2.4. Sizes

Little is known about the detailed structure of the clusters. Even with the superior optics of the refurbished HST, the two SSCs in the relatively nearby galaxy NGC 1569 (d 2.5 Mpc) still remain unresolved (Leitherer, these proceedings). Given these limitations, it is impossible to characterize the size of the clusters by conventional parameters such as the core or tidal radius. Instead, the observationally most straightforward parameter is the half-light radius (R_h), sometimes also referred to as the effective radius. R_h also has the advantage of being relatively insensitive to evolutionary or environmental effects (van den Bergh, Morbey, & Pazder 1991). Unfortunately, even such a simple parameter is difficult to determine with accuracy. As discussed by Meurer et al. (1995), the measurement of cluster radii from HST images is complicated by severe crowding and background confusion in many cases, rendering the sizes highly uncertain for galaxies more distant than a few Mpc. Meurer et al. note a tendency to systematically overestimate the sizes of distant clusters located on high-surface brightness backgrounds. In view of these caveats, the half-light radii of well-resolved SSCs seem to fall comfortably within the range of Galactic globular clusters, whose median R_h 3 pc (van den Bergh et al. 1991). The apparent tendency for the R_h distribution of SSCs to be skewed toward somewhat larger sizes is probably not significant for the reasons mentioned above (Meurer et al. 1995). In particular, Meurer (1995) has shown that the large radii reported by Whitmore et al. (1993) for the clusters in the Antennae have probably been overestimated, thus obviating van den Bergh's (1995) other major objection to the young globular cluster hypothesis for the SSCs in this system.

The compact sizes of SSCs can be used as supporting evidence that the clusters are most likely gravitationally bound. Any conceivable form of perturbation that may lead to the dispersal of the stars will traverse the clusters on a timescale shorter than 1 Myr, whereas the estimated ages are in some cases up to two orders of magnitude larger.