We thank Eugene Churazov and Torsten Ensslin for helpful discussions. We acknowledge support from SAO grant GO0-1129X, the National Science Foundation grants AST 9529170 and AST 9876887, and NASA under grant NAG5-6337. YC wishes to thank support from the MOST through the National R & D program for women's universities. CSR thanks support from Hubble Fellowship Grant HF 01113.01-98A.