A newly discovered young massive star cluster at the far end of the Galactic Bar

We present a near-infrared study of the candidate star cluster Mercer 81, located at the centre of the G338.4+0.1 H II region and close to the TeV gamma-ray source HESS 1640-465. Using Hubble Space Telescope/NICMOS imaging and VLT/ISAAC spectroscopy, we have detected a compact and highly reddened cluster of stars, although the bright stars in the centre of the field are in fact foreground objects. The cluster contains nine stars with strong P alpha emission, one of which we identify as a WolfRayet (WR) star, as well as an A-type supergiant. The line-of-sight extinction is very large, A(V) similar to 45, illustrating the challenges of locating young star clusters in the Galactic plane. From a quantitative analysis of the WR star, we argue for a cluster age of 3.7(-0.5)(+0.4) Myr, and, assuming that all emission-line stars are WR stars, a cluster mass of greater than or similar to 10(4) M-circle dot. A kinematic analysis of the clusters surrounding H II region shows that the cluster is located in the Galactic disc at a distance of 11 +/- 2 kpc. This places the cluster close to where the far end of the Bar intersects the Norma spiral arm. This cluster, as well as the nearby cluster [DBS2003]179, represents the first detections of active star cluster formation at this side of the Bar, in contrast to the near side which is well known to have recently undergone a similar to 10(6) M-circle dot starburst episode.