2013ApJ...769L..17T

The recently detected TeV γ-ray source HESS J1923+141 coincides with supernova remnant (SNR) W51C and the star-forming region W51B of the W51 complex. We construct H I absorption spectra of SNR W51C, H II regions G49.2-0.35 and G49.1-0.38 in W51B, and a nearby compact extragalactic source. Our study detects high-velocity (HV) H I clouds (above 83 km/s) that coincide with W51B, but finds that the clouds are behind W51B. Both W51C and G49.2-0.35 have similar highest velocity absorption features at ∼70 km/s. The H II region G49.1-0.38 is behind the SNR because its H I absorption spectrum has a feature at 83 km/s. These new results argue against previous claims that the SNR has shocked the HV H I clouds. Therefore, the TeV emission from the complex should not be associated with the HV H I 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.