The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell Publishing DOI : 10.1111/j.1365-2966.2004.07197.xWe discuss the systematic uncertainties inherent to analyses of observed (broad-band) Spectral Energy Distributions (SEDs) of star clusters with evolutionary synthesis models. We investigate the effects caused by restricting oneself to a limited number of available passbands, choices of various passband combinations, finite observational errors, non-continuous model input parameter values, and restrictions in parameter space allowed during analysis. Starting from a complete set of UBVRIJH passbands (respectively their Hubble Space Telescope/WFPC2 equivalents) we investigate to which extent clusters with different combinations of age, metallicity, internal extinction and mass can or cannot be disentangled in the various evolutionary stages throughout their lifetimes and what are the most useful passbands required to resolve the ambi- guities. We find the U and B bands to be of the highest significance, while the V band and near-infrared data provide additional constraints. A code is presented that makes use of luminosities of a star cluster system in all of the possibly available passbands, and tries to find ranges of allowed age-metallicity-extinction-mass combinations for individual members of star cluster systems. Numerous tests and examples are pre- sented. We show the importance of good photometric accuracies and of determining the cluster parameters independently without any prior assumptions

Abstract. We present low-resolution spectroscopy of the ionized gas in a sample of optical knots located along the tidal features of 14 interacting galaxies previously selected as candidate Tidal Dwarf Galaxies (TDGs). From redshift measurements, we are able to confirm their physical association with the interacting system in almost all cases. For most knots, the oxygen abundance does not depend on the blue luminosity. The average, 12 + log(O/H) = 8.34 ± 0.20, is typical of TDGs and is comparable to that measured in the outer stellar disk of spirals from which they were formed. A few knots showing low metallicities are probably pre-existing low-mass companions. The estimated Hα luminosity of the TDG candidates is higher than that of typical individual H  regions in spiral disks and is comparable to the global Hα luminosity of dwarf galaxies. We find several instances of velocity gradients with amplitudes apparently larger than 100 km s −1 in the ionized gas in the tidal knots and discuss various possible origins for the large velocity amplitudes. While we can exclude tidal streaming motions and outflows, we cannot rule out projection effects with the current resolution. The velocity gradients could be indicative of the internal kinematic characteristic of self-gravitating objects. Higher resolution spectra are required to confirm whether the tidal knots in our sample have already acquired their dynamical independence and are therefore genuine Tidal Dwarf Galaxies.

We analyse multiwavelength Hubble Space Telescope (HST) observations of a large number of star clusters in the nearby (post‐)starburst dwarf galaxy NGC 1569. Their spectral energy distributions (SEDs) cover at least the wavelength range from U to I in equivalent HST filters, in most cases supplemented by near‐infrared data. Using our most up‐to‐date evolutionary synthesis models of the Göttingen galev code we determine ages, metallicities, extinction values and masses for each individual cluster robustly and independently. We confirm the youth of most of these objects. The majority were formed in a very intense starburst starting around 25 Myr ago. While there are two prominent ‘super star clusters’ present in this galaxy, with masses of (5–16) ×105 M⊙, almost all remaining clusters are significantly less massive than an average Milky Way globular cluster, and are generally consistent with open cluster‐type objects. We determine the cluster mass function from individual cluster masses derived by scaling the model SEDs of known mass to the observed cluster SEDs for each individual cluster. We find signs of a change in the cluster mass function as the burst proceeds, which we attribute to the special conditions of star cluster formation in this starburst dwarf galaxy environment.

The large majority of extragalactic star cluster studies performed to date have essentially used two-or three-passband aperture photometry, combined with theoretical stellar population synthesis models, to obtain age, mass and extinction estimates, and sometimes also metallicities. The accuracy to which this can be performed depends on the choice of (broad-band) passband combination and, crucially, also on the actual wavelengths and the wavelength range covered by the observations. Understanding the inherent systematic uncertainties (the main aim of this paper) is of the utmost importance for a well-balanced interpretation of the properties of extragalactic star cluster systems.We simultaneously obtain ages, metallicities and extinction values for ∼300 clusters in the nearby starburst galaxy NGC 3310, based on archival Hubble Space Telescope observations from the ultraviolet (UV) to the near-infrared (NIR). We show that for ages 6 log(age yr −1 ) 9, and if one can only obtain partial coverage of the spectral energy distribution (SED), an optical passband combination of at least four filters including both blue and red passbands results in the most representative age distribution, as compared with the better constrained ages obtained from the full UV-NIR SED coverage. We find that while blueselected passband combinations lead to age distributions that are slightly biased towards lower ages due to the well-known age-metallicity degeneracy, red-dominated passband combinations should be avoided.NGC 3310 underwent a (possibly extended) global burst of cluster formation ∼3 × 10 7 yr ago. This coincides closely with the last tidal interaction or merger with a low-metallicity galaxy that probably induced the formation of the large fraction of clusters with (significantly) subsolar metallicities. The logarithmic slope of the V-band cluster luminosity function, for clusters in the range 17.7 F606W 20.2 mag, is α F606W −1.8 ± 0.4. The observed cluster system has a median mass of log(m/M ) 5.25 ± 0.1, obtained from scaling the appropriate model SEDs for known masses to the observed cluster SEDs.

High‐resolution Hubble Space Telescope (HST) imaging observations of star cluster systems provide a very interesting and useful alternative to spectroscopic studies for stellar population analyses with 8‐m class telescopes. Here, we assess the systematic uncertainties in (young) cluster age, mass and (to a lesser extent) extinction and metallicity determinations, based on broad‐band imaging observations with the HST. Our aim here is to intercompare the results obtained using a variety of commonly used modelling techniques, specifically with respect to our own extensively tested multidimensional approach. Any significant differences among the resulting parameters are due to the details of the various, independently developed, modelling techniques used, rather than to the stellar population models themselves. Despite the model uncertainties and the selection effects inherent to most methods used, we find that the peaks in the relative age and mass distributions of a given young (≲109 yr) cluster system can be derived relatively robustly and consistently, to accuracies of σt≡Δ〈log(age/yr)〉≤ 0.35 and σM≡Δ〈log(Mcl/M⊙)〉≤ 0.14, respectively, assuming Gaussian distributions in cluster ages and masses for reasons of simplicity. The peaks in the relative mass distributions can be obtained with a higher degree of confidence than those in the relative age distributions, as exemplified by the smaller spread among the peak values of the mass distributions derived. This implies that mass determinations are mostly insensitive to the approach adopted. We reiterate that as extensive a wavelength coverage as possible is required to obtain robust and internally consistent age and mass estimates for the individual objects, with reasonable uncertainties. Finally, we conclude that the actual filter systems used for the observations should be used for constructing model colours, instead of using conversion equations, to achieve more accurate derivations of ages and masses.