基金Supported by the National Natural Science Foundation of China.Acknowledgements We thank the anonymous referee for insightful comments and constructive suggestions. We thank Xinwu Cao for helpful discussions and suggestions. This work is supported by the NSFC under grants 10373019, 10633010 and 10703009. The National Radio Astronomical Observatory is operated by Associated Universities, Inc., under a cooperative agreement with the National Science Foundation. MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of PPARC. This research made use of the NASA/IPAC Extragalactic Database （NED）, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, un- der contract with the National Aeronautics and Space Administration. This work also made use of Astrophysical Catalogs Support System （CATS） maintained by the Special Astrophysical Observatory, Russia.