Astronomy and Astrophysics – Astrophysics
Scientific paper
2007-09-21
Astronomy and Astrophysics
Astrophysics
13 pages, 7 figures
Scientific paper
10.1111/j.1365-2966.2007.12497.x
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(O/H) = 8.29 +/- 0.09. According to continuum subtracted 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 +/- 1 Myr for cluster 1, from which a photometric mass of (1.1 +/- 0.3) x 10^6 Msun 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 sigma ~ 23 +/- 1 km/s for cluster 1 and sigma ~ 26 +/- 1 km/s for cluster 6. Combining sigma 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) x 10^6 Msun and (9.1 +/- 0.8) x 10^6 Msun 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.
Crowther Paul A.
de Grijs Richard
Mengel Sabine
Moll Sarah L.
Smith Lleweilun J.
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