Dynamical mass estimates for two luminous star clusters in galactic merger remnants

We present high-dispersion spectra of two extremely massive star clusters in galactic merger remnants, obtained using the UVES spectrograph mounted on the ESO Very Large Telescope. One cluster, W30, is located in the ∼500 Myr old merger remnant NGC 7252 and has a velocity dispersion and effective radius of σ = 27.5 ± 2.5 km s⁻¹ and R_eff = 9.3 ± 1.7 pc, respectively. The other cluster, G114, located in the ∼3 Gyr old merger remnant NGC 1316, is much more compact, R_eff = 4.08 ± 0.55 pc, and has a velocity dispersion of σ = 42.1 ± 2.8 km s⁻¹. These measurements allow an estimate of the virial mass of the two clusters, yielding M_dyn(W30) = 1.59(±0.26) × 10₇ M_⊙ and M_dyn(G114) = 1.64(±0.13) × 10⁷ M_⊙. Both clusters are extremely massive, being more than three times heavier than the most massive globular clusters in the Galaxy. For both clusters we measure light-to-mass ratios, which when compared to simple stellar population (SSP) models of the appropriate age, are consistent with a Kroupa-type stellar mass function. Using measurements from the literature we find a strong age dependence on how well SSP models (with underlying Kroupa or Salpeter-type stellar mass functions) fit the light-to-mass ratio of clusters. Based on this result we suggest that the large scatter in the light-to-mass ratio of the youngest clusters is not due to variations in the underlying stellar mass function, but instead to the rapidly changing internal dynamics of young clusters. Based on sampling statistics we argue that while W30 and G114 are extremely massive, they are consistent with being the most massive clusters formed in a continuous power-law cluster mass distribution. Finally, based on the positions of old globular clusters, young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) and dwarf-globular transition objects (DGTOs) in κ-space we conclude that 1) UCDs and DGTOs are consistent with the high mass end of star clusters and 2) YMCs occupy a much larger parameter space than old globular clusters, consistent with the idea of preferential disruption of star clusters.