Warped diffusive radio halo around the quiescent spiral edge-on galaxy NGC 4565
Journal article, 2019

Context. Cosmic rays play a pivotal role in launching galactic winds, particularly in quiescently star-forming galaxies where the hot gas alone is not sufficient to drive a wind. Except for the Milky Way, not much is known about the transport of cosmic rays in galaxies.
Aims. In this Letter, we present low-frequency observations of the nearby edge-on spiral galaxy NGC 4565 using the LOw-Frequency ARray (LOFAR). With our deep 144 MHz observations, we obtain a clean estimate of the emission originating from old cosmic-ray electrons (CRe), which is almost free from contamination by thermal emission.
Methods. We measured vertical profiles of the non-thermal radio continuum emission that we fitted with Gaussian and exponential functions. The different profile shapes correspond to 1D cosmic-ray transport models of pure diffusion and advection, respectively. Results. We detect a warp in the radio continuum that is reminiscent of the previously known H I warp. Because the warp is not seen at GHz-frequencies in the radio continuum, its minimum age must be about 100 Myr. The warp also explains the slight flaring of the thick radio disc that can otherwise be well described by a Gaussian profile with an FWHM of 65 arcsec (3.7 kpc).
Conclusions. The diffusive radio halo together with the extra-planar X-ray emission may be remnants of enhanced star-forming activity in the past where the galaxy had a galactic wind, as GHz-observations indicate only a weak outflow in the last 40 Myr. NGC 4565 could be in transition from an outflow-to an inflow-dominated phase.

cosmic rays

galaxies: individual: NGC 4565

galaxies: spiral

radio continuum: galaxies

galaxies: halos

galaxies: magnetic fields


V Heesen

Universität Hamburg

L. Whitler

Arizona State University

P. Schmidt

Max Planck Society

A. Miskolczi

Ruhr-Universität Bochum

S. S. Sridhar

Netherlands Institute for Radio Astronomy (ASTRON)

Cathy Horellou

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

R. Beck

Max Planck Society

G. Gurkan

CSIRO Astronomy and Space Science

E. Scannapieco

Arizona State University

M. Bruggen

Universität Hamburg

G. H. Heald

CSIRO Astronomy and Space Science

M. Krause

Max Planck Society

R. Paladino

Istituto nazionale di astrofisica (INAF)

B. Nikiel-Wroczynski

Jagiellonian University in Kraków

Leiden University

A. Wilber

Universität Hamburg

R-J Dettmar

Ruhr-Universität Bochum

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 628 L3

Subject Categories

Astronomy, Astrophysics and Cosmology

Fusion, Plasma and Space Physics

Condensed Matter Physics



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