Deep radio continuum study of NGC 4631 and its Faraday tomography

Abstract

NGC4631 is a nearby edge-on spiral galaxy with a spectacular radio halo, one of the largest observed among edge-on galaxies. This makes the galaxy an ideal target to study the properties of synchrotron halos, the propagation of cosmic rays from the disk into the halo and to investigate the configuration of the magnetic field out of the plane. Radio continuum observations of NGC4631 were performed with the Karl G. Jansky Very Large Array at C-band in the C & D array configurations, and at L-band in the B, C & D array configurations. To recover the large-scale emission the data were combined with single-dish Effelsberg data. In order to determine the best approach to separate the thermal free-free and nonthermal synchrotron components of the total radio emission in the context of edge-one galaxies, we derive the thermal emission of NGC4631 on a pixel-by-pixel basis by using three different methods to account for dust extinction. We conclude that the most promising approach to estimate the thermal emission for NGC4631 and other edge-on galaxies consists of using calibrations that combine Halpha and 24 micron data. We analyze the nonthermal synchrotron emission by deriving vertical scale heights, studying its spectral index distribution and by estimating energy losses of cosmic rays. This is the first time the distribution of synchrotron emission and synchrotron spectral indices have been derived for an edge-on galaxy. In addition, we present the distribution of polarized emission, Faraday depths, and the intrinsic magnetic field orientation of NGC4631. The Faraday depths in the northern halo of NGC4631 vary smoothly from 200 rad/m-2 to about -150 rad/m-2 in a quasi-periodic pattern, indicating that the magnetic field in the sky plane has a systematic change in direction on scales of about 1.9 kpc. This is the first detection of such features in the Faraday depth distribution of an edge-on galaxy.