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The formation and evolution of massive clusters in extragalactic environments

Hollyhead, KE (2017) The formation and evolution of massive clusters in extragalactic environments. Doctoral thesis, Liverpool John Moores University.

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Abstract

There are a host of open questions in the study of massive clusters relating to cluster formation and evolution. Understanding these processes can be useful in studying their host galaxies and how they have evolved. The formation of globular clusters is also a well debated topic, which is yet undecided and requires many more observations to constrain the theories. Here I present the work carried out during my PhD, with the goal of furthering our understanding of cluster formation and evolution using observations of massive clusters of various ages. Firstly, HST WFC3 data of the well studied face-on spiral galaxy M 83, combined with an existing cluster catalogue, was used to investigate the timescale by which young massive clusters become free of gas. This has implications for globular cluster formation theories, in addition to the survival of clusters at young ages. The presence of Wolf-Rayet stars was also investigated within the clusters and the unreliability of Hα photometry in young cluster age and mass fitting was explored. Secondly, the cluster population of NGC 1566 was used to investigate the cluster mass function and disruption in the galaxy. Whether the mass function has a truncation in the form of a Schechter function and whether disruption is environmentally and mass dependent are two questions that still persist in this area. For NGC 1566 I find that the mass function does show a truncation and using the observed luminosity function in conjunction with models, that an underlying Schechter mass function fits the observations well. Additionally the galaxy shows evidence for environmentally dependent disruption as the average timescale for the disruption of a 10⁴ msun cluster varies with galactocentric radius. A difference in age in radial bins is also indicated in a colour change found with U-B between consecutive bins, that shows more young clusters towards the centre of the galaxy and fewer at the edge. Finally, low resolution FORS2 spectra of two intermediate age massive clusters in the SMC (Lindsay 1 and Kron 3, 6-8 Gyr old) were used to look for the signatures of multiple populations (MPs), as observed in ancient GCs. The main driver behind this project was to investigate the possibility that YMCs can be considered young GCs and used to constrain their formation, and also to explore the role cluster age has in determining the presence of MPs. A subpopulation of N-enriched stars was found in each cluster, consistent with the presence of MPs. This indicates that MPs are not limited to ancient GCs and their formation mechanism must be operating until a redshift of at least 0.65, much later than the peak of GC formation at ≈ 3. It hints at a common formation mechanism between massive clusters of varying ages, including GCs, and suggests that YMCs can be used to constrain GC formation. The publications from these projects have contributed mainly to constraining GC formation theories and provides evidence for commonality in the formation mechanism used to produce GCs and YMCs alike.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: stellar clusters; galaxy:individual:ngc1566; galaxy:individual:m83; galaxy:individual:smc
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Date Deposited: 09 Jan 2018 09:22
Last Modified: 21 Jul 2018 00:50
DOI or Identification number: 10.24377/LJMU.t.00007730
Supervisors: Bastian, N and Davies, B
URI: http://researchonline.ljmu.ac.uk/id/eprint/7730

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