CHANG-ES XII

A LOFAR and VLA view of the edge-on star-forming galaxy NGC 3556

¹ Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany
e-mail: miskolczi@astro.rub.de
² Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
³ Dept. of Space, Earth and Environment, Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
⁴ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ CSIRO Astronomy and Space Science, PO Box 1130 Bentley, WA, 6102, Australia
⁶ Astronomical Observatory, Jagiellonian University, ul. Orla 171, Kraków, 30-244, Poland
⁷ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, , 11 rue de l’Université, 67000 Strasbourg, France
⁸ Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
⁹ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
¹⁰ Department of Astronomy, University of Massachusetts, 710 North Pleasant St., Amherst, MA, 01003, USA

Received: 23 July 2018
Accepted: 4 November 2018

Abstract

Context. Low-frequency radio continuum studies of star-forming edge-on galaxies can help to further understand how cosmic-ray electrons (CRe) propagate through the interstellar medium into the halo and how this is affected by energy losses and magnetic fields.

Aims. Observations with the Very Large Array (VLA) from Continuum Haloes in Nearby Galaxies – an EVLA Survey (CHANG-ES) are combined with those with the LOw Frequency ARray (LOFAR) from the LOFAR Two-metre Sky Survey (LoTSS ) to identify the prevailing mode of cosmic-ray transport in the edge-on spiral galaxy NGC 3556.

Methods. We mapped the radio spectral index, magnetic field strength, and orientation using VLA 1.5 and 6 GHz and LOFAR 144 MHz data, and we fit 1D cosmic-ray propagation models to these maps using SPINNAKER (Spectral Index Numerical Analysis of K(c)osmic-ray electron radio emission) and its interactive wrapper SPINTERACTIVE.

Results. We find that the spectral index in the galactic midplane is, as expected for young CRe, α ≈ −0.7 and steepens towards the halo of the galaxy as a consequence of spectral ageing. The intensity scale heights are about 1.4 and 1.9 kpc for the thin disc, and 3.3 and 5.9 kpc for the thick disc at 1.5 GHz and 144 MHz, respectively. While pure diffusion cannot explain our data, advection can, particularly if we assume a linearly accelerating wind. Our best-fitting model has an initial speed of 123 km s⁻¹ in the galactic midplane and reaches the escape velocity at heights between 5 kpc and 15 kpc above the disc, depending on the assumed dark matter halo of the galaxy. This galactic wind scenario is corroborated by the existence of vertical filaments seen both in the radio continuum and in H α in the disc-halo interface and of a large-scale reservoir of hot, X-ray emitting gas in the halo.

Conclusions. Radio haloes show the existence of galactic winds, possibly driven by cosmic rays, in typical star-forming spiral galaxies.