DWARF IRREGULAR GALAXIES ARE OLD SYSTEMS

A large old to intermediate-age stellar population has been found in a couple of dwarf irregular (dIr) galaxies in the Local Group. The blue colours of dIr galaxies, their large numbers of HII regions, Ha flux, blue stars and relatively large amount of gas have led them to be referred to as young objects and to be preferred targets for the study of young stellar populations and recent star formation. The presence and the age of a stellar population older than a few thousands of millions of years in dIrs had been questioned, even though an underlying population of red stars has been detected in the nearest of them (often called Baade's sheet). But the presence of Baade's sheet is not, by itself, a proof of old age since it may be produced by stars just a few thousands of millions of years old. 

Following a new approach to the problem (based on the comparison of observedand model colour-magnitude [CM] diagrams), star formation histories (SFHs) for old, intermediate and young ages have been obtained on the INT for three dIr galaxies: NGC6822, Pegasus and LGS3. Model CM diagrams are built up for different input SFHs and an accurate simulation of observational effects is a majoringredient. Although the resolution worsens in the determination of the SFH as we go to older ages, this work gives a definite answer to the question of the age of these dIrs in the sense that not only dIrs they begin to form stars about 15,000 million years ago but also that they show SFHs enhanced in the first half of their lives. 

This result is a new step towards the understanding of the mechanisms of the
formation and evolution of galaxies. The dIrs seem now to be as old as the other
kind of dwarf galaxies, the dwarf ellipticals (dE), which show few or no traces of
recent star formation but which might have a stronger relationship with dIrs than is usually assumed.