Late-type dwarf galaxies are playing an increasingly central role in understanding galaxy evolution, because their proximity allows one to examine in detail important issues, like the occurrence of galactic winds, the chemical enrichment of the interstellar and intergalactic media, the photometric evolution of galaxies. Besides, their low level of evolution, as implied by the low metallicity and the high gas content, makes these systems the most similar to primeval galaxies and, therefore, the most useful to infer the primordial galaxy conditions. Furthermore, they have been suggested to represent the building blocks of larger galaxies. Understanding how late-type dwarfs evolve and what were their conditions at early epochs is then crucial also for cosmological purposes. It is thus fundamental to derive the star formation history (SFH) in a number of representative systems of the major morphological sub-classes: blue compact galaxies, dwarf irregulars, giant irregulars (Tosi 2001b). To this aim we are undertaking a long term project (in collaboration with Aloisi, Clampin, Leitherer and Nota, Baltimore, USA) to study, from deep and accurate photometric data (ESO-VLT and HST), the stellar populations of a number of dwarfs known to show evidence of galactic winds. So far we have sampled IZw18, NGC 1569 and NGC 1705. HST-ACS time has been allocated to observe SBS1415+437. The resolved stars allow us to derive the intensity as a function of time of the star formation activity and the IMF of these galaxies back to fairly old epochs with the method of synthetic CMDs pioneered by our group and amply tested and applied by the international community.
In June 2001, the international workshop on Observed HR diagrams and stellar evolution: the interplay between observational constraints and theory, held in Coimbra (Portugal) has provided the opportunity to compare the results of the synthetic CMD method obtained by our group with those obtained by all the other groups active in this field. All the groups had to derive, with their own procedures, the SFH of an LMC field observed with HST-WFPC2, whose resulting CMD was provided to all groups together with all the information on the photometric errors and incompleteness of the data (Tosi et al 2002a). A blind comparison (see Gallart & Skillman 2002) has demonstrated that, despite fairly different approaches, the method provides results consistent with each other within a factor of two. When critical arguments are considered, even better agreement is reached.
The procedure of deriving the SFH from synthetic CMDs is ultimately based on the relation between the mass of the stellar populations and the number of stars counted on the CMD. The connection between the total mass in stars and the star counts in various regions of the CMD has been investigated from a purely theoretical point of view (Greggio 2002), to the end of providing basic relations between the mass in stars and the stars' counts for stellar populations with an age spread. It is found that the bright portion of the CMD allows to recover the star formation history with a fair degree of detail up to look back times of approximately 0.3 Gyr. For older stellar populations, the counts in selected regions of the CMD (RGB, He burning clump, Bright AGB) make it possible to estimate the mass in stars within a factor of 3, for an adopted Initial Mass Function slope.
In 2001 we completed the analysis of the red stellar populations in NGC 1569, by analyzing both the field (Aloisi et al. 2001) and the stellar cluster (Origlia et al. 2001) populations. We found a complex stellar population, covering the whole age range from a few Myr to a Hubble time. The intensity of the most recent SF activity is somewhat higher than in the past and mainly concentrated around the super star clusters, while the older stellar populations have a more uniform distribution within the galaxy.
We also completed the analysis of the HST-WFPC2 and HST-Nicmos VIJH data on NGC 1705 (Tosi et al. 2001). These data have allowed us to derive its distance with great accuracy from the red giant branch tip, and to show that this blue compact galaxy has a large fraction of old stars (hence has been strongly active also in remote epochs) and has a population age gradient, with decreasing age for decreasing galactocentric distance. We have started to apply the synthetic CMD method to concentric regions from the galaxy center to study their SFHs (Annibali et al. 2001a, 2001b).
Within the collaboration with Schulte-Ladbeck (Univ. of Pittsburg) and Hopp (Univ. of Munich), we have completed the analysis of HST data of Leo A and NGC 4214 (Drozdovky et al. 2002, Schulte-Ladbeck et al. 2002). In both cases we detect a considerable fraction of old stars, and a centrally concentrated young/ intermediate age component. These cases add to the numerous other examples of dwarf galaxies which experienced star formation since the very early epochs. The results are currently under publication on the Astronomical Journal
Numerical chemical evolution models have been computed (Recchi et al. 2002) for the blue compact galaxy IZw18, based on the SFHs derived by us (Aloisi et al. 1999) applying the synthetic CMD method to HST data. These new generation models take into account the effects of the supernovae explosions on the hydrodynamics of their interstellar medium and the possible onset of galactic winds. Analogous models will be applied in the future to NGC 1705 and the other late-type dwarfs analyzed with our method.