Nuclear star clusters in the Virgo Cluster of galaxies

Date

2018-11-23

Authors

Spengler, Chelsea

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Abstract

It is readily accepted that many galaxies are inhabited by dense, compact objects deep in their centres, manifesting as supermassive black holes (SMBHs) and/or nuclear star clusters (NSCs). Their widespread presence and apparent similar scaling relations with properties of their hosts implies that SMBHs and NSCs are two related flavours of central massive object (CMO) that play essential roles in their hosts' evolution. However, the formation conditions required for CMOs, the exact behaviour of these scaling relations, and the interplay among CMOs, their hosts, and the environment remain open questions, and are particularly poorly understood in lower-mass galaxies where NSCs are the dominant CMO. This thesis contributes to the answers to these questions through a study of ultraviolet, optical, and near-infrared imaging of NSCs and galaxies provided by three recent surveys of the Virgo Cluster: the ACS Virgo Cluster Survey (ACSVCS), Virgo Redux, and the Next Generation Virgo Cluster Survey (NGVS). The analysis of the masses, ages and metallicities for a choice sample of 39 nucleated early-type galaxies with the complete wavelength coverage provided from all three surveys supports complex formation scenarios for the NSCs, involving a stochastic mix of dissipative and dissipationless processes. However, trends in the structural parameters of the NSCs show that the brightest NSCs tend to be flattened, suggesting that NSC formation may be dominated by dissipative processes in more massive systems, compared to dissipationless star cluster infall dominating in less massive galaxies. A comparison of these photometrically-derived stellar population parameters with those from available high quality optical spectra shows that estimated metallicities from the two samples are consistent, which is encouraging for using broadband photometry to derive stellar population parameters when spectroscopy is not feasible. Probing the effects of environment with the unprecedented sample available in the NGVS first requires a method to identify distinct environments through the detection of substructures in an objective, self-consistent way. I introduce a novel clustering algorithm and validate its performance using NGVS and 12 Virgo analogues from the Illustris simulations. This validation also permits a test of the lambda cold dark matter (ΛCDM) model's ability to replicate observed structures on cluster-sized spatial scales. The algorithm successfully recovers already-known Virgo substructures along with multiple intriguing new substructure discoveries, verified using available recessional velocities and distances from surface brightness fluctuations. Additional tests tentatively suggest that ΛCDM does not reproduce the level of substructure in Virgo; however, an expanded sample of observed clusters is necessary for a statistically robust conclusion. Lastly, I expand the analysis of structural and photometric parameters to encompass all NSCs and galaxies with measured parameters in the NGVS, and combine this with the substructure identifications to explore how the properties and relationships of NSCs, nucleated galaxies, and non-nucleated galaxies change throughout the environments of the Virgo Cluster. I detect a clear dependence on environmental density for the NSC occupation fraction, but, interestingly, the sizes, shapes, masses, colours, and scaling relations of NSCs and their hosts appear unaffected by environment. I also reaffirm that nucleated galaxies are consistently more concentrated and rounder than their non-nucleated counterparts, as well as redder in the luminosity ranges where NSCs are most abundant. These relationships also remain constant throughout different environments. One possible interpretation of these results is that environment is important only for the initial creation of an NSC; alternatively, all NSC growth mechanisms may be influenced equally by the environment.

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Keywords

astronomy, galaxy evolution, stellar populations, star clusters

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