Probing the Complexities of ‘Simple’ Stellar Populations in Compact Stellar Systems

Abstract

In this Thesis, we present an unprecedentedly large sample of compact stellar system integrated light spectra and spectral line measurements, much of which has been obtained for this thesis through new observations or reduced for the first time with custom reduction pipelines. We present the results of the analysis of the stellar populations of several hundred compact stellar systems ranging in size from globular clusters to massive compact ellipticals and beyond. We adopt previously used analysis techniques as well as developing our own with the aim to recover “simple” parameters such as age, metallicity and alpha abundance. Based on comparison with resolved star studies of Milky Way globular clusters, we find that age is poorly constrained while metallicity is returned accurately and [α/Fe] is returned reliably under specific conditions.

Problems with age measurements are a common problem in integrated light studies of compact stellar systems, usually manifested in globular clusters as an underestimation in age. We suggest this is due to populations of hot blue horizontal branch stars at low metallicity ([Fe/H] ≤ -1.0) and blue straggler stars (for higher metallicities of [Fe/H] ≥ -1.0) which are unaccounted for in standard simple stellar population models. In order to better constrain the ages of the predominately metal-rich, massive, compact stellar system population, we therefore create a new set of models using a simple algorithm to add a fraction of younger star light to sMILES models to account for blue straggler stars. We find that these models produce better age estimations for Milky Way globular clusters and increase age measurements in general for all objects in our sample, while having minimal effect on metallicity or alpha measurements. Under the assumption that all compact stellar systems host populations of blue stragglers we therefore suggest the common usage
of such models when studying at least compact stellar populations, if not galaxies in general

We explore other parameters of compact stellar systems including relations between their mass and metallicity and dive into specific chemical abundances in the form of nitrogen enhancements. We suggest possible formation scenarios for many objects based on their derived star formation histories, metallicity spreads, measured ages and metallicities. We find nitrogen enhancement in many objects from globular clusters to the most massive galaxies hinting at a possible nitrogen - density correlation. We also identify another UCD which, based on its position in the mass-size plane, high metallicity, and high integrated velocity dispersion, is highly likely to host a supermassive black hole. Finally we identify potential signatures of star formation in a red and dead red nugget galaxy and signatures of potentially the first active galactic nucleus in a compact elliptical galaxy.