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- Della Bruna, Lorenza, 1992-
(författare)
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Star Formation and feedback at key physical scales for galaxy evolution
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Feedback from young, massive stars plays an essential role in the self-regulation of star formation in galaxies, and in shaping the galaxies' global properties. This phenomenon originates at small scales, surrounding the stars, but has been observed to be effective up to galactic-wide scales. The exact mechanism which allows the ionising radiation to escape the star-forming regions (HII regions), initially still embedded in their natal molecular hydrogen gas, is still unknown. Constraining the escape of ionising photons from HII regions is also relevant in order to explain the origin of the diffuse ionised gas (DIG) that is observed to contribute up to 50% to the Ha luminosity of nearby galaxies. I present the results of the study of stellar feedback in two nearby galaxies (NGC 7793 and M83), at spatial scales that critically connect the sources of ionisation with their immediate surroundings. We determine the fraction of DIG and study its properties and origin. We find that in NGC 7793 ionising sources located in the DIG are producing a sufficient amount of hydrogen-ionising (LyC) photons to explain the diffuse gas emission. In M83, on the other hand, the DIG is ionised by a mixed contribution of photoionisation and shocks. We investigate the link between LyC leakage from HII regions and their stellar and gas properties. We find that the age spread of the stellar population in the region does not seem to imply a higher leakage. Also the ionisation structure of the regions (e.g. the presence of "channels" that are transparent to the LyC photons) appears to be uncorrelated with escape in our sample. In M83, we also study the relative importance of different types of stellar feedback. We find that the pressure exerted by the ionised gas is always dominant over the direct radiation pressure. When the total HII region pressure is compared to the environmental pressure, we observe that regions near the galactic centre are in equilibrium with the surroundings, whereas regions in the disk are overpressured and are therefore expanding. We also find that changes in the local environmental conditions are the dominant factor in setting the ionised gas pressure, and that the pressure terms are linked to the physical properties (age and mass) of the young star clusters powering the regions. In the near future, observations from the James Webb Space Telescope will allow us to study the most embedded star-forming regions with a resolution comparable to the present one.
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| 2. |
- Sirressi, Mattia, 1995-
(författare)
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Star clusters as engines of galaxy evolution
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 3. |
- Messa, Matteo, 1988-
(författare)
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Young Star Clusters and Clumps in the Local Universe : The effect of galactic environment on star formation
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Stars do not form in isolation, but rather out of a hierarchical structure set by the turbulence of the interstellar medium. At the densest peak of the gas distribution, the star formation process can produce young star clusters (YSCs), which are gravitationally bound systems of stars with mass between ~100 and 106 MSun and typical size of few parsecs. At larger scales, clusters are themselves arranged into cluster complexes, on scales of hundreds of parsecs and up to kiloparsec scales, which are usually referred to as ‘star-forming clumps’.Observations of local star-forming galaxies show that YSCs form over a wide range of galactic environment. However, it is not yet clear if and how the galactic environment relates to the properties of star clusters. I present the results obtained by studying the YSC population of the nearby spiral galaxy M51. We find that the cluster mass function, dN/dM, can be described by a power-law with a -2 slope and an exponential truncation at 105 MSun, consistent with what is observed in similar galaxies in the literature. The shape of the mass function is similar when looking at increasing galactocentric distances. We observe significant differences, however, when comparing clusters located in the spiral arm with those the inter-arm environments. On average, more massive clusters are formed in the spiral arms, as also previously found for the YSC progenitors, the giant molecular clouds (GMCs). Finally, we see that clusters are more quickly disrupted in denser environments, as expected if their disruption is mainly caused by tidal interaction with dense gas structures like the GMCs.I have also undertaken the analysis of the interplay between galactic scale properties and larger star forming units, the stellar clumps. The analysis has been conducted in a sample of 14 low-redshift starburst galaxies, the Lyman-Alpha Reference Sample (LARS). The elevated star formation rate densities of such galaxies allow to form clumps with densities comparable to clumps at high-redshift, typically more massive and denser than what is normally observed in the local universe. The clumps in the LARS galaxies contribute to a large fraction to the UV flux of the galaxy itself (in many galaxies > 50%), resulting in galaxies which appear ‘clumpy’. In agreement with formation theories we observe that clumpiness is higher in galaxies with higher SFR surface density and dominated by turbulent gas motion.
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