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tokens: ACKNOWLEDGEMENTS We wish to thank Michael Opitsch for useful discussion during the preliminary phase of this work. LC thanks the Department of Physics and Astronomy of the Padova University for hospitality while this paper was in progress. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.