Young Stellar Clumps and Clusters in the Local Universe

Abstract: The thesis introduces the process of star formation at the scale of galaxies, where a turbulent--dominated interstellar medium (ISM) can locally collapse under self-gravity. This collapse results in a star formation process which is hierarchical both in space and time. At the high-density peaks of this hierarchical ISM is possible to form star clusters, collections of stars which can stay gravitationally bound for several Myr. Young star clusters (YSCs) are a common product of star formation in the local universe and, in fact, all star forming galaxies have been shown to host a YSC population. They have masses ranging from few hundreds \msun\ to ~107 Msun, and radii of few parsecs (up to ~10 pc). It has been proposed that the most massive of these systems could survive for several Gyr, forming populations of globular clusters.The majority of star formation happens in a clustered fashion and studying the properties of YSC is therefore of fundamental importance to understand the star formation process across galaxies. YSCs are much more luminous than single stars and can therefore be studied to larger distances. They are usually host a stellar population with uniform properties, in particular age and metallicity, which make them a useful probe of the host galaxy properties.In addition, due to the hierarchy of star formation, YSCs are themselves clustered into cluster complexes, or clumps. Such huge clumps (R ~1-3 kpc, M ~108-109 Msun) are major blocks of the structure of high-z galaxies and their study can shed light on star formation and galaxy evolution over cosmological times.In my papers, I used the new HST UV observations obtained from the Legacy Extragalactic UV Survey (LEGUS), combined with archival HST BVI observations to study the YSC population of the nearby interacting galaxy M51. In the first paper I built a catalog of the YSC population and studied its properties in the entire galaxy. A truncated mass function was found for the population, meaning that there is a physical mechanism that inhibits the formation of clusters above a specific mass. The disruption of YSC was found to be very moderate, in line with what observed in similar galaxies. The measured fraction of star formation happening in bound clusters was ~20%. These results were compared to studies of YSC populations in local galaxies and to models.In the second paper, instead, the YSC properties were studied in function of the environment inside M51. The shape of the mass function and its truncation were found uniform at different galactocentric radii, while the age function showed more variability. The biggest differences in YSC properties were recovered when the spiral arm and inter-arm environments are compared, suggesting a difference which can already set in when the giant molecular clouds, progenitor of YSCs, start forming.