The discovery of a molecular cavity in the Norma near arm associated to H.E.S.S gamma-ray source located in the direction of Westerlund 1

TL;DR

This paper reports the discovery of a molecular cavity in the Norma near arm associated with gamma-ray emission, likely caused by a pulsar, revealing new insights into the region's structure and energetic processes.

Contribution

It identifies a previously unknown molecular cavity and links it to gamma-ray emission and a pulsar, providing new understanding of the region's dynamics.

Findings

A molecular cavity with a diameter of ~100 pc was discovered.

The cavity is expanding at 6-8 km/s with an energy of ~10^51 erg.

Gamma-ray emission coincides with the cavity's inner wall, not the dense gas around Westerlund 1.

Abstract

We report on the discovery of a molecular cavity in the Norma near arm in the general direction of Westerlund 1 (Wd1), but not associated with it. The cavity has a mean radial velocity of -91.5 kms^{-1}, which differs by as much as ~40 kms^{-1} from the mean radial velocity of the Wd1 stars. The cavity is surrounded by a fragmented molecular shell of an outer diameter of about 100 pc and 10^{6} M_odot, which is expanding at velocities of 6 to 8 kms^{-1}. The amount of kinetic energy involved in the expanding shell is ~10^{51} erg. Inside this cavity the atomic HI gas surface density is also the lowest. Structure of the extended Very High Energetic (VHE) gamma-ray emission, recently reported by the H.E.S.S. collaboration Ohm et al. 2009, coincides with the cavity. The observed morphology suggests that the inner wall of the molecular shell is the zone of the gamma-ray emission, and not the dense gas surrounding massive stars of Wd1 as had been speculated by the H.E.S.S. collaboration. A likely candidate responsible for creating the observed cavity and the gamma-ray emission is the pulsar PSR J1648-4611.