High velocity HI is not associated with TeV supernova remnant W51C

TL;DR

This study uses HI absorption spectra to investigate the association between high-velocity HI clouds and the supernova remnant W51C, concluding that the HV HI clouds are behind W51B and not related to the TeV gamma-ray emission.

Contribution

The paper provides new HI absorption measurements that clarify the spatial relationship between high-velocity HI clouds and W51C, challenging previous claims of their association.

Findings

HV HI clouds are behind W51B and not associated with W51C

W51C's distance is about 4.3 kpc, within the Sagittarius arm

TeV emission is not linked to HV HI clouds

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.