Other
Scientific paper
Aug 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........10h&link_type=abstract
Thesis (PHD). HARVARD UNIVERSITY , Source DAI-B 58/02, p. 749, Aug 1997, 212 pages.
Other
22
Vlbi, Compact Sources, Black Holes
Scientific paper
The results of a multi-epoch program of VLBI observations of the sub-parsec maser disk in NGC 4258 are presented. Chapter 1 provides background on NGC 4258 and megamasers in general, and on the megamaser in NGC 4258 in particular. Chapter 2 considers the technical challenges of performing broad bandwidth VLBI on compact sources, and reports a new position for NGC 4258 based on a fringe rate analysis. In Chapter 3, we describe a global disk-fitting algorithm and demonstrate that the positions and velocities of the high-velocity features, together with their lack of LOS accelerations, effectively rule out thin, flat disk solutions. We find a statistical scatter of about 1.7 kms-1 (0.2%) in the velocity of the high-velocity features about the Keplerian rotation curve. The thickness of the maser layer is unresolved and less than 5 μas (H/R< 0.0013). In Chapter 4 we propose that the warp in the disk, together with its orientation, can explain the persistent, relative faintness of the blue-shifted maser emission. Chapter 5 discusses the VLBI detection of 22 GHz continuum emission. The results provide compelling support for standard models associating compact radio emission with relativistic nuclear jets. The continuum emission is also discussed in the context of maser amplification models. In Chapter 6 we report a non-detection of both circular and linear polarization in the high-velocity masers, and place upper limits of 220 and 100 mG on B| and B⊥, respectively. Chapter 7 describes efforts to measure a geometric distance to NGC 4258 from accelerations and proper motions of individual maser features. Using a Bayesian, discrete cross correlation algorithm, we derive a distance of 7.3 ± 0.4 Mpc, consistent with all available Cepheid-based distances. The proper motion and acceleration distances are consistent with one another, suggesting that we have indeed detected the space motions of maser features entrained in the rotating molecular disk. Concluding remarks and future prospects are presented in Chapter 8.
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