Cluster and nebular properties of the central star - forming region of NGC 1140 ⋆

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 + logO/H = 8.29 ± 0.09. According to continuum subtracted Hα 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 ± 1 Myr for cluster 1, from which a photometric mass of (1.1 ± 0.3)× 10 M⊙ is obtained. For this age and photometric mass, the modelling predicts the presence of ∼ 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(WR)/N(O) ∼ 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 σ ∼ 23 ± 1 km s for cluster 1 and σ ∼ 26 ± 1 km s for cluster 6. Combining σ with half-light radii of 8 ± 2 pc and 6.0 ± 0.2 pc measured from the F625W ACS image implies virial masses of (10 ± 3) × 10 M⊙ and (9.1 ± 0.8) × 10 6 M⊙ 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.