We use empirical techniques to interpret the near-infrared colours of a sample of 5800 galaxies drawn from Sloan Digital Sky Survey (SDSS) main spectroscopic sample with YJHK photometry from the UK Infrared Deep Sky Survey (UKIDSS) data release one. We study correlations between near-IR colours measured within SDSS fibre and physical parameters derived from the spectra. These parameters include specific star formation rate, stellar age, metallicity and dust attenuation. All correlations are analyzed for samples of galaxies that are closely matched in redshift, in stellar mass and in concentration index. Whereas more strongly star-forming galaxies have bluer optical colours, the opposite is true at near-IR wavelengths -- galaxies with higher specific star formation rate have redder near-IR colours. This result agrees qualitatively with the predictions of models in which Thermally Pulsing Asymptotic Giant Branch (TP-AGB) stars dominate the H and K-band light of a galaxy following a burst of star formation. We also find a surprisingly strong correlation between the near-IR colours of star-forming galaxies and their dust attenuation as measured from the Balmer decrement. Unlike optical colours, however, near-IR colours exhibit very little dependence on galaxy inclination. This suggests that the correlation of near-IR colours with dust attenuation arises because TP-AGB stars are the main source of dust in the galaxy. Finally, we compare the near-IR colours of the galaxies in our sample to the predictions of three different stellar population models: the Bruzual & Charlot 2003 model, a preliminary version of a new model under development by Charlot & Bruzual, which includes a new prescription for AGB star evolution, and the Maraston 2005 model.
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