A study of stellar populations in the last 5 billion years using Integral Field Spectroscopy

Thesis

In this thesis, I study the properties of quiescent and star-forming galaxies with integral field spectroscopy. The study of quiescent galaxies focuses on inferring the low-mass end of the stellar initial mass function (IMF). To accomplish this, we make spatially resolved measurements of a number of “gravity-sensitive” absorption features in the spectra of three nearby early-type galaxies. These vary in strength between otherwise identical (low-mass) dwarf and giant stars, allowing for measurement of the dwarf-to-giant ratio and hence the low-mass IMF.

In NGC 1277 and IC 843, I use the Oxford SWIFT instrument to study the variation of these features as a function of radius. I find strong gradients in NaI0.82 absorption but flat FeH0.99 profiles in both cases, although the data cannot distinguish between chemical abundance gradients or a gradient in the IMF being the cause. I then fit models to the integrated spectrum of each galaxy, finding an IMF slope of x = 2.27±0.17 IC 843 and x = 2.69±0.11 in NGC 1277. Next, I use MUSE to make quantitative inferences about an IMF gradient in NGC 1399. I use full spectral fitting in 18 annular bins to measure a super-Salpeter IMF slope in the galaxy’s centre, which is radially constant out to 0.7 effective radii before becoming consistent with a Milky-Way IMF at R_e. These findings add weight to the theory of a two-stage formation process for massive elliptical galaxies.

I then introduce K-CLASH, which uses KMOS to observe star-forming galaxies in four galaxy clusters from the CLASH survey. I describe the observations, data reduction and analysis process, and present a novel probabilistic method of determining whether a target resides in a cluster or field environment. I compare and contrast the properties of field and cluster galaxies, finding that the cluster sample follow a star-formation rate “Main Sequence” relation which is offset to smaller SFR by 0.18 dex. Furthermore, I measure the size of galaxies in optical light and Hα emission and find the average ratio r_50Hα/r_50Rc to be significantly smaller in cluster galaxies than in the field comparison sample. I find no significant differences in ISM conditions. These observations are evidence for environmental reduction of star-formation, al- though deeper observations are required to discriminate between a global effect (due to ISM heating) or outside-in processes (such as ram-pressure stripping).

Authors: Vaughan, S

• DOI: 10.5287/ora-mzqdwbq6d
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Integral Field Spectrogphy; Stellar Populations; Galaxy Quenching; Initial Mass Function; Galaxy Environment; Galaxy Evolution; Galaxy Formatiom
• Subjects: Astrophysics; Physics