• galaxies evolution
• dwarf galaxies
• NGC 5474
• stellar content
• SFH
• synthetic CMD method
• stellar populations

Dwarf galaxies have crucial cosmological relevance. According to the Cold Dark Matter (CDM) cosmological model, the low mass halos collapsed first, and massive systems like giant galaxies should be assembled by subsequent merging of these less massive fragments: the so called bottom-up scenario. One of the most effective approaches to test the validity of the bottom-up scenario is to observe the resolved stellar populations of giant and dwarf galaxies and compare their properties at fixed age. If chemical abundances of massive galaxies are consistent with those of dwarfs at some time t, then the former can be the result of the successive merging of the latter. Thus, investigating the formation and evolution of tiny systems is the key to understand the formation of bigger systems like spiral and elliptical galaxies. Moreover, by studying the star formation process in dwarf galaxies, where large-scale mechanisms (e.g., density waves) are less important than in giant galaxies, we can try to assess the importance of stochastic processes (e.g., supernova feedback, ram pressure, gravitational interactions). The study of star formation histories (SFHs) of galaxies is an attempt to build a "time machine" to view the past. While observations at high redshift provide a large picture view of the history of the Universe, they do not have the ability to follow the evolution of a single galaxy over the many billions of years since its formation. On the other hand, thanks to the predictions of stellar evolution models, stellar ages over the entire Hubble time can be reliably recovered for any nearby galaxy (d < 10$ Mpc). The aim of this master thesis is to study the SFHs of dwarf galaxies in the local Universe using the synthetic color-magnitude diagram (CMD) approach. In particular, we focused on the peculiar dwarf galaxy NGC 5474, as part of the Hubble Space Telescope (HST) Treasury program LEGUS (Legacy Extragalactic Ultraviolet Survey). NGC 5474 is a star forming dwarf galaxy, belonging to the M101 Group, at a distance of about 7 Mpc. It shows a seemingly circular compact bulge, apparently off-setted by 1 kpc with respect to the kinematic center of its HI disk. Young stars (< 100 Myr) follow an irregular but clear and wide spiral pattern, whose center of symmetry roughly coincides with the kinematic center of the disk. Moreover, the galaxy shows the presence of a significant stellar overdensity to the South-West of the optical bulge, a structure that extends for almost 1 Kpc^2. These multiple dynamical structures are generally attributed to the interaction with the nearby M 101. However, recent studies seems to suggest that the bulge could be instead an independent early-type dwarf galaxy orbiting around NGC 5474. The study of the detailed SFH for all its major components (bulge, stellar disk, and South-West overdensity) will allow to better constraint the interaction history of NGC 5474. For this task, I developed a new synthetic CMD code, based on the FORTRAN code SFERA (Star Formation Evolutionary Recovery Algorithm), a synthetic CMD technique implementing a genetic algorithm. A synthetic CMD is the expected fitting 2D distribution of stars, given a particular combination of SFH, chemical enrichment, initial mass function, photometric errors, incompleteness (estimated using extensive artificial star tests), distance modulus, binary fraction, and interstellar extinction. In my approach, the SFH of any galaxy is parameterized as a combination of quasi-simple stellar populations (bursts). The most likely SFH behind the data is the one whose synthetic CMD most closely resembles the observed CMD. During the fitting process, the metallicity is free to vary at any epoch, allowing to recover the SFH and chemical enrichment simultaneously. My contribution to the new code, named SFERA 2.0, has been threefold. First, I upgraded SFERA using the latest Padova PARSEC-COLIBRI stellar models, which cover all the evolutionary phases of a star, from the pre-main sequence to the end of the asymptotic giant branch. Then, I implemented a new Python-based adaptive interpolation scheme, able to carefully sample all "fast" stellar evolutionary phases (e.g., the upper main sequence, the intermediate mass He-burning phase, the asymptotic giant branch), as well as the "slow" ones (e.g., the main-sequence, the red giant branch, the horizontal branch). This new synthetic CMDs generation technique represents a drastic step forward with respect to the previously used Monte Carlo approach. The last improvement concerns the models metallicity. The previous code used a fixed number of metallicities for each burst, whereas the new one adopts a very fine grid of metallicities. Thanks to these upgrades, the new code is much more robust than the previous, as confirmed by extensive tests performed on mock galaxies. We confirmed for the first time on quantitative basis that the bulge is not a genuine old population. Instead, this region shows evidence of a prolonged star formation (SF) activity (at least up to 10 Myr ago). This, together with its peculiar position in the galaxy, might suggest that this structure is not a real bulge, but rather something else (e.g., pseudobulge, another dwarf galaxies). The SFH of the South-West overdensity shows a dominant episode of star formation around 1-3 Gyr ago, but also signs of a fairly continuous SF activity from 13.5 Gyr up to 10 Myr ago. The SFH of NGC 5474's disk has been fairly constant over the lifetime of the galaxy, with the star formation rate at the top of the peaks only a factor of 2 higher than the bottom of the dips.