Star Cluster Systems in Interacting and Starburst Galaxies: A Multicolour Approach

The large majority of extragalactic star cluster studies done to date have essentially used two or three-passband aperture photometry, combined with theoretical stellar population synthesis models, to obtain age estimates. The accuracy to which this can be done depends on the number of observations through different (broad-band) filters available as well as, crucially, on the actual wavelength range covered. I show, based on the examples of the nearby starburst galaxies NGC 3310 and M82, that the addition of, in particular, near-infrared passbands to a set of optical filters greatly enhances our ability to disentangle age, metallicity and extinction parameters for star clusters with ages younger than a few Gyr. In addition, for the intermediate-age star cluster system in the fossil starburst region of M82 we find (i) a well-defined burst of cluster formation at slightly older ages than derived from previous estimates based on optical fluxes alone, and (ii) that by considering only the clusters originating in this burst of cluster formation, we uncover the first conclusive observational evidence for an unambiguous turn-over in the luminosity function of a coeval star cluster system with an age as young as ∼ 1 Gyr. 1 The cluster luminosity function as diagnostic The distribution of cluster brightnesses, known as the Cluster Luminosity Function (CLF), is one of the most important diagnostics for the study of extragalactic compact star cluster populations. For the old globular cluster (GC) systems the CLF shape is well-established: it is roughly Gaussian, with the peak or turn-over magnitude at M V ≃ −7.4 and a FWHM of ∼ 3 mag (Harris 1991, Whitmore et al. 1995, Harris et al. 1998). The well-studied young star cluster (YSC) population in the Large Magellanic Cloud, on the other hand, displays a power-law CLF (Elson & Fall 1985, Elmegreen & Efremov 1997). Hubble Space Telescope (HST) observations have provided CLFs for young compact cluster systems in more distant galaxies and are continuing to do so. Although incompleteness effects often preclude detection of a turn-over in the CLF (e.g., Whitmore & Schweizer 1995, Schweizer et al. 1996, Miller et al. 1997), in galaxies with YSC systems for which deep observations are available there is little evidence for an intrinsic turn-over (but see de Grijs, Bastian & Lamers 2002b). In most of these cases, the CLF shapes are consistent with power laws down to the completeness threshold (but see Miller et al. 1997, de Grijs et al. 2001, 2002b). GC formation models suggest that the distribution of the initial cluster masses (and, therefore, of the initial cluster luminosities) is closely approxi-