The Role of Gas in Star Cluster Formation

Abstract

Stars form in clustered environments embedded inside giant clouds of molecular gas across galaxies throughout the observable Universe. These clouds are turbulent entities that can go on to collapse into a collection of dense filamentary structures, along which, star formation is expected. Stars form along these filaments and coalesce into small sub-clusters that eventually merge with one another inside the cloud leading to the growth of a star cluster. In this thesis, we perform a suite of simulations that model the evolution of clusters as they grow inside their host cloud through accretion of the surrounding gas and mergers with other clusters. We model our systems as collections of stars and gas using the AMUSE framework and the ASURA+BRIDGE code. We first consider gas accretion onto the cluster in the form of a background gas distribution and dense filaments with velocities directed towards the cluster centre. Both of these processes cause a change to the cluster structure and filaments in particular efficiently provide the cluster with bound, star-forming gas. Movement through an ambient background environment causes the cluster to lose a fraction of its bound gas that is dependent on the velocity of the cluster, and the density of the medium. We then consider sub-cluster mergers inside a background gas environment whose distribution we inherit from previously run, larger scale star cluster formation simulations that were unable to adequately resolve individual stars. By resolving the individual stars in our simulations, we are able to track the dynamical evolution of the clusters as they merge. We find that mergers result in clusters that are anisotropically expanding and rotating. Both of these signatures are consistent with recent observations of gas-free star clusters. The clusters that merge remain gravitationally bound because of the high mass of background gas present (≈ 10^4 − 10^5M⊙) which also lowers the amount of unbound stars produced from mergers to < 3%. We show that gas is necessary in promoting the increase in cluster mass through mergers by simulating a merger without background gas. This simulation results in a non-monolithic cluster contrary to the simulation that does include background gas which results in a monolithic cluster after the merger. Lastly, we improve our simulation physics through the use of the ASURA+BRIDGE code which allows us to simulate stars and gas simultaneously while also including prescriptions for stellar feedback and the formation of new stars. We rerun a simulation from our previous work with this new framework to constrain the effects of stellar feedback and star formation and find that new star formation contributes to the mass growth of the cluster in two key ways: star formation from gas that is compressed by the merger process, and star formation from nearby filamentary gas that becomes accreted onto the merged cluster. Star formation also enhances the anisotropic expansion and rotation inherited from the cluster merger such that they are still present after the cluster has removed its background gas through feedback and star formation. We find that dynamical signatures that the merger took place are still present after the cluster has removed most of its background gas and argue that these signatures will have an effect on the long term evolution of the cluster.