| 1. |
- Adamo, Angela, 1981-
(författare)
-
Super Star Clusters in Blue Compact Galaxies : Evidence for a near-infrared flux excess and properties of the starburst phase
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Luminous Blue compact galaxies (BCGs) are metal-poor actively star-forming systems, characterised by bright ultraviolet and blue luminosities. Hubble Space Telescope high-resolution data have revealed that the luminous star-forming knots in these galaxies are composed of hundreds of young massive star clusters. In this work we present a systematic study of the star cluster populations in BCGs with important implications for the formation history of their host systems. The studied galaxies show recently increased star formation rates and a high fraction of massive clusters, probably as a result of minor/major merger events. The age distributions have a peak of cluster formation at only 3 - 4 Myr, unveiling a unique sample of clusters still partially embedded. A considerable fraction of clusters (30 - 50 %), mainly younger than 10 Myr, shows an observed flux excess between 0.8 and 2.2 μm. This so-called near-infrared (NIR) excess is impossible to reproduce even with the most recent spectral synthesis models (that include a self-consistent treatment of the photoionized gas). The origin of the NIR excess, which still remains unexplained, challenges our understanding of the cluster formation process under extreme conditions. The results achieved in this work have produced important insights into the cluster formation process in BCGs. We suggest that the BCG environment has most likely favoured the compression and collapse of giant molecular clouds into compact massive star clusters. The cluster formation efficiency (i.e., the fraction of star formation happening in star clusters) in BCGs is higher than the reported 8 - 10 %, for quiescent spirals and local star-forming galaxies. Luminous BCGs have a cluster formation efficiency comparable to luminous infrared galaxies and spiral starburst nuclei (the averaged value is about 30 %), suggesting an important role of the merger event in the cluster formation.
|
|
| 2. |
- Messa, Matteo, 1988-
(författare)
-
Young Stellar Clumps and Clusters in the Local Universe
- 2017
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)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.
|
|
