People involved at OAB: Origlia, Romano, Tosi.
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, such as 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 2003a,b).
To this aim we are undertaking a long term project in collaboration with Angeretti (Astron. Dept., University of Bologna), Greggio (INAF-Padova Obs.), Annibali (Sissa), Monelli (Univ. Tor Vergata), Aloisi, Clampin, Leitherer and Nota (Baltimore, USA), and Tolstoy (Groningen, NL) to study, from deep and accurate photometric data (VLT and HST), the stellar populations of a number of dwarfs known to show evidence of galactic winds. So far we have studied IZw18, NGC 1569 and NGC 1705 and observed SBS1415+437 (with ACS@HST) and DDO210 (with FORS@VLT). Time has been allocated at the ACS@HST to study some young clusters in the SMC. 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.
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In 2003 we have published (Annibali et al. 2003) the study of the SFH in NGC1705 from WFPC2@HST and Nicmos@HST VIJH data (see Fig. 4). These data have allowed us to derive the galaxy 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 applied the synthetic CMD method to concentric regions from the galaxy center to study their SFHs and found that the SF has been fairly continuous everywhere, but with significant ups and downs in the rate and with clear short quiescent intervals. Our study has made us discover that NGC1705 is experiencing now a new strong SF activity. We have completed the study of the U and B WFPC2@HST images of NGC1705 confirming the evolutionary scenario mentioned above and providing the catalogue of its candidate star clusters (Monelli et al. in preparation).
The synthetic CMD method has been applied also to the Nicmos@HST data on NGC1569 (Angeretti et al. in preparation). This has allowed us to derive the SFH at epochs older than those covered by optical data. We have found that this galaxy has been extremely active also in the past.
We have reduced the VLT data on the Local Group dwarf DDO210; we are now applying extensive artificial star tests to these images to derive the photometric properties (errors, incompleteness, blending) to be inserted in the synthetic CMD code to derive its SF history.
New numerical chemical evolution models have been computed (Recchi et al. 2004) 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 supernova explosions on the hydrodynamics of their interstellar medium and the possible onset of galactic winds. Meanwhile, classical chemical evolution models are being computed for IZw18, NGC1569, NGC1705, also assuming the SF history and IMF derived with the synthetic CMD method. These simpler models, once compared with the observed abundances and masses of the examined galaxies, provide useful information on the various parameters and can provide indications on how the yet unknown part of the gas dynamics could behave.