Surface-brightness fluctuations in stellar populations

Abstract

Aims. A new theoretical calibration of surface-brightness fluctuations (SBF) for single age, single metallicity stellar populations is presented for the optical and near-IR broad-band filters, as well as for the HST WFPC2 and ACS filters. Methods. The IAC-star code is used. Two Padua and the Teramo stellar evolution libraries have been considered. A set of single-burst stellar populations (SSP) with a wide range of ages (3 Gy−15 Gy) and metallicities (Z = 0.0001−0.03) have been computed using each one of the three considered stellar evolution libraries. For each SSP, color indexes and SBF magnitudes are given for the filters U, B, V, R, I, J, H, K, F218W, F336W, F439W, F450W, F555W and F814W, and for the first time, an uncertainty has been estimated for the SBF theoretical calibration. Results. Although some differences might be addressed, the Padua and Teramo stellar evolution libraries provide comparable SBF results. A detailed comparison of the present SBF calibrations with both previous calibrations and observational data is also presented. Comparing the different models with observational data, Padua based models reproduce fairly well the optical data for globular clusters, while Teramo based models fits both optical galaxies and globular clusters data, as well. In the near-IR wavelengths, the Teramo based models provide the only SBF theoretical calibration to date able to properly reproduce the observational data for superclusters, with intermediate-to-low metallicity. As a conclusion, Teramo based models work better than any other calibration reproducing observational data for the near-IR wavelengths. Furthermore, the age-metallicity degeneracy is broken for low metallicity (Z ≤ 0.0037) stellar populations. Finally, a clear relation between the B SBF absolute magnitude of a stellar population and its metallicity is found for intermediate to old populations, so the B-band fluctuation magnitude is proposed as a metallicity tracer. The present theoretical calibration shows that the analysis of SBF provides a very powerful tool in the study and characterization of unresolved stellar populations.