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HI observations of NGC 4258 (van Albada 1978, 1980) reveal a rather
flat rotation curve (
200 kms-1) in the outer
parts of the galaxy, but a steep rise in the nuclear region. The latter prompts
van Albada (1978) to speculate ``that the rotational velocities near the nucleus
may be significantly higher than measured, implying a larger central mass
concentration.'' Streaming motions in the HI velocity field are
attributed to the weak bar in the disk of NGC 4258 (type SXS4, see Table 2).
NGC 4258 is also well known for its anomalous arms, visible only in H
and radio continuum, which are usually interpreted in terms of collimated
nuclear outflow or jets (Cecil, Wilson & Tully 1992; Dettmar & Koribalski
1990, and references therein).
As NGC 4258, the galaxies NGC 3079, NGC 1068, NGC 4945, and Circinus each host
an H20 megamaser (apparent isotropic luminosity > 10 L).
A position-velocity diagram of the brightest H2O masers in the central
parsec of NGC 1068 (Gallimore et al. 1996) shows a similar, but much less
extreme phenomenon than NGC 4258. Other megamaser galaxies are currently
investigated. Weaker H20 masers are also known in NGC 253, M82 and a
few other galaxies (see Greenhill et al. 1990, and references therein).