Extending the baseline: Spitzer mid-infrared photometry of globular cluster systems in the Centaurus A and Sombrero Galaxies

Spitzer IR Array Camera (IRAC) mid-infrared photometry is presented for the globular cluster (GC) systems of the NGC 5128 ('Centaurus A') and NGC 4594 ('Sombrero') galaxies. Existing optical photometric and spectroscopic are combined with this new data in a comprehensive optical-to-mid-IR colour catalogue of 260 GCs. Empirical colour–metallicity relationships are derived for all optical-to-mid-IR colour combinations. These colours prove to be very effective quantities to test the photometric predictions of simple stellar population (SSP) models. In general, four SSP models show larger discrepancies between each other and the data at bluer wavelengths, especially at high metallicities. Such differences become very important when attempting to use colour–colour model predictions to constrain the ages of stellar populations. Furthermore, the age-substructure determined from colour–colour diagrams and 91 NGC 5128 GCs with spectroscopic ages from Beasley et al. is inconsistent, suggesting any apparent GC system age-substructure implied by a colour–colour analysis must be verified independently. Unlike blue wavebands, certain optical-to-mid-IR colours are insensitive to the flux from hot horizontal branch stars and thus provide an excellent metallicity proxy. The NGC 5128 GC system shows strong bimodality in the optical R band to mid-IR colour distributions, hence proving it is bimodal in metallicity. In this new colour space, a colour–magnitude trend, a 'blue tilt', is found in the NGC 5128 metal-poor GC data. The NGC 5128 young GCs do not contribute to this trend. In the NGC 4594 GC system, a population of abnormally massive GCs at intermediate metallicities shows bluer optical-to-optical colours for their optical-to-mid-IR colours, suggesting they contain extended horizontal branches and/or are younger than typical GCs. Analysis of optical-to-mid-IR colours for a ultracompact dwarf galaxy suggests its metallicity is just below solar.