Stellar populations and surface brightness fluctuations: new observations and models

We investigate the use of surface brightness fluctuations (SBF) measurement s in optical and near-IR bandpasses for both stellar population and distanc e studies. New V-band SBF data are reported for five galaxies in the Fornax cluster and combined with literature data to define a V-band SBF distance indicator, calibrated against Cepheid distances to the Leo group and the Vi rgo and Fornax clusters. The colour dependence of the V-band SBF indicator is only similar to 15 per cent steeper than that found for the I band, and the mean 'fluctuation colour' of the galaxies is [(V) over bar - (I) over b ar macr] approximate to 2.4. We use new stellar population models, based on the latest Padua isochrones transformed empirically to the observational plane, to predict optical and near-IR SBF magnitudes and integrated colours for a wide range of populatio n ages and metallicities. We examine the sensitivity of the predicted SBF-c olour relations to changes in the isochrones, stellar transformations, and initial mass function. The new models reproduce fairly well the weak depend ence of V and I SBF in globular clusters on metallicity, especially if the more metal-rich globulars are younger. Below solar metallicity, the near-IR SBF magnitudes depend mainly on age, while the integrated colours depend m ainly on metallicity. This could prove a powerful new approach to the age-m etallicity degeneracy problem; near-IR SBF observations of globular cluster s would be an important test of the models. The models also help in understanding the ((V) over bar - (I) over bar) and ((I) over bar - (K) over bar) fluctuation colours of elliptical galaxies, with much less need for composite stellar populations than in previous mode ls. However, in order to obtain theoretical calibrations of the SBF distanc e indicators, we combine the homogeneous population models into composite m odels and select out those ones with fluctuation colours consistent with ob servations. We are able to reproduce the observed range of elliptical galax y (V - I) colours, the slopes of the V and I SBF distance indicators agains t (V - I) (fainter SBF in redder populations), and the flattening of the I- band relation for (V - I) less than or similar to 1.0. The models also matc h the observed slope of I-band SBF against the Mg-2 absorption index and ex plain the steep colour dependence found by Ajhar et al. for the HST/WFPC2 F 814W-band SBF measurements. In contrast to previous models, ours predict th at the near-IR SBF magnitudes will also continue to grow fainter for redder populations. The theoretical V-band SBF zero-point predicted by these models agrees well with the Cepheid-calibrated V-band empirical zero-point. However, the mode l zero-point is 0.15-0.27 mag too faint in the I band and 0.24-0.36 mag too faint in K. The zero-points for the I band (empirically the best determine d) would come into close agreement if the Cepheid distance scale were revis ed to agree with the recent dynamical distance measured to NGC 4258. We not e that the theoretical SBF calibrations are sensitive to the uncertain deta ils of stellar evolution, and conclude that the empirical calibrations rema in more secure. However, the sensitivity of SBF to these finer details pote ntially makes it a powerful, relatively unexploited, constraint for stellar evolution and population synthesis.