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Title:
Searching for the Outer Boundary of the Milky Way Disk Using the Green Bank Telescope
Authors:
Lockman, F. J.; Dickey, J. M.; McClure-Griffiths, N. M.
Affiliation:
AA(NRAO - Green Bank), AB(University of Minnesota), AC(Australia Telescope National Facility)
Publication:
American Astronomical Society, 200th AAS Meeting, #33.08; Bulletin of the American Astronomical Society, Vol. 34, p.688
Publication Date:
05/2002
Origin:
AAS
Bibliographic Code:
2002AAS...200.3308L

Abstract

We have used the new 100 meter Green Bank Telescope of the NRAO to search for the outer boundary of the Milky Way disk in the 21-cm line of neutral hydrogen. We apply three observing techniques to the 21-cm emission in the second and third quadrants at low latitudes: on-the-fly raster mapping of small areas, long scans in latitude at fixed longitude, and deep integrations on groups of five beam areas. Spectra were also taken in a few directions where UV absorption line measurements exist toward extragalactic objects. In all cases we are studying the wings of the emission profile at very low levels, in the range 10 to 100 mK antenna temperature. The GBT's large, fully unblocked aperture brings several unique advantages to this study. Stray radiation correction is unnecessary at these levels, and receiver baselines are usually very stable. Our longer integrations (two to three hours) reach rms of 5 mK, and the spectra are repeatable at this level. The absence of a cusp or tail in the low latitude emission spectra beyond +/-200 km s-1 in the third (second) quadrant gives strong indication that the Milky Way HI disk ends somewhere between three and four times the solar circle radius. We see no evidence for small ``islands'' or high velocity clouds beyond this cutoff. Although we do not know the rotation curve at these large radii, its shape becomes just a parameter in our analysis, like the solar circle radius and the LSR orbital velocity. The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. This research has been supported in part by NSF grant AST 97-32695 to the University of Minnesota.
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