Nearby galaxies which have an unusually large amount of neutral hydrogen gas can be studied by radio astronomical techniques. The objectives of the observations will be to define the gaseous structures of large, gas-rich galaxies and to see if there are dynamical processes at work now that might have been important in the past. A collaborative program that uses the new receiving systems at the Westerbork (The Netherlands) Synthesis Radio Telescope (WSRT) will be initiated to study the highly redshifted 21 cm wavelength absorption line of hydrogen gas. The instrument is uniquely suited to this type of experiment. The optical survey work on the galaxies which is already in progress will provide candidates for study at radio wavelengths. It is important to emphasize that radio astronomy techniques complement the optical data by providing: (1) Very Long Baseline Interferometric (VLBI) measurements of the spatial extent of the absorbing gas, (2) the neutral hydrogen gas (HI) temperature estimates, and (3) details of the kinematics in the high column density clouds at the heart of the absorption system. This program will be a collaborative effort with an astronomer from the University of Pittsburgh and Netherland astronomers. VLBI spectral line experiments will be carried out to compete with the WSRT. The galaxy system in PKS 0458-02 (redshift z = 2.04) is well-suited to more detailed study. There is radio structure in the background source on a wide range of scales, and the absorption itself is very strong. The precision of the spectral line VLBI observations of this system will be improved and intermediate-length baseline experiments will be attempted, which should detect the structure in the absorber on typical galactic scales. A four-station VLBI experiment including WSRT, Arecibo, Green Bank, and Jodrell Bank would be very useful since the WSRT-Jodrell Bank baseline would be short enough to define the absorption in front of the large (about 2 arcsecond) structure. The intercontinental baselines would detect the fine structure in the absorber against the more compact components. Both Arecibo and Green Bank are already equipped with receivers. Jodrell Bank is planning to build a low noise preamplifier in order to include the 250-foot telescope in the experiment. Since receiving systems have improved by a factor of about 4 in sensitivity since the previous experiment was performed, the VLBI observations of the lower redshift systems (z = 0.69) in the spectrum of 3C286 will be repeated. The inclusion of WSRT and Jodrell Bank in this experiment would be of great interest since the background radio source in 3C286, like PKS 0458-020, has structure on a wide range of scales. The results of the project should improve our understanding of the structure and evolution of galaxies.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Millissa J. Evans
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University of Pittsburgh
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