基金Acknowledgements We thank the referee for very careful reading and very detailed comments, Dr. Hui Shi for kind helps in the RM collection from some papers, Dr. Tracy Clark for providing RM data from her PhD thesis and Dr. Bin Cui for his helps and advice in development of the webpage for the RM catalog. J. Xu would especially like to thank X. Y. Gao and P. F. Wang for helpful discussions. The authors are financially supported by NAOC135 Grants and the Strategic Priority Research Program ＂The Emergence of Cosmological Structures＂ （Grant No. XDB09010200） of the Chinese Academy of Sciences. This research has extensively used the databases of the National Radio Astronomy Observatory VLA Sky Survey （NVSS）, VLA Faint Images of the Radio Sky （FIRST）, the set of Identifications, Measurements and Bibliography for Astronomical Data （SIMBAD） and 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.
基金This work was supported by the National Natural Science Foundation of China with project No. 50906080 and the National Basic Research Program of China numbered 2007CB210104.Acknowledgements This work was supported by the National Natural Science Foundation of China with project No. 50906080 and the National Basic Research Program of China numbered 2007CB210104. These supports are gratefully acknowledged.