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Extragalactic archaeology using spatially-resolved spectroscopy from large-scale galaxy surveys

Student thesis: Doctoral Thesis

Our current working theory of the Universe explains galaxy evolution in the context of hierarchical structure formation. However, the details underpinning galaxy assembly times, star formation, and chemical enrichment remain to be understood. There is currently a hot debate on the universality of the stellar initial mass function (IMF) in galaxies. The IMF is a crucial player in galaxy evolution, and it strongly influences feedback and chemical enrichment in galaxies.
Using data from the integral eld unit survey Mapping Nearby Galaxies at Apache Point Observatory, part of the Sloan Digital Sky Survey, we investigate the IMF and chemical abundances as a function of galaxy mass, type, and radius, for the largest sample of galaxies to-date. Spectra are stacked in elliptical annuli in order to increase the signal-to-noise ratio and mitigate problems due to sky residuals. A range of absorption features in the optical and near-infrared are used to break degeneracies between stellar population parameters.
We derive radial gradients out to the half-light radius of 366 early-type galaxies with masses 9:9 - 10:8 log M/M. They display at gradients in age and negative gradients in metallicity, consistent with the literature. We find that C, Mg, and Ti trace each other both as a function of galaxy radius and galaxy mass, and obtain shallow radial gradients. Conversely, N and Ca are generally oset to lower abundances. The under-abundance of Ca compared to Mg implies delayed enrichment of Ca through Type Ia supernovae, whereas the correlated behaviour of Ti and the lighter elements, C and Mg, suggest contributions to Ti from Type II supernovae. Furthermore, we measure strong negative radial gradients for [N/Fe] and [Na/Fe], of up to -0:25±0:05 and -0:29±0:02 dex/Re respectively. These gradients become shallower with decreasing galaxy mass. These are strongly correlated with the total metallicity, suggesting metallicity-dependent Na enrichment, and secondary N production in higher mass early-type galaxies.
We find a gradient in the IMF slope, with a bottom-heavy IMF in the centre (typical mass excess factor of 1.5) and a Milky Way-type IMF at the half-light radius. This pattern is mass-dependent with the lowest mass galaxies in our sample featuring only a shallow gradient around a Milky Way IMF. Our results imply that the local IMF- relation within galaxies is steeper than the global relation, and hint towards the local metallicity being the dominating factor behind the IMF variations. We also employ dierent stellar population models in our analysis and show that a radial IMF gradient is found independently of the stellar population model used.We then extend our analysis in mass and morphology with an increased
sample size of 1900 galaxies, spanning 8:6 - 11:3 log M/M. We find negative age and metallicity gradients for late-type galaxies, and the age gradients steepen with increasing galaxy mass. The abundances of elements show similar gradients with the local velocity dispersion, independent of morphology, hence these relations appears to be driven by galaxy mass or velocity dispersion rather than galaxy type. Additionally, we compare IMF-sensitive indices such as NaI and TiO, to provide clues as to how the IMF might behave in galaxies of different types.
Differences in the stellar population gradients for early- and late-type
galaxies provide clues regarding their formation scenarios and our results
place stringent constraints on future models of galaxy formation and chemical
evolution.
Original languageEnglish
Awarding Institution
Supervisors/Advisors
  • Daniel B. Thomas (Supervisor)
Award dateMay 2019

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