1. Department of Astronomy, Beijing Normal University, Beijing 100875, P. R. China
2. Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
3. Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, P. R. China
4. School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, P. R. China