A colour composite image of polarized radio emission of our own Galaxy detected with the LOFAR radio telescope in 3C196 field. Different colors show emission detected at different Faraday depths. These observations cover the frequency range 115 - 178 MHz.
The Milky Way Working Group aims to increase our knowledge of magnetic fields in our Galaxy using LOFAR, primarily through the following channels: (1) diffuse polarized synchrotron emission and its Faraday rotation; (2) Faraday rotation measured towards pulsars; and (3) Faraday rotation measured towards extragalactic radio sources.
Measuring the frequency-dependent polarization of diffuse synchrotron emission allows us to map out the emission by the amount of Faraday rotation it has experienced; this process is called Faraday tomography. By measuring how much Faraday rotation is present for different lines-of-sight, and how the polarized emission is distributed by different amounts of Faraday rotation, we can explore properties of the small-scale magnetic field inside our Galaxy. This naturally enables collaboration with the other groups within the MKSP because every target field is expected to show Galactic synchrotron foreground emission, due to LOFAR’s large field-of-view.
Faraday rotation measures determined towards polarised radio sources, including pulsars and extragalactic sources, are an excellent probe of the large-scale Galactic magnetic field. Pulsars are valuable because they are distributed throughout the Galaxy, and both Faraday rotation measures and dispersion measures can be determined, providing an estimate of the average magnetic field in the ISM (parallel to the line-of-sight). Extragalactic sources provide complementary Faraday rotation information about lines-of-sight through (at least) our entire Galaxy, and this work also prompts collaboration with the other MKSP working groups.
An image of the Faraday cube of the IC342 field (Van Eck et al, in prep) made using LOFAR data covering the frequency range 115 to 178 MHz. To collapse the 3D cube to a 2D image, for each pixel the intensity and Faraday depth of the peak polarized flux density were identified. The color corresponds to the Faraday depth of the peak (red is -3 rad/m^2, green is around -1 rad/m^2, light blue is around zero, purple is +2 rad/m^2), while brightness corresponds to the polarized flux density. The most striking feature is the blue region in the lower left: it is surrounded on three sides by green emission with a sharp boundary, but the intensity is smooth across this boundary; this indicates that the emission comes from a single continuous region but there is a sharp enhancement in the Faraday rotation inside this blue region. The light blue point sources are artifacts caused by instrumental polarization leakage from total intensity into polarization.
The gas rich galaxy M33, at a distance of only 840 kpc, permits a coherent survey at high spatial resolution. Its very extended synchrotron outer disk makes it an ideal target for the study of the interstellar medium and its interaction with star formation processes, and for the study of the cosmic rays propagation mechanisms. No polarization has so far been detected with LOFAR in any nearby galaxy, due to strong Faraday depolarization. However, due to its location in the second galactic quadrant, in the Fan region, where Galactic diffuse synchrotron emission has been already detected with LOFAR, we expect to detect polarization from the Galactic foreground, which will be useful for studies of magnetism in the Milky Way. The displayed wide-field, low resolution (160 arcsec) total intensity image was obtained by using only core stations and by stacking images over 1 MHz bandwidth at 116 MHz. Reduction is in progress to both obtain high resolution Stokes I images to compare with the high frequency WSRT data and to properly characterize the Galactic polarized signal.
Located towards at the edge of the 1st Galactic quadrant (l,b)~(94,+35), Abell 2255 is a rich galaxy cluster at redshift z=0.08. Previous investigations by Pizzo et al. (2011) studied both the total and polarized power at short and long wavelengths (18, 21, 25, and 85 cm). While the radio galaxies and the highly polarized filaments at the edges of the central radio halo are detected in the high-frequency RM cube (obtained by combining the data at 18, 21, and 25 cm) the 85 cm RM cube is dominated by the Galactic foreground. A LOFAR HBA observation carried out in 2013 is currently being processed. The displayed wide-field, low resolution (160 arcsec) Stokes I image was obtained by using only core stations and by stacking images over the bandwidth 110-190 MHz. The noise level is 0.6 mJy/beam. Reduction is in progress to both obtain high resolution total intensity images to compare with the high frequency WSRT data and to properly characterize the Galactic polarized signal already detected in LOFAR HBA commissioning observations in 2011.