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   72093
   Molecular absorption line studies and X-ray absorption observations
   toward extragalactic sight lines indicate that molecular gas is as
   abundant as neutral atomic gas in the Solar vicinity. At larger
   Galactic radii, H2 exceeds HI in some locations, and if this is
   generally true, it implies a dramatic revision in the picture of
   neutral gas in disk galaxies. To determine if H2 dominates the ISM at
   large radii, we propose to measure the X-ray absorption column toward
   AGNs projected behind the outer part of the Galactic disk, which is
   expected to be about 1E22/cm2. The H2 column is the difference between
   the X-ray absorption column and the observed HI 21 cm column.
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