We present new high spatial resolution HST /ACS imaging of NGC 1140 and high spectral resolution VLT/UVES spectroscopy of its central star-forming region .
The central region contains several clusters , the two brightest of which are clusters 1 and 6 from Hunter , O ’ Connell & Gallagher , located within star-forming knots A and B , respectively .
Nebular analysis indicates that the knots have an LMC-like metallicity of 12 ~ { } + ~ { } \log { \mathrm { O / H } } = 8.29 \pm 0.09 .
According to continuum subtracted \mathrm { H } \alpha ACS imaging , cluster 1 dominates the nebular emission of the brighter knot A. Conversely , negligible nebular emission in knot B originates from cluster 6 .
Evolutionary synthesis modelling implies an age of 5 \pm 1 Myr for cluster 1 , from which a photometric mass of ( 1.1 \pm 0.3 ) \times 10 ^ { 6 } \mathrm { M } _ { \odot } is obtained .
For this age and photometric mass , the modelling predicts the presence of \sim 5900 late O stars within cluster 1 .
Wolf-Rayet features are observed in knot A , suggesting 550 late-type WN and 200 early-type WC stars .
Therefore , N ( \mathrm { WR } ) / N ( \mathrm { O } ) \sim 0.1 , assuming that all the WR stars are located within cluster 1 .
The velocity dispersions of the clusters were measured from constituent red supergiants as \sigma \sim 23 \pm 1 \mathrm { km s } ^ { -1 } for cluster 1 and \sigma \sim 26 \pm 1 \mathrm { km s } ^ { -1 } for cluster 6 .
Combining \sigma with half-light radii of 8 \pm 2 pc and 6.0 \pm 0.2 pc measured from the F625W ACS image implies virial masses of ( 10 \pm 3 ) \times 10 ^ { 6 } \mathrm { M } _ { \odot } and ( 9.1 \pm 0.8 ) \times 10 ^ { 6 } \mathrm { M } _ { \odot } for clusters 1 and 6 , respectively .
The most likely reason for the difference between the dynamical and photometric masses of cluster 1 is that the velocity dispersion of knot A is not due solely to cluster 1 , as assumed , but has an additional component associated with cluster 2 .