IX. Radio scale heights and scale lengths of a consistent sample of 13 spiral galaxies seen edge-on and their correlations
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69,
2 Dept. of Physics, Engineering Physics, & Astronomy, Queen’s University, Kingston, Ontario K7L 3N6, Canada
3 Astronomisches Institut, Ruhr Universität Bochum, 44780 Bochum, Germany
4 Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109-1107, USA
5 CSIRO Astronomy and Space Science, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
6 Department of Physics and Astronomy, University of New Mexico, 800 Yale Boulevard, NE, Albuquerque, NM 87131, USA
7 Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
8 Department of Astronomy, New Mexico State University, PO Box 30001, MSC 4500, Las Cruces, NM 88003, USA
9 Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
10 National Radio Astronomy Observatory, 520 Egmont Road, Charlottesville, VA 22903, USA
Accepted: 27 November 2017
Aim. The vertical halo scale height is a crucial parameter to understand the transport of cosmic-ray electrons (CRE) and their energy loss mechanisms in spiral galaxies. Until now, the radio scale height could only be determined for a few edge-on galaxies because of missing sensitivity at high resolution.
Methods. We developed a sophisticated method for the scale height determination of edge-on galaxies. With this we determined the scale heights and radial scale lengths for a sample of 13 galaxies from the CHANG-ES radio continuum survey in two frequency bands.
Results. The sample average values for the radio scale heights of the halo are 1.1 ± 0.3 kpc in C-band and 1.4 ± 0.7 kpc in L-band. From the frequency dependence analysis of the halo scale heights we found that the wind velocities (estimated using the adiabatic loss time) are above the escape velocity. We found that the halo scale heights increase linearly with the radio diameters. In order to exclude the diameter dependence, we defined a normalized scale height h˜ which is quite similar for all sample galaxies at both frequency bands and does not depend on the star formation rate or the magnetic field strength. However, h˜ shows a tight anticorrelation with the mass surface density.
Conclusions. The sample galaxies with smaller scale lengths are more spherical in the radio emission, while those with larger scale lengths are flatter. The radio scale height depends mainly on the radio diameter of the galaxy. The sample galaxies are consistent with an escape-dominated radio halo with convective cosmic ray propagation, indicating that galactic winds are a widespread phenomenon in spiral galaxies. While a higher star formation rate or star formation surface density does not lead to a higher wind velocity, we found for the first time observational evidence of a gravitational deceleration of CRE outflow, e.g. a lowering of the wind velocity from the galactic disk.
Key words: galaxies: spiral / galaxy: halo / galaxies: magnetic fields / galaxies: ISM / radio continuum: general / radio continuum: galaxies / radio continuum: galaxies
© ESO 2018