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J. M. Dickey (University of Minnesota), F. J. Lockman (NRAO - Green Bank), N. M. McClure-Griffiths (Australia Telescope National Facility)
We are using the NRAO Green Bank Telescope to study the outer edge of the Milky Way neutral hydrogen disk. In this poster we consider various possibilities for the rotation curve of the outer disk, combined with various functional forms for the radial dependence of the HI surface density, to model 21-cm profiles for comparison with observations. The modelling indicates what longitudinal dependence to expect, and to some extent where to look for the most significant data. It is particularly useful to study directions where the Galactic warp does not cause the midplane to depart significantly from latitude zero in the outer galaxy.
Our preliminary results suggest that it is difficult for the surface density of the Galactic HI to have an exponential scale length longer than about one times the solar circle radius. If the surface density is constant with radius and then cuts off to zero, the cutoff radius cannot be more than four times the solar circle radius. Both of these numbers are upper limits. If the rotation curve begins to drop in the outer galaxy then these numbers must be smaller. Including the random velocity distribution of HI will also force these upper limits down.
The Green Bank Telescope makes it possible to take Galactic 21-cm spectra that are reliable and repeatable at levels of 10 to 100 mK. This capability allows us to refine the parameters of the outer Galaxy through comparison with models like these.
If you would like more information about this abstract, please follow the link to http://www.astro.umn.edu/~john/GBTresults.html. This link was provided by the author. When you follow it, you will leave the Web site for this meeting; to return, you should use the Back comand on your browser.
The author(s) of this abstract have provided an email address for comments about the abstract: john@astro.umn.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.