• synthetic CMD
• star formation history
• Small Magellanic cloud

The study of galaxy formation and evolution is a complex endeavor, due to the intricate interplay of various phenomena shaping these celestial objects. Constructing a single comprehensive theory to predict and explain their diversity is challenging. The ΛCDM model offers a cosmological framework that successfully explains many aspects of the Universe, including the cosmic microwave background, the growth of cosmic structures, and galaxy clustering. Observations reveal the existence of galaxies from very old ages, and superclusters are only forming today, hinting at the role of merging events among small components in the formation of large structures (the bottom-up scenario). Among galaxies, dwarf galaxies, the most prevalent type in the cosmos, are the best candidates to be the fundamental building blocks of larger structures. Our knowledge about this type of galaxies primarily comes from those within the Local Group, most of them satellite systems of the largest galaxies of the group (the Milky Way and the Andromeda galaxy).
This thesis focuses on the Small Magellanic Cloud (SMC), one of the closest dwarf galaxies to the Milky Way (MW) (located at a distance of ~60 kpc). The SMC belongs to the Magellanic Spiral Irregular class in the Hubble classification and stands as a a unique astrophysical laboratory for studying its spatially-resolved star formation history thanks to its proximity and size. The SMC and its bigger companion, the Large Magellanic Cloud (LMC), are a pair of gravitationally interacting dwarf galaxies that are slowly merging with the MW, forming a dynamic three-body system. The MCs show the scars of the gravitational interaction as distinct structures, such as the Magellanic Stream, the Leading Arm and the Magellanic Bridge. The SMC exhibits a bar structure spanning about two-thirds of its optical size, where the star-forming activity is concentrated. The older population is more uniformly spread across the galaxy, in a spheroidal/ellipsoidal structure. The actual shape and depth of the SMC have been subjects of debate over the past few decades. Several studies found varying depth along different lines-of-sight, with the eastern part of the SMC deeper than the western region. Moreover, the MCs' interaction with the MW remains an evolving topic, as their orbital history is still unclear. Recent high-precision measurements of the proper motion suggest the MCs are in their first encounter with the MW, explaining their unique properties among the satellite galaxies of the MW.
Early investigations of the Magellanic Clouds suffered of limited spatial coverage and low resolution. The Hubble Space Telescope provided high-resolution snapshots of small regions of the galaxy, while extensive ground-based surveys (e.g.: MCPS, OGLE, VMC, SMASH) have played a pivotal role in the last decades to reconstruct the global SFH. We further investigate on this topic, employing the synthetic CMD method on new data. The Small Magellanic Cloud in Time: Evolution of a Prototype interacting late-type dwarf galaxy (STEP) survey employed the VLT Survey Telescope (VST) to gather extensive photometric data of the entire galaxy and of the Magellanic Bridge in SDSS photometric filters. STEP reaches stars below the turn-off of the oldest population, even in crowded regions, and may provide a far better age resolution respect to any previous survey. This thesis is the initial part of a long-term project aimed at the recovery the global SFH of the SMC using all 53 STEP tiles. In this work we focus on the analysis of two tiles (of 1 deg2 each): tile 3_7, relatively close to the center of the galaxy and pointing towards the LMC, and tile 3_2, that lies in the opposite direction, in the eastern periphery. Their large separation (~5 kpc) was chosen to study different environments of the SMC, with tile 3_7 experiencing stronger gravitational interactions due to its proximity to the LMC. The choice of widely separated tiles also minimizes any potential correlations in the recovered SFH resulting from their proximity.
We utilized a colour–magnitude diagram reconstruction approach to obtain the best-fitting star formation rate. In this approach, we model the SFH of a galaxy as the combination of quasi-simple stellar populations (also called partial models). These partial models are convolved with the specific characteristics of the observed data, accounting for reddening, distance modulus, photometric errors, and incompleteness (estimated using extensive artificial star tests). The goal is to find the SFH that best matches the observed CMD. In order to do this, we employed the FORTRAN code SFERA (Star Formation Evolutionary Recovery Algorithm, Cignoni et al. 2015), a synthetic CMD code that employs a genetic algorithm to converge to the best-fit solution. We upgraded SFERA building a library of partial models covering the whole Hubble time using the latest Padova PARSEC-COLIBRI stellar models. We avoided the imposition of an a priori age-metallicity relation (AMR), instead leaving the algorithm free to choose the best fitting metallicity for each partial model (in the range [Fe/H]=[-2.0, 0]), enabling the simultaneous recovery of the galaxy's SFH and its chemical evolution.
The analysis of Tile 3_7 revealed that the star formation rate (SFR) reached a peak ~5.5 Gyr ago, followed by a decline and a smaller peak ~2.75 Gyr ago. Subsequently, the SFR remained relatively low until around 200 Myr ago. From this point, the SFR began to rise again, continuing up to the present day. The comparisons with previous studies prove the higher age resolution we are able to reach. Tile 3_2 shows a peak in SFR ~4.5 Gyr ago, followed by declining rates, with smaller peaks at ~2.75 Gyr and ~450 Myr ago. Unlike tile 3_7, there is no significant star-forming activity in the last 100 Myr. The recovered age-metallicity relation shows a relatively constant metallicity until around 3-4 Gyr ago, followed by a steep enrichment. 50% of the mass was formed prior to an age of ~5.3 Gyr in both the analyzed tiles, while 90% was formed around 2.3 Gyr in tile 3_2 and 1.4 Gyr ago in tile 3_7. Our findings appear to be more consistent with a chemical enrichment model that involves bursting star formation, as well as inflow and outflow of gas respect to simpler closed-box models assuming continuous star formation and chemical homogeneity. The recovery of the global SFH with STEP data may help constraining the orbital history of the MCs and give us a more complete picture of the history of the magellanic system.

The thesis is structured as follows:
Chapter 1 provides a comprehensive review of the cosmological background and delves into the origin and evolution of density perturbations in the universe. This chapter also briefly discusses the morphological classification of galaxies and the processes that shape them over time, with a particular emphasis on dwarf galaxies.
Chapter 2 offers a concise overview of the primary stellar evolutionary phases. It describes the formation of stars from molecular clouds and their evolutionary tracks on the Hertzsprung-Russell diagram. Additionally, this chapter introduces the color-magnitude diagram and explores the concept of simple and composite stellar populations.
Chapter 3 focuses on the Magellanic system, with specific attention given to the Small Magellanic Cloud. It presents an overview of our current knowledge regarding the SMC's morphology and dynamics, along with a summary of the results obtained in previous studies on the global star formation history of the galaxy.
In Chapter 4, we present the photometric data from the STEP survey and describe the artificial star test conducted to assess data completeness and quality. This chapter also introduces the two tiles analyzed in this thesis, providing a qualitative examination of their stellar populations.
Chapter 5 details the synthetic color-magnitude diagram technique employed to recover the SFH. It also discusses the tests performed to evaluate the age resolution achievable with our data, along with assessments against mock galaxies with known SFH and age-metallicity relations to determine the algorithm's reliability.
In Chapter 6, we present the primary findings of our study on two SMC tiles and their comparison with the orbital and chemical evolution models in the literature.
In Chapter 7 we discuss the concluding remarks and future prospects.