In recent years, globular clusters have been thought of as one of the possible sources of the reionization of the inter-galactic medium. They have ages consistent with the reionization epoch and can potentially produce massive quantities of ionizing photons. In this thesis I present new tests of how globular clusters form and determine the role they played during the epoch of reionization.
This work is based on the Aquarius simulations which are the first suite of simulations capable of resolving the full mass range of potential globular cluster formation sites. With a particlemass mp = 1.4⇥104 M⊙, darkmatter concentrations (halos) as small as 106 M⊙ contain a minimum of ⇠100 particles. Thus, this simulation provides one with a unique opportunity to study in detail the evolution of globular clusters. This thesis is broken down into two major works.
In my first major work, I use the Aquarius simulation to test a model of metal-poor globular cluster formation based on collapse physics. In this model, globular clusters form when the virial temperatures of haloes first exceed 104K as this is when electronic transitions allow the gas to cool efficiently. I calculate the ionizing flux from the stars in these first clusters and stop the formation of new clusters when all the baryonic gas of the galaxy is ionized. The model is successful in that it predicts ages (peak age ⇠ 13.3 Gyrs) and spatial distributions of metal-poor globular clusters which are consistent with the observed populations of the Milky Way. I also investigate a merger model for metal-rich cluster formation in central gas disks. I find it is possible for younger (⇠ 7–13.3 Gyrs), more centrally-located clusters to form which are consistent with the Galactic metal-rich population.
In my second major work, I present detailed radiative transfer simulations of the reionization history of the Milky Way by metal-poor globular clusters. I determine their contribution to reionization by modelling their radiation using a three dimensional nonequilibrium chemistry code. I also allow additional globular clusters to form in regions that were originally suppressed as they were ionized, if they later become neutral. This spatial treatment of the ionization field leads to drastically di↵erent numbers and spatial distributions when compared to models where globular cluster formation is simply truncated at a given redshift. The resulting radial distributions are not consistent with those of Milky Way metal-poor globular clusters, unless an additional ionization source is added at later times (z ⇠ 10).
I estimate that metal-poor globular cluster contributions to the reionization of the local (i.e. 23 h−3 Mpc3 centred on the host galaxy) volume and mass by redshift 10 could have been as high as 98% and 90%, respectively. This means globular clusters are important contributors to the reionization process on local scales at high-redshift until more photon-rich sources dominate the photon budget at later times. The surviving clusters in all models have a narrow average age range (mean = 13.34 Gyr, σ = 0.04 Gyr) consistent with current ages estimates of the Milky Way metal-poor globular clusters.
I also test a simple dynamical destruction model and estimate that ⇠60% of all metalpoor globular clusters that formed at high redshift have since been destroyed via tidal interactions with the host galaxy.