Extragalactic H2O masers and X-ray absorbing column densities J. S. Zhang, C. Henkel, M. Kadler, L. J. Greenhill, N. Nagar, A. S. Wilson and J. A. Braatz A&A, 450 3 (2006) 933-944 Published online: 19 April 2006 DOI: 10.1051/0004-6361:20054138