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The structure of the accretion disk in NGC 4258

Published online by Cambridge University Press:  01 March 2007

James M. Moran ,
Elizabeth Humphreys ,
Lincoln Greenhill ,
Mark Reid  and
Alice Argon
James M. Moran
Affiliation:
Harvard-Smithsonian Center for Astrophysics† email: [email protected]
Elizabeth Humphreys
Affiliation:
Harvard-Smithsonian Center for Astrophysics† email: [email protected]
Lincoln Greenhill
Affiliation:
Harvard-Smithsonian Center for Astrophysics† email: [email protected]
Mark Reid
Affiliation:
Harvard-Smithsonian Center for Astrophysics† email: [email protected]
Alice Argon
Affiliation:
Harvard-Smithsonian Center for Astrophysics† email: [email protected]
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Abstract

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A wealth of new information about the structure of the maser disk in NGC 4258 has been obtained from a series of 18 VLBA observations spanning three years, as well as from 32 additional epochs of spectral monitoring data from 1994 to the present, acquired with the VLA, Effelsberg, and GBT. The warp of the disk has been defined precisely. The thickness of the maser disk has been measured to be 12 micro-arcseconds (FWHM), which is slightly smaller than previously quoted upper limits. Under the assumption that the masers trace the true vertical distribution of material in the disk, from the condition of hydrostatic equilibrium the sound speed is 1.5 km s−1, corresponding to a thermal temperature of 600K. The accelerations of the high velocity maser components have been accurately measured for many features on both the blue and red side of the spectrum. The azimuthal offsets of these masers from the midline (the line through the disk in the plane of the sky) and derived projected offsets from the midline based on the warp model correspond well with the measured offsets. This result suggests that the masers are well described as discrete clumps of masing gas, which accurately trace the Keplerian motion of the disk. However, we have continued to search for evidence of apparent motions caused by “phase effects.” This work provides the foundation for refining the estimate of the distance to NGC 4258 through measurements of feature acceleration and proper motion. The refined estimate of this distance is expected to be announced in the near future.

Keywords

NGC 4258galaxies: activeaccretion disksmasersgalaxies: distances and redshiftsblack hole physics
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 3 , Symposium S242: Astrophysical Masers and their Environments , March 2007 , pp. 391 - 398
Copyright
Copyright © International Astronomical Union 2008

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