Multifrequency observations of the radio continuum emission from NGC 253. 1: Magnetic fields and rotation measures in the bar and halo

Details Multifrequency observations of the radio continuum emission from NGC 253. 1: Magnetic fields and rotation measures in the bar and halo Multifrequency observations of the radio continuum emission from NGC 253. 1: Magnetic fields and rotation measures in the bar and halo

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...292..409b&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 292, no. 2, p. 409-424

Astronomy and Astrophysics

Astronomy

58

Astronomical Models, Barred Galaxies, Faraday Effect, Galactic Halos, Interstellar Magnetic Fields, Ionized Gases, Polarization, Radio Astronomy, Radio Emission, Spiral Galaxies, Astronomical Polarimetry, Galactic Winds, Luminous Intensity, Magnetic Field Configurations, Very Large Array (Vla)

Scientific paper

Radio continuum observations of the spiral galaxy NGC 253 with the Effelsberg and Very Large Array (VLA) telescopes reveal polarized emission from the bar and halo regions. Within the bar Faraday depolarization is strong at 1.5 and 5 GHz, due to ionized gas with ne approximately equal 0.1 - 3/cu cm which is mixed with turbulent magnetic fields of approximately equal 17 microG estimated strength. Even at 10 GHz the degree of polarization in the bar is low (only approximately equal 5% east and approximately equal 2% west of the nucleus) due to beam depolarization by unresolved tangled fields. In contrast, the magnetic fields in the halo are highly uniform, as indicated by fractional polarizations up to 40% at 10 GHz. Faraday depolarization in the halo at 1.5 GHz calls for a warm, clumpy gas component with ne approximately equal 0.02/cu cm and approximately equal 6 microG turbulent fields. We detected Faraday rotation in the bar, with rotation measures absolute value of RM approximately equal 100 rad/sq m (between 10 and 5 GHz) having different signs east and west of the nucleus. Below 5 GHz Faraday rotation is strongly reduced by the limited transparency for polarized emission in the bar. Faraday rotation in the halo in two regions at approximately 5 kpc above and below the plane with RM approximately equal -7 rad/sq m between 10 and 1.5 GHz can be ascribed to hot gas with mean value of ne approximately equal 0.002/cu cm and uniform fields along the line of sight of mean value of Bu parallel approximately equal -2 microG. The magnetic field structure in the bar and halo of NGC 253 is best described by the quadrupole-type dynamo mode SO, with a ring-like field in the bar and a field mainly parallel to the plane in a co-rotating halo. A major perturbation occurs in the east where the field is perpendicular to the plane and follows a 'spur'. The galactic wind is suppressed by the dominating plane-parallel field, except along the spur.

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