Previous abstract Next abstract

Session 109 - Seyferts.
Oral session, Thursday, January 16
Piers 4/5,

[109.01] Recent Results from the Masers in NGC4258: Detection of Nuclear Continuum Emission, and Progress in Deriving a Geometric Distance

J. Herrnstein, L. Greenhill, J. Moran (Harvard-Smithsonian CfA), P. Diamond (NRAO), M. Miyoshi (Mizusawa Astrogeodynamics Observatory), N. Nakai, M. Inoue (Nobeyama Radio Observatory)

VLBI observations of the water masers in the nucleus of the weakly active Seyfert 2 galaxy NGC4258 reveal a nearly edge-on disk in almost perfect Keplerian rotation. The disk dynamics provide compelling evidence that the central mass is a massive blackhole. The disk is extremely thin and appears to be slightly warped. This contribution presents two recent results: the detection of 1.3 cm continuum emission in the vicinity of the dynamical center of the maser disk, and progress in determining a geometric distance to NGC4258.

22 GHz continuum emission has been detected in 3 of 4 VLBA+phased VLA observations. These data, together with a program of VLA snapshots, indicate that the continuum emission is approximately 100% variable on timescales of weeks, and has an average flux density of 3 mJy. Most of the emmision is located 0.5--1.0 mas (0.015--0.03 pc) north of the dynamical center of the disk, along the projected disk rotation axis. This axis is itself well-aligned with large scale jets extending to kpc scales. The northern emission is unresolved and has a brightness temperature of >10^7 K. We conclude that we have detected synchrotron emission from the base of the northern jet. The offset from the dynamical center is consistent with that predicted for a mildy relativistic jet. The southern continuum emission is also along the rotation axis, but is consistently >3 times fainter. This may be due to thermal absorption in the disk.

In principle, the masers can provide a purely geometrical distance to the NGC4258 which is independent of all other distance indicators. The distance is determined by observing accelerations and proper motions of the systemic features as they rotate with the disk. We estimate that distances accurate to 5% are achievable given current uncertainties in the disk fitting process. Proper motions and accelerations have been detected from the VLBI data, and they are consistent with the expected motions in the disk. Our progress in deriving a geometrical distance from these data will be discussed at the meeting.

Program listing for Thursday