Among various approaches to indirect detection of dark matter (DM), synchrotron emission due to secondary electrons/positrons produced in galactic weakly interacting, massive particles annihilation is raising an increasing interest. In this Letter, we propose a new method to derive bounds in the m(chi)- <sigma(A)upsilon > plane by using radio continuum observations of Messier 33, paying particular attention to a low emitting radio cavity. The comparison of the expected radio emission due to the galactic DM distribution with the observed one provides bounds which are comparable to those obtained from a similar analysis of the Milky Way. Remarkably, the present results are simply based on archival data and are thus largely improvable by means of specifically tailored observations. The potentiality of the method compared with more standard searches is discussed by considering the optimistic situation of a vanishing flux (within the experimental sensitivity) measured inside the cavity by a high-resolution radio telescope like Atacama Large Millimeter Array. Under the best conditions, our technique is able to produce bounds which are comparable to the ones expected after 5 years of Fermi Large Area Telescope (LAT) data taking for a hadronic annihilation channel. Furthermore, it allows us to test the hypothesis that space telescopes such as Pamela and FermiLAT are actually observing electrons and positrons due to galactic DM annihilation into leptons.
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