Recently, the Low-Frequency ARray (LOFAR) has been used to search for the elusive dark matter in dwarf spheroidal galaxies using radio continuum observations. One of the leading dark matter candidates are the weakly interacting massive particles (WIMPs), which can produce via several annihilation channels cosmic-ray electrons. With the micro-Gauss magnetic field strengths we expect in galaxies, these electrons generate non-thermal synchrotron emission that peaks in the range of hundreds of MHz. With a new 8-hour observation at 150 MHz as part of the LOFAR Two-metre Sky Survey (LoTSS), upper limits for the radio continuum emission in the dwarf spheroidal galaxy Canes Venatici I (CVnI) were obtained. These can be converted into new upper limits for the cross-section of WIMPs. LOFAR has quite good sensitivity, so that for a reasonable choice of model parameters such as magnetic field strength and comic-ray diffusion coefficient, the limits are comparable with those set by the Fermi Large Area Telescope using gamma-ray observations of this particular galaxy. The benchmark limits of this new work exclude already several thermal WIMP realizations in the [2, 20]-GeV mass range.

Figure. (a) 150-MHz map of Canes Venatici I observed with LOFAR, where the large circle shows the extent of the stellar disc and the small circles show the position of background sources; these were subtracted from the map to search for the diffuse emission from annihilating WIMPs. (b) new resulting cross-section of WIMPs annihilating into electron–positron pairs. Several models with various magnetic field strengths and diffusion coefficients are presented.

Reference: Vollmann et al. (2020), MNRAS, 496, 2663