Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253 III. Helical magnetic fields in the nuclear outflow

Abstract

Magnetic fields are a good tracer for gas compression by shock waves, which can be caused by interaction of star-formation driven outflows from individual star formation sites as described in the chimney model. We study the magnetic field structure in the central part of the nuclear starburst galaxy NGC 253 with spatial resolutions between 40 and 150 pc to detect any filamentary emission associated with the nuclear outflow. New VLA observations at 3 cm with 7.5" resolution were combined with archive data at 20 and 6 cm. We find filamentary radio continuum emission in a geometrical distribution that we interpret as the boundary of the northwestern nuclear outflow cone. The scaleheight of the continuum emission is 150+/-20 pc, regardless of the observing frequency. The equipartition magnetic field strength is 46+/-10 microG for the total field and 21+/-5 microG for the regular field in the filaments. The ordered magnetic field is aligned along the filaments, in agreement with amplification due to compression. The perpendicular diffusion coefficient across the filaments is kappa_perp = 1.5 x 10^28 cm^2 s^-1 E(GeV)^(0.5+/-0.7). In the SE part of the nuclear outflow cone the magnetic field is pointing away from the disc in form of a helix, with an azimuthal component increasing up to at least 1200 pc height, where it is about equal to the total component. The ordered magnetic field in the disc is anisotropic within a radius of 2.2 kpc. At larger radii, the large-scale field is regular and of even parity. The magnetic field is able to collimate the outflow, which can explain the observed small opening angle of ~26 degree. Due to angular momentum conservation, the field lines are frozen into the plasma and are wound up into a helix. Strong adiabatic losses of the cosmic-ray electrons can partly explain why the radio luminosity of the nucleus lies below the radio-FIR correlation.