OLD POPULATIONS AND THE STAR FORMATION HISTORY OF THE LOCAL GROUP DWARF IRREGULAR GALAXY NGC 6822

Abstract

ABSTRACT A method for studying the full star formation history (SFH) of nearby galaxies,as a function of time, has been developped and implemented. For the first time, the full SFH of a nearby dwarf irregular galaxy (DIG) has been retrieved from the photometry of its resolved stars. NGC~6822 has been observed in UBVRI at the 2.5m INT telescope, at the Roque de los Muchachos Observatory and photometry of the resolved stellar content has been obtained [1]. The VRI frames are very deep and contain about 18000 well measured stars. Most of them are old stars populating the red-giant branch (RGB) and the asymptotic giant branch (AGB) phase, which has been well sampled. Using the new BVRI magnitudes for the Cepheids, the first self-consistent BVRI multi-wavelength Cepheid distance and reddening has been derived for NGC~6822 [1]. The values obtained are (m-M)0=23.49± 0.08 and E(B-V)=0.24±0.03, respectively. The comparison of the [(V-I),I] and [(V-R),V] colour-magnitude (CM) diagrams with theoretical isochrones allowed the qualitative understanding of the stellar populations present in NGC~6822 ([1] and [4]). Two structures, the red-tangle and the red-tail ({tocho} and {colita}, as we originally called them) have been identified as the locus of the old and intermediate-age stars (age >1 Gyr), and therefore, as the structures to be studied to retrieve information of the old and intermediate-age SFH. Since stars older and younger than 1 Gyr are situated in clearly distinct areas of the CM diagram, and because of the different time-resolution on which these populations can be studied, the old and intermediate-age SFH and the young SFH have been derived separately ([2] and [3]). Nevertheless, the tool has been in both cases the same: the comparison of the observed [(V-I),I] CM diagram with a set of model CM diagrams, computed assuming different evolutionary scenarios. This is the main core of this thesis. The comparison has been performed through a number of indicators relative to the stars' distribution over the CM diagram. The model CM diagrams have been computed in collaboration with the Padova Group of stellar evolution. The reliability of the model CM diagrams computed is a result of three main components : i) a reliable set of stellar evolutionary models covering the needed range of ages and metallicities, ii) the interpolation of the stellar evolutionary tracks of fixed mass and metallicity to determine the precise, {smooth} distribution of stars of any age, mass, and metallicity on the CM diagram, and iii) a realistic simulation of the observational effects on the synthetic CM diagram. Significant constraints have been put on the SFH of NGC~6822. The main conclusions reached are the following: --Star formation beginning at Ti≤ 6 Gyr can be ruled out. Star formation beginning at Ti≃ 6 Gyr would only be possible if the galaxy started its early star formation from gas already enriched, or if a prompt metal enrichment occurred in the galaxy. --NGC~6822 has most likely begun forming stars at a very early epoch (about 15-12 Gyr ago), from low metallicity gas. --An SFR close to constant or declining in the last few Gyr of the galaxy's lifetime seems best to reproduce the observations. Short-time and small-amplitude fluctuations of the SFR may have occurred. -- An overall enhancement of the star formation activity has occurred in the last 100-200 Myr. The strength of this enhancement has been somewhat different from one region of the galaxy to another. Although applied to the case of a DIG, the fundamentals of the method developed in this thesis can be applied to galaxies of any morphological type, provided that deep enough photometry is available. This can be achieved, from the ground, for all Local Group galaxies, and using Hubble Space Telescope, for galaxies within 4--5 Mp.