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.

radio continuum: galaxies

galaxies: ISM

cosmic rays

galaxies: halos

galaxies: individual: NGC 891

galaxies: magnetic fields


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

CSIRO Astronomy and Space Science

V. Heesen

Universität Hamburg

Cathy Horellou

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Extragalactic Astrophysics

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 Astrofisico 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 A98

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics

Fusion, Plasma and Space Physics



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