1. CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, P.R. China
2. School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, P. R. China
3. Helmholtz-Institut für Strahlen-und Kernphysik, Universität Bonn, Nussallee 14-16, D-53115 Bonn, Germany
4. Charles University in Prague, Faculty of Mathematics and Physics, Astronomical Institute, V Holesovickach 2, CZ-180 00 Praha 8, Czech Republic