Gatto, Massimiliano (2022) Unveiling the evolutionary history of the Magellanic Clouds and beyond through detection and analysis of stellar over-densities: clusters and streams. [Tesi di dottorato]

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One of the key questions in astrophysics concerns how galaxies evolve, or more precisely, which are the physical processes that drive the morpho-chemo-physical evolution in a galaxy during its cosmic life, from the moment of its formation till the present time. In the framework of the hierarchical building block described by the $\Lambda$CDM paradigm, the evolution of a galaxy is also affected by the manifold of close encounters, interactions, and mergers it undergoes with its neighbor galaxies. Thus, reconstructing the whole evolutionary path of a galaxy, including its interaction history, is pivotal to link the galaxy properties we observe at high red-shift to their physical characteristics observed in the local Universe, and to improve galaxy formation and evolutionary models.\par The Magellanic Clouds (MCs), composed of the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), are the closest example of a three-body interacting system composed of the Milky Way (MW), the LMC and SMC. Therefore, the unique opportunity provided by their relative proximity allowed us to analyse with matchless detail the dynamical and morphological evolution that a galaxy experience as a consequence of the mutual gravitational interaction with its neighbors. In addition, the MCs likely represent the massive members of a group of galaxies in the first infall into the MW halo and thus they are the ideal test-bed to investigate the pre-processing of groups prior the capturing by a massive galaxy, such as the MW.\par In this thesis, I made use of astrometric, kinematics (proper motions and radial velocities) and photometric data, with the main goal of unveiling the evolutionary path of the MCs and their intense interaction history. To this aim, I developed new algorithms or improved existing techniques to take full advantage of the datasets I worked with. The majority of the works presented in this thesis are based on two complementary deep and highly accurate surveys which explored more than 160 square degrees within the MCs and their outskirts: the STEP and YMCA survey (PI: V. Ripepi), which were carried out with the VLT survey telescope (VST) since Dec. 2011 up to the last year, when the last tile has been observed. The photometry obtained with these surveys allows us to resolve the MC stars singularly down to about 1.5-2 magnitudes below the main-sequence turn-off of the oldest stellar population, permitting us to probe even the early stages of MCs evolution. The deep photometry is combined with the wide spatial coverage of the two surveys, which, in the case of YMCA also cover a significant part of the peripheries of the MCs, regions that have not been explored before employing deep and wide-field photometry. When I started my Ph.D. project, the STEP survey was already completed and the catalog available, while the YMCA survey was just starting the observations. As part of my thesis, I carried out the photometric measurements for the YMCA survey, including the construction of the procedures needed to calibrate and make available the catalog. We tackled the reconstruction of the MCs evolutionary history by using two complementary approaches. The first one, which concerns the major part of this thesis, consisted of a deep analysis of the properties of the MCs star cluster (SC) system to get insights into their past evolution. We take advantage of the exclusive YMCA observations to try to obtain for the first time a complete census of SCs, till the smallest masses, in the areas imaged by the survey. To this aim, we developed a procedure to discover even the faintest, low-luminous, and sparse still unknown SCs. Our SC finding algorithm needs only the star coordinates as input parameters, and searches for local over-densities in the sky, based on density maps obtained through a Kernel Density Estimation (KDE) algorithm. In practice, we search for small regions in the sky (of the order of tens of arcsec) with a number of stars significantly higher than the local average field. The procedure also automatically derives the centre of each over-density and estimates its radius. This algorithm has been tested on a suite of Montecarlo simulations and artificial generated SCs to demonstrate its robustness. On this basis, we estimated an efficiency of the procedure of about 90\%, achieved even in the worst possible case, that is poorly populated SCs embedded in regions with a high density of field stars. We ran the searching algorithm in the outermost and poorly studied regions of the LMC by taking advantage of the catalog provided by the majority of the YMCA tiles (65 and 12 sq. deg. towards East and West of the LMC, respectively) and to the STEP tiles close to the West of the LMC. As result, we detected 55 high reliable candidate SCs, of which 35 (60\% of the sample) were new discoveries, in a total area of 79 square degrees in the LMC surrounding in a range of distances between $\sim 4$~kpc and $\sim 13$~kpc. We exploited the colour-magnitude diagrams (CMDs) of the detected SCs, after having properly de-contaminated from field or foreground stars with a customized procedure, to estimate their properties, such as age, reddening and metallicity via isochrone fitting. The age distribution of the newly discovered SCs revealed a peak at about 2-3 Gyr which strongly indicates that a past close encounter between the LMC and SMC enhanced the star cluster formation activity. Even more interestingly, we detected for the first time the presence of tens of candidate SCs formed during the so-called ``age-gap'', an interval ranging from 4 to 10 Gyr in the LMC, which was thought to be almost totally devoid of SCs. Among the known SCs included in our photometry, the new age estimate obtained for KMHK~1762, based on the accurate and deep YMCA photometry, indicated that it is much older than previously evaluated, making it one of the very few (the third) confirmed LMC age gap SC ever discovered. Our findings allow us to suggest a possible solution for the age gap problem, which has puzzled the researchers for three decades. We propose that the age gap is not a real physical feature, but instead, the result of an observational bias, originated by the combination of too shallow photometry carried out by past surveys and a limited investigation of the LMC outskirts, where the lower field stellar density makes it easier to spot even the faint and sparse SCs.