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K. L. O'Neil (NRAO)
The 21-cm neutral hydrogen line is possibly the most commonly observed spectral line by centimeter wavelength telescopes. Yet a brief perusal through any collection of HI observations on a gas-rich galaxy will often find flux measurements varying by 5% - 100%, or more. The reasons behind these discrepancies are many-fold, including differences in telescope beam sizes and shapes, random pointing offsets, baseline variations, rapid temperature fluctuations, etc. As a result, astronomers wishing to obtain accurate flux values for a given galaxy are often forced to average the various measurements found in published catalogs, invoke a selection effect on the previous measurements by assuming some measurements are more accurate than others, or simply re-observe the objects of interest with the hope that their data will be be more reliable, on average, than that found in the literature. While all of the above methods are sound and should be encouraged, they can also be unpractical, particularly when the researcher is interested in comparing their own results against a consistent, well-defined catalog with readily understood errors. This is not meant to imply that catalogs of HI measurements found in the literature are in general unreliable, simply that as the purpose of the majority of existing catalogs is to look for statistical information regarding the neutral hydrogen content in the studied galaxies, the errors associated with the measurements for an individual galaxy can be high.
With the upgrade of the Arecibo 305m telescope and the construction of the Robert C. Byrd Green Bank Telescope (GBT) we now have access to unprecedented stable baselines, increased sensitivity, and extremely good pointing at 21-cm. Taking advantage of this, we are constructing an extremely reliable catalog of extragalactic HI observations. Our project has already observed over 100 sources with the Arecibo telescope, determining extremely reliable line measurements. Follow-up observations with the GBT are underway and should be completed shortly. The resultant catalog will be extremely useful as a well defined reference for anyone interested in 21-cm spectral line work.
The results and data products from this survey will be made publicly available as soon as all observations are complete.
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The author(s) of this abstract have provided an email address for comments about the abstract: koneil@nrao.edu
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.