The light emitted by galaxies is one of the most stimulating evidence on the evolution of matter in the Universe. Stars are the building blocks of galaxies; understanding their evolution and their properties is an essential requirement to increase our knowledge on galaxies formation and evolution. At the basis of the study of stellar evolution is the comparison of observed Color-Magnitude Diagrams (CMD) with synthetic ones. CMD analysis is a powerful tool to characterize a stellar system (for example, to know its age, chemical composition, mass distribution, etc.). However, beyond the Local Group galaxies the horizontal branch (HB), the sub-giant branch, and the turnoff point fall far below even the HST detection and crowding limit. This clearly means that star-by-star analysis is limited to the vicinity of the Local Group, and we need to learn how to study distant unresolved stellar populations through their integrated light.

The task is to determine how many independent parameters give the emitted galaxy light, and how to extract them from observations. To tackle this problem is one of the purposes of the stellar population synthesis techniques. The comparison between observations and models of stellar populations helps understanding the formation and evolutionary processes of stars. The method can be applied, in principle, to any stellar system in the observable Universe, from Galactic systems to high redshift galaxies, that is both to resolved star clusters, and to unresolved systems.

The expertise of our group on the numerical simulation of stellar populations, directly follows from the long experience in stellar evolution theory. The study of star's life, coupled with the know-how in the field of stellar photometry, lead our group to develop a numerical code which - after being tested on resolved stellar systems - allowed us to make detailed studies aimed to shed light on:

1. Distance estimations, through the Main-Sequence Fitting, RR-Lyrae, and Cepheids methods;

2. Analysis of peculiar HB-morphology in Galactic Globular Clusters,

3. Disentangle the properties of unresolved stellar systems, such as Globular Clusters in distant galaxies, or stellar populations in external galaxies.

On this line, the SPoT group developed also Surface Brightness Fluctuations (SBF) models, in order to provide accurate SBF predictions for resolved and unresolved stellar populations. The SBF technique is widely adopted as distance indicator for ellipticals and bulges of spirals out to ~100 Mpc, as well as for Local Group stellar systems. More recently, the comparison between observations and theory showed that SBF can cope with the long-standing problem of the age/metallicity degeneracy, possibly useful as a tracer of the physical and chemical properties of stars in unresolved stellar systems.

SBF predictions can be obtained starting from the synthetic luminosity function of a properly simulated stellar population. The method we developed to predict SBF is based on Monte Carlo techniques, thus providing the unique opportunity of simulating the pixel-to-pixel variations observed in real galaxies. At the same time, by relying on a code originally devoted to simulate integrated quantities as well as synthetic CMDs, we can provide fully consistent simulations of synthetic CMDs, integrated magnitudes and colors, and SBF.

Our models CMD, integrated data, and SBF models have been checked through the comparison with data either taken form literature, or measured (for the first time) by our group.

On the observational side, we have started several collaborations devoted:

• to detect possible SBF radial-gradients in galaxies, using data from high performance telescopes (HST, LBT)

• to study SBF of resolved stellar systems, in order to enlight possible effects of small numbers (of stars) statistics on the SBF signal.

The goal of this web site is to give a global description of the SPoT models, also allowing the download of models and publications. Here you can access the SBF values and the integrated color indices, as well as the synthetic CMDs of the stellar population responsible for the integrated light originally studied. Please, have a look to the Readme file for the details on the models. It will be a pleasure to receive comments, questions or requests.

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