High-velocity H2O masers associated massive star formation regions

We report on the results of (CO)-C-12 (1-0) emission associated with H2O masers and massive star formation regions to identify high-velocity H2O masers. Several masers have a large blueshift, even up to 120 km.s(-1), with respect to the CO peak, but no large redshifted maser appears. This result suggests that high-velocity H2O masers can most probably occur in high mass star-forming regions and quite a number of masers stem from the amplifications of a background source, which may enable those undetectable weak masers to come to an observable level.