SIMBAD references

We examine the characteristics of warping modes in the maser disc at the centre of NGC 4258. We find that the velocity field associated with the modes is approximately vertical and that its amplitude decreases with radius. This characteristic accounts for the observed position and velocity distribution of the high-velocity maser features of NGC 4258. We also find that the position-velocity diagram calculated for a disc with a warping perturbation pattern is consistent with the observed redward offset of the low-velocity maser features, if the maser emission arises from the upper-half part of the disc.