A newly-discovered young massive star cluster at the 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 HII region, and close to the TeV
gamma-ray source HESS 1640-465. Using HST/NICMOS imaging and VLT/ISAAC
spectroscopy we have detected a compact and highly extincted cluster of stars,
though the bright stars in the centre of the field are in fact foreground
objects. The cluster contains nine stars with strong Paschen-alpha 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, $A_{V}\sim 45$,
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$^{+0.4}_{-0.5}$\,Myr, and, assuming that all emission-line stars are
WRs, a cluster mass of $\ga 10^4$\msun. A kinematic analysis of the cluster's
surrounding HII-region shows that the cluster is located in the Galactic disk
at a distance of 11$\pm$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, represent 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 $\sim 10^6$\msun\ starburst episode.
