From young massive star cluster to old globular: the L_v-σ0 relationship as a diagnostic tool

We present a new analysis of the properties of the young massive star clusters (YMCs) forming profusely in intense starburst environments, which demonstrates that these objects are plausible progenitors of the old globular clusters (GCs) seen abundantly in the Local Group. The method is based on the tight relationship for old GCs between their V-band luminosities, L_V, and (central) velocity dispersions, σ0. We improve the significance of the relationship by increasing the GC sample size and find that its functional form, L_V/L_⊙∝σ₀ ^1.57±0.10 (km s^-1), is fully consistent with previous determinations for smaller Galactic and M31 GC samples. The tightness of the relationship for a GC sample drawn from environments as diverse as those found in the Local Group implies that its origin must be sought in intrinsic properties of the GC formation process itself. We evolve the luminosities of those YMCs in the local Universe which have velocity dispersion measurements to an age of 12 Gyr, adopting a variety of initial mass function (IMF) descriptions, and find that most YMCs will evolve to loci close to, or to slightly fainter luminosities than the improved GC relationship. In the absence of significant external disturbances, this implies that these objects may potentially survive to become old GC-type objects over a Hubble time. The main advantage of our new method is its simplicity. Whereas alternative methods, based on dynamical mass estimates, require one to obtain accurate size estimates and to make further assumptions, the only observables required here are the system's velocity dispersion and luminosity. The most important factor affecting the robustness of our conclusions is the adopted form of the IMF. We use the results of N-body simulations to confirm that dynamical evolution of the clusters does not significantly alter our conclusions about the likelihood of individual clusters surviving to late times. Finally, we find that our youngest observed clusters are consistent with having evolved from a relation of the form L_V/L_⊙σ₀ ^2.1+0.5-0.4 (km s^-1). This relation may actually correspond to the origin of the GC fundamental plane.