Query : 2020A&A...642A.118K

2020A&A...642A.118K - Astronomy and Astrophysics, volume 642A, 118-118 (2020/10-1)

The magnetized disk-halo transition region of M 51.


Abstract (from CDS):

The grand-design face-on spiral galaxy M 51 is an excellent laboratory for studying magnetic fields in galaxies. Due to wavelength-dependent Faraday depolarization, linearly polarized synchrotron emission at different radio frequencies yields a picture of the galaxy at different depths: observations in the L-band (1-2GHz) probe the halo region, while at 4.85GHz (C-band) and 8.35GHz (X-band), the linearly polarized emission mostly emerges from the disk region of M 51. We present new observations of M 51 using the Karl G. Jansky Very Large Array at the intermediate frequency range of the S-band (2-4GHz), where previously no high-resolution broadband polarization observations existed, to shed new light on the transition region between the disk and the halo. We present the S-band radio images of the distributions of the total intensity, polarized intensity, degree of polarization, and rotation measure (RM). The RM distribution in the 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 multi-frequency polarization data of M 51 between 1-8GHz. The model makes distinct predictions of a two-layer (disk-halo) and three-layer (far-side halo "disk" near-side halo) system. Since the model predictions strongly differ within the wavelength range of the S-band, the new S-band data are essential for distinguishing between the different systems. A two-layer model of M 51 is preferred. 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 18µG and 24µG, while the regular field strengths are 8-16µG. In one inter-arm region, the regular field strength of 18µG exceeds that of the turbulent field of 11 µG. The regular field strengths in the halo are 3-5µG. 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.

Abstract Copyright: © M. Kierdorf et al. 2020

Journal keyword(s): galaxies: general - galaxies: magnetic fields - galaxies: individual: M 51 - galaxies: ISM

VizieR on-line data: <Available at CDS (J/A+A/642/A118): list.dat fits/*>

Simbad objects: 8

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Number of rows : 8
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2022
1 M 74 G 01 36 41.772 +15 47 00.46 10.52 10.00 9.46 9.16   ~ 1500 1
2 NGC 4631 GiP 12 42 08.009 +32 32 29.44   9.78 9.19 9.10   ~ 1004 2
3 ICRF J131337.8+545823 Sy2 13 13 37.85316548 +54 58 23.8993959     21.735 20.374   ~ 31 1
4 M 51 GiP 13 29 52.698 +47 11 42.93   9.26 8.36 8.40   ~ 3975 4
5 NGC 5195 GiP 13 29 59.590 +47 15 58.06 10.76 10.45 9.55 9.31   ~ 721 3
6 3C 286 Sy1 13 31 08.2885060664 +30 30 32.960825107   17.51 17.25     ~ 3948 2
7 QSO B1404+286 BLL 14 07 00.3933203928 +28 27 14.695133019   16.13 15.35 10.29   ~ 711 2
8 NGC 6946 H2G 20 34 52.332 +60 09 13.24   10.5       ~ 2335 2

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