Star clusters as engines of galaxy evolution

Abstract: 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.