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Session 25 - Seyfert Galaxies.
Oral session, Wednesday, January 07
Georgetown,
The water masers in NGC 4258 trace a thin, nearly edge-on disk in Keplerian rotation around a 3.5\times10^7\,M_ødot central mass. The disk rotation of about 1000 km\,s^-1 is expected to cause the masers along the near edge of the disk (the systemic masers) to drift in both velocity and position by amounts that depend on the distance to the galaxy. Single-dish monitoring programs have indeed detected accelerations in these features of \sim9 km\,s^-1\,yr^-1, and these are consistent with the a priori distance to NGC 4258 of about 7 Mpc. Here, we report the detection of \sim30\,\muas\,yr^-1 proper motions amongst 14 of the systemic masers in a sequence of five VLBA epochs spaced over three years. These motions, together with accelerations also detected in the VLBA data, yield a geometric, distance-ladder-independent distance to NGC 4258 of 7.3\pm0.3 Mpc. This distance is discussed in the context of other primary and secondary distance indicators. We note that distances derived from the accelerations and proper motions independently agree to better than 4%. This, together with the lack of detectable proper motions and accelerations for the masers along the midline of the disk (and therefore in the plane of the sky), represents powerful confirmation that we are indeed tracking the 1000 km\,s^-1 rotation of a 0.1 pc disk.