\par In the context of the analysis of the YMCA tiles through the cluster searching algorithm, we also discovered a peculiar SC likely belonging to the LMC (YMCA-1), which, based on the YMCA data appeared distant, old and very metal-poor. The follow-up deep photometry ($g\sim$27 mag) with FORS2@VLT which we obtained for YMCA-1 allowed us to confirm that it is old and metal-intermediate ($t \sim 11.7$~Gyrs, $Fe/H \sim -1.12$~dex). It is placed at about 55 kpc from the sun, which is sufficiently close to the LMC to suggest it belongs to this galaxy, even if we cannot rule out completely its belonging to the MW halo. It is also a faint and very compact ($L_g = 10^{2.1 \pm 0.3} L_{\odot}$, $r_h=3.5 \pm 0.3$~pc). These characteristics make YMCA-1 an exceptional object among the old LMC SC population. Indeed, YMCA-1 has age and metallicity similar to those of the other 15 known old LMC GCs, but structural characteristics are completely dissimilar, as it is two or three order of magnitudes fainter. In particular, it resides in a transition region of the M$_V$-r$_h$ plane, in between the ultra-faint dwarf galaxies and the classical old clusters, and close to SMASH-1, another faint stellar system recently discovered in the LMC surroundings. Regarding the SMC SC system, we took advantage of the STEP survey photometric catalogue to perform a deep analysis of 170 already known SMC SCs located in the main body of the galaxy. We derived their surface brightness profiles (SBPs) and obtained their main structural parameters by fitting their SBPs with Elson, Fall \& Freeman (EFF) and King models. In addition, we also exploited their CMDs to estimate the ages for a subsample of 134 SCs. It was the first time that such a large sample of SCs in the SMC was homogeneously characterized in terms of their sizes, ages, luminosities and masses, exploring also a wider region of the parameter space, down to hundreds of solar masses. The outcomes of this work confirmed the existence of a physical mechanism that induces an increase of the core radius after 0.3-1.0 Gyr. In the light of the very large mass interval investigated in our work, we suggested that the process driving the inner expansion is mass-dependent, as none of the SCs having $\log (M/M_{\odot}) \leq 3.5$~dex analysed in this work undergoes the expansion. This result represents a strong constraint for the different physical mechanisms proposed to explain such an expansion.\par In addition to the study of the past evolution of the LMC with SCs, we also focused our attention on the detection of faint low-luminous sub-structures originated by recent or past tidal stripping events. These stellar tides represent coherent structures in both position and velocities, and thus they can be revealed as over-densities in the phase-space of the LMC stars. In this context, we exploited the exquisite results from the {\it Gaia} early data release 3 (EDR3) to search for over-densities in the six-dimensional parameters space constituted by photometric, kinematics and astrometric information by means of an unsupervised Gaussian Mixture Modelling (GMM) clustering algorithm. The GMM was able to disentangle the LMC, SMC and MW stellar populations, thus returning a genuine sample of LMC stars which was used to investigate the low surface outer regions of the LMC. In this context, we reported the discovery of a new diffuse stellar sub-structure protruding up to $\sim 20$ degrees from the LMC centre to its North-Eastern side, which we called the North-East structure (NES). Particularly noteworthy is that the NES is placed in a region where N-body simulations predict a bending of the LMC disc due to tidal stresses induced by the MW. In the final part of this thesis, we studied the possibility of recovering the faint tidal signatures of merger events for more distant systems, where it is not possible to resolve the galaxies' individual stars any longer. To this aim, we developed the COld STream finder Algorithm (COSTA) that, through a deep friend-of-friend approach, analyze the position and radial velocity of discrete tracers (as could be globular clusters and/or planetary nebulae) to look for cold kinematic substructures in the phase-space. We applied COSTA to the relatively distant Fornax cluster since previous photometric surveys demonstrated that many interactions between the galaxy members are still ongoing. COSTA detected 13 cold substructures and for the first time it kinematically confirmed a stream already discovered in the deep photometry of the Fornax cluster. It is worthwhile to point out that the procedure developed and described in this thesis can be easily generalized for any stellar system, from the MW to the edge of the local Universe ($\sim$~50 Mpc), to decipher the evolutionary path of the systems investigated and unveil their history of mass assembly through the manifold interactions they experienced.

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