New Near-Infrared Surface Brightness Fluctuation Models

We present new theoretical models for surface brightness fluctuations in the near-infrared. We show the time evolution of near-infrared brightness fluctuation properties over large age and metallicity ranges, i.e., from 12 Myr to 16 Gyr, and from Z/Zsun=1/50 to Z/Zsun=2.5, for single age, single metallicity stellar populations. All the stellar models are followed from the zero age main sequence to the central carbon ignition for massive stars, or to the end of the thermally pulsing regime of the asymptotic giant branch phase for low and intermediate mass stars. The new models are compared with observed near-infrared fluctuation absolute magnitudes and colours for a sample of Magellanic Cloud star clusters and Fornax Cluster galaxies. For star clusters younger than ~3 Gyr, the predicted near-infrared fluctuation properties are in a satisfactory agreement with observed ones over a wide range of stellar population metallicities. However, for older star clusters, the agreement between the observed and predicted near-IR brightness fluctuations depends on how the surface brightness absolute magnitudes are estimated. The computed set of models are not able to match the observed near-IR fluctuation absolute magnitudes and colours simultaneously. We argue that the observed discrepancies between the predicted and observed properties of old MC superclusters are more likely due to observational reasons