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The peculiar galaxy NGC 4258: A black hole candidate.

NGC 4258 is a nearby (D = 6.4 Mpc) active spiral galaxy with high luminosity H₂O maser emission. VLBI observations by Greenhill et al. (1995) and Miyoshi et al. (1995) find the masers residing in a very fast-rotating ( $v_{\rm sys} \pm$900 kms^-1), nearly edge-on toroid/disk of radius 0.2 pc. This provides the most compelling evidence so far for a black hole in the centre of a galaxy. -- The masers are probably formed in relatively dense gas clouds that are forming new stars.

HI observations of NGC 4258 (van Albada 1978, 1980) reveal a rather flat rotation curve ( $v_{\rm rot} \sim$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$\alpha$ 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$_{\odot}$). 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).