New near-infrared surface brightness fluctuation models

We present new theoretical models for surface brightness fluctuations (SBFs) in the near-infrared (near-IR). We show the time evolution of near-IR brightness fluctuation properties over large age and metallicity ranges, i.e. from 12 Myr to 16 Gyr and from Z/Z circle dot = 1/50 to 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 (AGB) phase for low- and intermediate-mass stars. The new models are compared with observed near-IR fluctuation absolute magnitudes and colours for a sample of Magellanic Cloud (MC) star clusters and Fornax cluster galaxies. For star clusters younger than similar to 3 Gyr, the predicted near-IR fluctuation properties are in 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.