While the HSTHST imaging studies discussed up to now have opened a new vista on the importance of young star clusters, we still do not know how common the cluster phenomenon truly is. Are SSCs only found in extreme environments such as mergers and very luminous starbursts, or do they also occur in more quiescent settings? The examples given in Section 1 suggest a more widespread occurance, but it is difficult to be quantitative without access to a proper control sample.
Fortunately, there are a number of snapshot imaging surveys of nearby galaxies being conducted with HST, and some of these may be useful for addressing the statistical properties of SSCs. Here, I will briefly mention a recently completed UV imaging survey with HST (see Maoz et al. 1996b for details). In brief, UV (F220W filter; effective wavelength ~ 2270 Å) images of the central 22" x 22" were obtained with the Faint Object Camera (FOC) for 110 nearby galaxies. The images were taken prior to the HST refurbishment mission, and hence suffer from the effects of spherical aberration. The final pixel scale is 0".0225, and the core of the point-spread function has a full width at half maximum (FWHM) of ~ 0".05. The targets were selected randomly from a complete sample of 240 large (D > 6') and nearby (cz < 2000 km s-1) galaxies as listed in the UGC and ESO catalogs, and hence constitute a well-defined, unbiased group suitable for a variety of statistical studies. The UV passband is especially useful for studying regions of recent star formation.
Even a cursory glance at the images in the Maoz et al. (1996b) catalog reveals that the UV emission exhibits a diverse assortment of morphologies. Of relevance in the present context are those images showing compact, pointlike sources.
Figure 1 illustrates two examples. NGC 3077 is a nearby amorphous galaxy interacting with M81 and M82 (Barbieri et al. 1974; Yun, Ho, & Lo 1995). From ground-based images, it is known that its central region contains a single bright knot (Price & Gullixson 1989), which is the dominant feature in the FOC image. The central UV morphology strongly resembles that of NGC 1705 and NGC 5253 (Meurer et al. 1995). Assuming a distance of 3.6 Mpc to M81 (Freedman et al. 1994), the central cluster has L2200 = 1.0 x 1036 ergs s-1 Å-1, or M2200 = -10.9 mag (Meurer et al. 1995). According to Meurer et al.'s definition, this cluster would not qualify as an SSC; but like other SSCs, its emission is very compact - the half-light radius measures 0.4 pc. A number of fainter clusters are also visible in the frame, the faintest of which may be individual O and B stars.
Figure 1. Sample F220W images from the FOC snapshot survey (Maoz et al. 1996b).
The morphology of NGC 7462, on the other hand is much more complicated. Many compact clusters dot the extent of the large swath of diffuse UV emission running parallel to the major axis of the disk. Assuming a distance of 13 Mpc (Tully 1988), the clusters have typical sizes of Rh 1.5-6 pc and luminosities of L2200 1 x 1036 ergs s-1 Å-1.
Although the full analysis of the data base is yet to be completed (Ho et al. 1996), clusters have been identified in approximately 40% of the sample. This fraction is merely a lower limit, since presumably some of the images with no UV emission suffer from high extinction. The vast majority of the galaxies in the survey, strictly speaking, are not considered starbursts. The central regions of a substantial fraction of nearby galaxies undergo some level of current star formation (Ho 1995; Ho, Filippenko, & Sargent 1996b), albeit with quite modest star-formation rates. Moreover, the sample objects generally show no outstanding morphological peculiarities indicative of recent interactions. An immediate conclusion that can be drawn is that cluster formation is very commonplace and does not require very extreme physical conditions.