ within the wavelength range of the S-band, the new data are essential. The parameters of the model are adjusted to fit to the data of polarization fractions in a few selected regions. In three spiral arm regions, the turbulent field in the disk dominates with strengths between 18muG and 24muG, while the regular field strengths are 8-16muG. In one inter-arm region, the regular field strength of 18muG exceeds that of the turbulent field of 11muG. The regular field strengths in the halo are 3-5muG. The observed RMs in the disk-halo transition region are probably dominated by tangled regular fields, as predicted from models of evolving dynamos, and/or vertical fields, as predicted from numerical simulations of Parker instabilities or galactic winds. Both types of magnetic fields have frequent reversals on scales similar to or larger than the beam size (550pc) that contribute to an increase of the RM dispersion and to distortions of any large-scale pattern of the regular field. Our study devises new ways of analyzing and interpreting broadband multi-frequency polarization data that will be applicable to future data from, for example, the Square Kilometre Array.
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