Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891
Journal article, 2018
Context. Cosmic rays and magnetic fields play an important role for the formation and dynamics of gaseous halos of galaxies. Aims. Low-frequency radio continuum observations of edge-on galaxies are ideal to study cosmic-ray electrons (CREs) in halos via radio synchrotron emission and to measure magnetic field strengths. Spectral information can be used to test models of CRE propagation. Free-free absorption by ionized gas at low frequencies allows us to investigate the properties of the warm ionized medium in the disk. Methods. We obtained new observations of the edge-on spiral galaxy NGC 891 at 129-163 MHz with the LOw Frequency ARray (LOFAR) and at 13-18 GHz with the Arcminute Microkelvin Imager (AMI) and combine them with recent high-resolution Very Large Array (VLA) observations at 1-2 GHz, enabling us to study the radio continuum emission over two orders of magnitude in frequency. Results. The spectrum of the integrated nonthermal flux density can be fitted by a power law with a spectral steepening towards higher frequencies or by a curved polynomial. Spectral flattening at low frequencies due to free-free absorption is detected in star-forming regions of the disk. The mean magnetic field strength in the halo is 7 +/- 2 mu G. The scale heights of the nonthermal halo emission at 146 MHz are larger than those at 1.5 GHz everywhere, with a mean ratio of 1.7 +/- 0.3, indicating that spectral ageing of CREs is important and that diffusive propagation dominates. The halo scale heights at 146 MHz decrease with increasing magnetic field strengths which is a signature of dominating synchrotron losses of CREs. On the other hand, the spectral index between 146 MHz and 1.5 GHz linearly steepens from the disk to the halo, indicating that advection rather than diffusion is the dominating CRE transport process. This issue calls for refined modelling of CRE propagation. Conclusions. Free-free absorption is probably important at and below about 150 MHz in the disks of edge-on galaxies. To reliably separate the thermal and nonthermal emission components, to investigate spectral steepening due to CRE energy losses, and to measure magnetic field strengths in the disk and halo, wide frequency coverage and high spatial resolution are indispensable.
cosmic rays
galaxies: ISM
radio continuum: galaxies
galaxies: individual: NGC 891
galaxies: magnetic fields
galaxies: halos
Author
D. D. Mulcahy
Max Planck Society
University of Manchester
A. Horneffer
Max Planck Society
R. Beck
Max Planck Society
M. Krause
Max Planck Society
P. Schmidt
Max Planck Society
A. Basu
Bielefeld University
Max Planck Society
K. T. Chyzy
Jagiellonian University in Kraków
R. -J. Dettmar
Ruhr-Universität Bochum
M. Haverkorn
Radboud University
G. Heald
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
V. Heesen
University of Hamburg
Cathy Horellou
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
M. Iacobelli
Netherlands Institute for Radio Astronomy (ASTRON)
B. Nikiel-Wroczynski
Jagiellonian University in Kraków
R. Paladino
Istituto nazionale di astrofisica (INAF)
A. M. M. Scaife
University of Manchester
Sarrvesh S. Sridhar
University of Groningen
Netherlands Institute for Radio Astronomy (ASTRON)
R. G. Strom
University of Amsterdam
Netherlands Institute for Radio Astronomy (ASTRON)
F. S. Tabatabaei
University of La Laguna
Instituto de Astrofísica de Canarias
T. Cantwell
University of Manchester
S. H. Carey
University of Cambridge
K. Grainge
University of Manchester
J. Hickish
University of Cambridge
Y. Perrot
University of Cambridge
N. Razavi-Ghods
University of Cambridge
P. Scott
University of Cambridge
D. Titterington
University of Cambridge
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 615 A98Subject Categories
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
Fusion, Plasma and Space Physics
DOI
10.1051/0004-6361/201832837