Histogram showing the number of H₂O kilomaser galaxies as a function of their nuclear column density derived from X-ray spectroscopy. Due to the higher angular resolution, Chandra data are taken whenever possible (with the exception of NGC 4051 for which the XMM-derived column density value was preferred to that of Chandra because of the large uncertainties reported). Otherwise, the most recent measurements are used (see Table 4). The arrows for NGC 2273 and NGC 6300 indicate lower limits to the column densities. Each color indicates the association of the water maser emission with either star formation or AGN activity, and the degree of confidence in such an association.