Abstract
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.
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