The study of stellar populations of non-resolved systems has greatly relied on the models derived from evolutionary populations synthesis technique. This approach is based on the spectrophometric properties of stars at, ideally, all evolutionary phases and takes into account all phenomena that largely affect the evolution of a star (e.g., mass-loss). Over the years, ever since the technique was first implemented (Tinsley 1968, 1972), a wide variety of models based on different ingredients have been constructed and used in the study of young and old stellar populations (among the most popular ones, Buzzoni 1989, Bressan, Chiosi & Fagotto 1994, Worthey 1994, Bruzual & Charlot 1993, Bruzual & Charlot 2003, Leitherer et al. 1999, Maraston 2005). Not surprisingly, most of the work done up to date has vastly focused in the optical spectrophotometric properties of stellar systems, and until relatively recently it has expanded to other wavelengths (as far as the detailed analysis of spectral features is concerned), and, in some cases, included the effects of an interstellar medium (Silva et al. 1998, Panuzzo et al. 2005).
At ultraviolet (UV) wavelengths, usually divided into two segments, the far-UV (1200-2000 Å) and the mid-UV (2000-3200 Å), the natural systems to look at are those whose underlying populations copiously emit and have their emission maxima in that window, i.e. star-forming systems. While these systems are extremely important in many astrophysical contexts (see, e.g., Buzzoni 2002), it was eventually realized that also old and intermediate age populations, which will be the main subject in this paper, deserve attention by their own right. As an example we can mention the countless studies motivated by the unexpected finding of a prominent far-UV flux excess in the bulge of Andromeda (Code 1969). Aside of this far-UV flux excess, the mid-UV still remains vastly unexplored, in spite of the early suggestions that this wavelength region can help in lifting the so-called age-metallicity degeneracy (AMD) that plagues the optical spectrophotometric properties of evolved populations and that prevents the univocal determination of these parameters (Worthey 1994, Dorman et al. 2003). Disentangling the effects of age and chemical composition is particularly important when attempting to evaluate the characteristics of distant red objects for which, through optical observations only feasible with the current generation of large telescopes, we can only access the rest-frame mid-UV flux (e.g., Dunlop et al. 1996).
Motivated by these three issues inherent to aged populations, the nature of the far-UV flux, the AMD, and the properties of instrisically red galaxies up to z ~ 2, we started a project aimed at providing complementary tools for their analysis. In what follows, we present a short (and necessarily incomplete) review of each of the above mentioned topics; we also briefly describe our project and present some preliminary results that are still under investigation.