The internal velocity dispersions of three young star clusters in the Large Magellanic Cloud

Abstract

We have measured the radial velocities of 11 to 37 stars in each of three rich young star clusters in the Large Magellanic Cloud (LMC): NGC 1866, NGC 2164, and NGC 2214. The observational errors (a few km s^-1^) are comparable to the observed velocity dispersions and the samples are probably contaminated by a few field stars. Under these circumstances, we can only set upper limits on the true velocity dispersions within the clusters. The methods presented here should be useful in a variety of contexts where velocity dispersions are difficult to measure. Our limits for all three LMC clusters are σ_v_ <~ 3 to 4 km s^-1^ at the 95% confidence level. Combining these results with the surface brightness profiles derived in an earlier study, we find upper limits on the total masses of 6 x 10^5^, 2 x 10^5^, and 4 x 10^5^ M_sun_, respectively, for NGC 1866, NGC 2164, and NGC 2214. The corresponding limits on the global mass-to-light ratios are 0.5, 0.7, and 1.3 in solar units. From the small velocity dispersion and large radial extent of NGC 1866, we infer that the cluster is not yet tidally limited by the LMC. This confirms a previous suggestion by Elson, Fall, and Freeman, which was based on mass-to-light ratios derived from stellar population models. The other clusters in our sample, NGC 2164 and NGC 2214, may also have unbound halos, but our limits on the velocity dispersions are not tight enough to draw definite conclusions.