Testing extragalactic H₂O masers against the thin disk model: the present and the future

Abstract

Water maser emission is detected towards the nuclei of 22 galaxies, all of them showing some sign of nuclear activity. Except for a couple of cases, maser features have been found only within 150 km s⁻¹ of the systemic galaxy recession velocity but are often offset from systemic within that window. Spectral features in maser-detected galaxies are monitored regularly and these observations show that velocity drifts like those seen in the systemic masers of NGC 4258 are rare. Hence the conclusion that either masers in other galaxies are generally located farther from the central black hole than in NGC 4258, that the black holes in those galaxies are less massive than the one in NGC 4258, or that the masers' acceleration is in the plane of the sky (as in the case of maser gas at the projected extremities of an edge-on disk). Assuming the disk model holds, the last of these options leads to the possibility that, despite the relative faintness of high velocity maser features in the case of NGC 4258, the brightest maser emission originates from the projected edges of the disk, in general. A selection effect would have caused a bias, then, in previous surveys whereby masers in larger, more slowly rotating disks are favorably detected. New wide-bandwidth capabilities at the 100-m Effelsberg telescope and the emergence of the GBT will help to overcome any such bias and provide new examples of maser sources.