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 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 P \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 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 10 ^ % { 4 } M _ { \normalsize \odot } .
A kinematic analysis of the cluster ’ s surrounding H ii -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 } M _ { \normalsize \odot } starburst episode .