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

We present new high spatial resolution Hubble Space Telescope/Advanced Camera for Surveys (ACS) imaging of NGC 1140 and high spectral resolution Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph 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. A nebular analysis indicates that the knots have a Large Magellanic Cloud-like metallicity of 12 + log O/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 ⁶ M_⊙ is obtained. For this age and photometric mass, the modelling predicts the presence of ∼5900 late O stars within cluster 1. Wolf-Rayet (WR) features are observed in knot A, suggesting 550 late-type nitrogen-rich (WNL) and 200 early-type carbon-rich (WCE) 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 ^-1 for cluster 1 and σ ∼ 26 ± 1 km s^-1 for cluster 6. Combining σ with half-light radii of 8 ± 2 and 6.0 ± 0.2 pc measured from the F625W ACS image implies virial masses of (10 ± 3) × 10⁶ and (9.1 ± 0.8) × 10⁶M_⊙ 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.