High velocity HI is not associated with TeV supernova remnant W51C
  The recently-detected TeV gamma-ray source HESS J1923+141 coincides with
Supernova Remnant (SNR) W51C and the star forming region W51B of the W51
complex. We construct HI absorption spectra to SNR W51C, HII regions G49.2-0.35
and G49.1-0.38 in W51B, and a nearby compact extragalactic source. Our study
detects high-velocity (HV) HI clouds (above 83 km/s) which coincide with W51B,
but finds that the clouds are behind W51B. Both W51C and G49.2-0.35 have have
similar highest-velocity absorption features at ~70 km/s. The HII region
G49.1-0.38 is behind the SNR because its HI absorption spectrum has a feature
at 83 km/s. These new results argue against previous claims that the SNR has
shocked the HV HI clouds. Therefore the TeV emission from the complex should
not be associated with the HV HI clouds. W51C has a distance of about 4.3 kpc,
smaller than the tangent point distance of 5.5 kpc in that direction, but still
in the Sagittarius spiral arm.
