Stars form and live in hierarchical structures such as star clusters on the smallest scales up to groups of galaxies on the largest scales. The role of young and massive star clusters in the evolution of the host galaxies is one of the most active fields of research in modern astronomy. The most massive stars are able to inject into the interstellar medium large amounts of energy and momentum through their radiation, their stellar winds and ultimately by exploding as supernovae. Also because of their clustered configuration, massive stars have an impact on the host galaxy with their energetic output, which is often referred to as a whole with the term feedback. Feedback processes are invoked by numerical simulations in order to reproduce galaxies with realistic properties. In the most extreme scenarios stellar feedback can launch galactic winds that may suppress the star formation and mix the gas content not only within the galaxy but also with the intergalactic medium.

In this thesis I describe the scientific background of my doctoral studies and present my research project on the local starburst galaxy Haro 11 (z=0.021). Together with my supervisors, we use multi-band photometry from the  Hubble Space Telescope for resolving the nuclear starburst into tens of star clusters with masses between  10⁵ and 10⁷ solar masses and ages younger than 20 Myr. We observe that star-formation is propagating across the galaxy from the eastern side to the western side. Additionally, we model aperture-matched ultraviolet and optical spectroscopy and derive the physical properties of the stellar populations in the three starburst knots (A, B, C). This also allow us to estimate the following stellar feedback quantities: photo-ionisation rate, energy of stellar winds and supernovae, and their power. We compare the above measurements with the information on the kinematics of the ionised gas within the three knots, extracted from optical spectroscopy. Combining our analysis with previous works, we find a multi-phase medium with maximum outflow velocities up to 400 km/s. The three knots have at different times all hosted powerful outflows, whose energetics is consistent with the quantified stellar feedback. This indicates that the stellar winds and SN explosions have driven the detected outflows. Our work is important not only for shedding light on the physics of feedback, but also because it combines and compares different methodologies (i.e. photometry and spectroscopy, ultraviolet and optical wavelengths). 

I conclude with an outlook on the upcoming projects of my doctoral studies, which will exploit the methodology that I developed during the first two years of my PhD and extend it to a whole sample of star-forming nearby galaxies. Such projects also connect with the science that will be enabled by the future astronomical observatories.

The work on Haro 11 is the subject of a paper recently submitted to the journal MNRAS. A copy of the paper can be found at the end of this thesis.

Feedback in star forming galaxies is the key process that regulates how many stars form given the available gas reservoir. The radiation, energy and momentum released by the stellar activity and the active galactic nuclei change the physical properties of the gas in the galaxy such as the ionisation state, the density and the kinematics. In the most extreme scenarios, feedback is able to launch powerful outflows that can bring significant portions of gas out into the intergalactic medium, directly suppressing the star formation in the galaxies and therefore influencing their evolution. For non-active galaxies the dominant source of feedback is represented by the ionising radiation, stellar winds and supernova explosions of massive stars. Young star clusters (YSCs) are the natural habitat of massive stars and inject large amounts of radiation, energy and momentum into the surrounding interstellar medium (ISM). The fractal geometry of stellar clustering amplifies the impact of the stellar feedback on the surrounding gas in the ISM. In fact, the total outward momentum acting on the gas around a star cluster is given by the sum of the contributions from the single stars, which being at the centre of the physical system never cancel each other outward momenta. 

The main goal of this thesis is to investigate how stellar feedback originates at small scales, as well as to quantify its impact on the ISM gas surrounding young stellar populations. We use far-ultraviolet (FUV) spectroscopy of YSCs to study both the physical properties of the stellar population and the gas kinematics in the ISM. The first study presented in the thesis focuses on the starburst galaxy Haro 11, characterized by three knots of star formation, each populated by young star clusters of slightly different ages. We measure the stellar feedback in terms of photo-ionisation rate, mechanical energy and mechanical luminosity and we study its relation to the outflows of ionised gas traced by optical emission lines. The second and third works are both part of a survey with a sample of 20 YSCs named CLUES (CLusters in the Uv as EngineS). We consistently derive with multiple methods the ages, metallicites, masses, attenuation for all clusters. We model the gas kinematics tracing both the neutral and ionised phase, and we detect an outflow in most targets. This survey reveals for the first time the properties of outflows driven by YSCs at scales between tens and a few hundreds pc. We find that the relation found in previous works between galactic outflows and the star-formation properties of host galaxies extends to smaller scales. Both the work on Haro 11 and the CLUES survey serve as benchmark studies for future investigations of high-redshift galaxies, based on observations of upcoming facilities, that will open a window on the star formation at different epochs in the history of the Universe.