A multi-wavelenght analysis of stellar populations hosted by the Local Group galaxy NGC 6822

The dwarf galaxies within the Local Group (LG) are proved to be ideal laboratories to constrain both cosmological and astrophysical issues. Concerning cosmology they are useful tools to test the so-called hierarchical scenario for galaxy formation and also the dark matter (DM) paradigm as they appear to be dark matter dominated. Whereas, from an astrophysical point of view the great variety of morphological types and the proximity of the LG dwarf galaxies, hence the chance to resolve them into stars, offers the unique possibility to study the galaxy and stellar evolution, the chemical enrichment, the star formation history and a number of other open issues in different regimes. Among them, the dwarf irregular galaxies (dIs) are often isolated and very complex systems that are rich in gas, both neutral and ionized, host stellar populations ranging from very young to intermediate-age and old stars, hold a quite low metallicity and in most cases show evidence of an ongoing star formation activity. Moreover, they are considered unique analogs of the star forming galaxies observed at high redshift and populating the universe at the peak of cosmic star formation history. Therefore, they allow to probe the star formation and evolution in metal-poor environments and the effects that their intrinsic properties may have on the evolution of such complex systems. NGC 6822 is the closest dwarf irregular to the Milky Way (MW) and thus one of the most investigated galaxy within the LG, revealing several interesting properties. This PhD work project aims at further constraining the identification and characterization of the different stellar populations and at assessing the structural parameters for each of them. We present accurate and deep optical multi-band (g, r, i) photometry of NGC 6822 . The images were collected with wide field cameras at 2m- (INT), 4m- (CTIO, CFHT) and 8m-class telescope (SUBARU) and cover a FoV of 2 square degrees across the centre of galaxy. We performed PSF photometry (DAOPHOT-ALLSTAR-ALLFRAME) over more than 7, 000 CCD images and the final catalogue includes more than 1 million objects with at least one measurement in two different photometric bands. Thus, it is the widest and most homogeneous photometric dataset ever collected for a nearby dI, but the Magellanic Clouds (MCs). Since NGC 6822 is located at low Galactic latitudes and hence affected by field contamination, we focused on the separation of candidate field and galaxy stars developing a new approach. Then, we performed a new estimate of the galaxy centre by using the old tracers of the candidate galaxy stars and we found that it differs by 1.15 (RA) and 1.53 (DEC) arcmin from previous estimates. To further constrain the stellar content of NGC 6822, we cross correlated the optical catalogue with NIR (UKIRT) and MIR (SPITZER, WISE) photometry available in the literature. We have selected young (Main Sequence, Red Supergiants), intermediate (Red Clump, Asymptotic Giant Branch [AGB]) and old (Red Giant Branch [RGB]) stellar populations and found that they display different radial distributions. The young stars are distributed along a well defined bar and form a disk-like structure, as also suggested by the HI density map from radio measurements, that is off-centre compared with the oldest stars. The spatial distribution of the old stellar population is more spherical and symmetric and extends at radial distances larger than previous estimates (∼1 degree). We also focused our attention on the identification of AGB stars and to the characterization of Oxygen (O) and Carbon (C) rich stars. We discuss pros and cons of the different approaches available in the literature and develope new diagnostics based on optical-NIR-MIR colour-colour diagrams (C-CDs). Thanks to this new diagnostics we found mean population ratios between Carbon and M-type (C/M) stars of 0.68 ± 0.12 (optical-NIR), 0.63 ± 0.04 (optical-MIR) and of 0.70 ± 0.08 (optical-NIR-MIR). Finally, we used the current C/M ratios and the empirical C/M-metallicity relations to estimate the mean metallicity of the AGB stars and found mean iron abundances ranging from [F e/H] ∼ −1.20 (σ = 0.04 dex) to [F e/H] ∼ −1.30 (σ = 0.03 dex), that agree quuite well with similar estimates available in the literature.