Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253

I. The distribution and transport of cosmic rays

¹ Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany e-mail: heesen@astro.rub.de
² Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 8 July 2008
Accepted: 28 November 2008

Abstract

Context. Nearby edge-on galaxies showing a synchrotron halo are nearly ideal objects for studying the transport of cosmic rays (CRs) in galaxies. Among them, the nearby starburst galaxy NGC 253 hosts a galactic wind indicated by various ISM phases in its halo.

Aims. The diffusive and convective CR transport from the disk into the halo is investigated using the local CR bulk speed. The connection between the CR transport and the galactic wind is outlined.

Methods. We observed NGC 253 with the VLA at λ6.2 cm in a mosaic with 15 pointings. The missing zero-spacing flux density of the VLA mosaic was filled in using observations with the 100-m Effelsberg telescope. We also obtained a new λ3.6 cm map from Effelsberg observations and reproduced VLA maps at λ20 cm and λ90 cm. The high dynamic range needed due to the strong nuclear point-like source was addressed with a special data calibration scheme for both the single-dish and the interferometric observations.

Results. We find a thin and a thick radio disk with exponential scaleheights of 0.3 kpc and 1.7 kpc at λ6.2 cm. The equipartition total magnetic field strength between 7 μG and 18 μG in the disk is remarkably high. We use the spectral aging of the cosmic ray electrons (CREs) seen in the vertical profiles of the spectral index to determine a lower limit for the global CR bulk speed as 170 ± 70 km s^-1. The linear correlation between the scaleheights and the CRE lifetimes, as evident from the dumbbell shaped halo, requires a vertical CR transport with a bulk speed of 300 ± 30 km s^-1 in the northeastern halo, similar to the escape velocity of 280 km s^-1. This shows the presence of a “disk wind” in NGC 253. In the southwestern halo, the transport is mainly diffusive with a diffusion coefficient of 2.0 ± 0.2 10²⁹ cm² s^-1.

Conclusions. Measuring the radio synchrotron scaleheight and estimating the CRE lifetime allow us to determine the bulk speed of the CR transport into the halo. The transport is convective and more efficient in the northeastern halo, while it is diffusive in the southwestern halo. The luminous material is transported by the disk wind, which can explain the different amounts of extra-planar H I, Hα, and soft X-ray emission in the two halo parts. Future low-frequency radio observations will provide the data to analyze the vertical velocity profile of galactic winds.