Star Formation in Merging Galaxies

Abstract

Star formation is detected in any galaxy with an appreciable amount of gas and the vast majority of stars form in embedded clusters. However, very few bound star clusters are detected in the Milky Way, which has led to the hypothesis that as many as 90% of these clusters are disrupted during the early stages of their evolution. Many of those that do survive are likely to be progenitors of globular clusters that are observed in elliptical galaxies, and in the bulge and halo regions of spiral galaxies. In order to understand how star clusters form and evolve, and the disruption mechanisms they encounter, it is necessary to observe star clusters during their earliest evolutionary stages. This is difficult to observe in quiescent galaxies like the Milky Way where only a few newly formed star clusters have currently been detected. Gas-rich interacting and merging galaxies however, host thousands of newly formed star clusters and are ideal targets to observe the evolution of star clusters.

In this thesis star clusters are observed in galaxy mergers for a range of evolutionary states using both photometric and spectroscopic data. We find evidence that tidal interactions have produced new cluster populations in two separate galaxy mergers. Analysis of the cluster populations in these mergers also suggest that tidal interactions do not destroy more clusters than they produce, in disagreement with simulations in the literature. Furthermore, we observe several star clusters that may be the product of merging between multiple clusters. These star clusters show spectral features consistent with multiple episodes of star formation. If these clusters remain bound for the next few Gyrs, they could explain the multiple stellar population feature observed in globular clusters in the Galaxy. We also explore the possibility of star clusters evolving to form the halo cluster population of their host merger before the progenitor disks coalesce. Star clusters are generally found in the halo population of a galaxy merger once the progenitor nuclei coalesce. However, some galaxy mergers like the Antennae harbour star clusters that may be forming their halo cluster population before their progenitor nuclei
merge. This suggests that star clusters begin to form the halo cluster population of a galaxy merger before the progenitor nuclei coalesce. We recommend future surveys of gas-rich major galaxy mergers to study young ( ∼ few hundred Myr) star clusters in dense and quiescent regions to further support our findings.