The stellar populations of low surface brightness galaxies

Near-infrared (NIR) K' images of a sample of five low surface brightness disc galaxies (LSBGs) have been combined with optical data, with the aim of constraining their star formation histories. Both red and blue LSBGs have been imaged to enable comparison of their stellar populations. For both types of galaxy strong colour gradients are found, consistent with mean stellar age gradients. Very low stellar metallicities are ruled out on the basis of metallicity-sensitive optical--NIR colours. These five galaxies suggest that red and blue LSBGs have very different star formation histories and represent two independent routes to low B-band surface brightness. Blue LSBGs are well described by models with low, roughly constant star formation rates, whereas red LSBGs are better described by a `faded disc' scenario.