Involved people at OAB: Bellazzini, Greggio, Origlia, Parmeggiani, Tosi.
Dwarf irregular 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 local counterpart of the faint blue galaxies found in excess in deep galaxy counts. Understanding how dwarf irregulars 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 in a number of representative systems of the major morphological sub-classes (blue compact galaxies, dwarf irregulars, giant irregulars) (Tosi 1999). To this aim we are undertaking a long term project (in collaboration with A.Aloisi, M.Clampin, C.Leitherer and A.Nota, at STScI in Baltimore, USA) to study, from deep and accurate photometric data (from ESO VLT and HST), the stellar populations of a number of dwarfs known to show evidence of galactic winds. The resolved stars allow us to derive the epochs and the intensity of the star formation activity and the IMF of these galaxies back to fairly old epochs with the method of synthetic color-magnitude diagrams created by our group (Tosi et al., 1991, AJ 102, 951) and amply tested and applied by the international community.
In 1999 we have completed (Aloisi, Tosi, Greggio 1999) the study of the star formation history of IZw18, based on HST-WFPC2 data, and we are now computing numerical models for its chemical evolution using such results as constraints. We have reduced the NIR data on NGC1569, acquired with HST-Nicmos, and we are completing the analysis of the HST-WFPC2 and HST-Nicmos data on NGC1705. Further HST orbits for optical imaging of NGC1705 have been allocated, but postponed to the end of 2000. The color-magnitude diagrams of these two galaxies will be interpreted in terms of IMF and star formation history in the next few months. Then, numerical chemical evolution models will be computed, taking also into account the effects of the supernovae explosions on the hydrodynamics of their interstellar medium and the possible onset of galactic winds.
In 1999 we have also obtained, in collaboration with E.Tolstoy (ESO) and others, deep images at the ESO-VLT of the Local Group irregulars Aquarius, Cetus and Phoenix (Tolstoy et al., 2000), which will be analyzed with the same way as described above.