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 Pα emission, one of which we identify as a Wolf-Rayet (WR) star, as well as an A-type supergiant. The line-of-sight extinction is very large, A V ∼ 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 Myr, and, assuming that all emission-line stars are WR stars, a cluster mass of ≳104 M⊙. A kinematic analysis of the cluster's 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 ∼106 M⊙ starburst episode.