| Abstract
| - Abstract. We introduce and discuss the properties of a theoretical (B−K)(J−K) integrated colour diagram for single-age, single-metallicity stellar populations. We show how this combination of integrated colours is able to largely disentangle the well-known age-metallicity degeneracy when the age of the population is greater than ∼300 Myr, and thus provides valuable estimates of both age and metallicity of unresolved stellar systems. We discuss in detail the effect on this colour-colour diagram of α-enhanced metal abundance ratios (typical of the oldest populations in the Galaxy), the presence of blue horizontal branch stars unaccounted for in the theoretical calibration and of statistical colour fluctuations in low-mass stellar systems. In the case of populations with multiple stellar generations, the luminosity-weighted mean age obtained from this diagram is shown to be heavily biased towards the youngest stellar components. We then apply this method to several data sets for which optical and near-infrared photometry are available in the literature. We find that Large Magellanic Cloud and M31 clusters have colours which are consistent with the predictions of the models, but these do not provide a sensitive test due to the fluctuations which are predicted by our modelling of the Poisson statistics in such low-mass systems. For the two Local Group dwarf galaxies NGC 185 and 6822, the mean ages derived from the integrated colours are consistent with the star formation histories inferred independently from photometric observations of their resolved stellar populations. The methods developed here are applied to samples of nearby early-type galaxies with high-quality aperture photometry in the literature. A sample of bright field and Virgo cluster elliptical galaxies is found to exhibit a range of luminosity-weighted mean ages from 3 to 14 Gyr, with a mean of ∼8 Gyr, independent of environment, and mean metallicities at or just above the solar value. Colour gradients are found in all of the galaxies studied, in the sense that central regions are redder. Apart from two radio galaxies, where the extreme central colours are clearly driven by the active galactic nucleus, and one galaxy which also shows a radial age gradient, these colour changes appear consistent with metallicity changes at a constant mean age. Finally, aperture data for five Virgo early-type dwarf galaxies show that these galaxies appear to be shifted to lower mean metallicities and lower mean ages (range 1-6 Gyr) than their higher luminosity counterparts.
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