Galaxy spectral analysis in the era of large-scale galaxy surveys

  • Oliver Steele

Student thesis: Doctoral Thesis


In this work I address two of the big questions in modern astrophysics; the role of environment as a driver of galaxy evolution, and the the role of mass in star formation and stellar population evolution. I use one of the most powerful tools available to the astrophysical community, large-scale galaxy spectroscopy, to contribute towards the answers to these dilemmas. I construct a data analysis pipeline based on the public codes gandalf and pPXF to extract gas and stellar dynamics, emission line statistics, absorption line indices and stellar population parameters from these galaxy spectra. I test and calibrate this pipeline against existing results for the Sloan Digital Sky Survey Data Release 7, and find it to provide accurate measurements.
I use the emission line results from this to probe the dependence of star formation and ionisation characteristics on stellar mass, local environment and global environment in the Galaxy AND Mass Assembly survey. I find that mass is the main driving factor behind the presence of star formation and determining different ionisation sources, and see a trend with increasing mass from star forming objects to those hosting active galactic nuclei via composites of the two. Local density plays a role only at the highest densities, and is considerably less significant than mass; global environment is found to have negligible impact. This suggests that star formation quenching is primarily a mass-driven process, with active galactic nucleus feedback being a likely candidate for the environment independent process involved in our sample.
I stack objects together from the Sloan Digital Sky Survey III: Baryon Oscillation Spectroscopic Survey in order to produce high-signal-to-noise spectra for the purpose of absorption line measurement and the subsequent modelling of stellar population parameters. I use this to investigate the dependence of age, metallicity and α/Fe on mass (using stellar velocity dispersion as a proxy for dynamical mass) and redshift. I find that light-averaged age, metallicity and α/Fe all increase with velocity dispersion, which are predictions of the downsizing paradigm, where the least massive galaxies form their stars later, over more extended timeframes and less effciently than more massive galaxies. Age is also seen to increase with redshift, which is simply the result of everything in the Universe getting older, whilst I see no evidence of metallicity or α/Fe changing with lookback time. Investigating how galaxies age when compared to the Universe, I find that more massive galaxies appear to age faster than the Universe whilst less massive galaxies age slower. I hypothesise that this is due to the different star formation histories of galaxies with differing masses, and test this by compiling models with varying stellar histories and comparing them to our observations. I find that as mass decreases, I require more extended periods of star formation that peak more recently. At the high-mass end, the relationship between the most massive bins is best reproduced by a passively evolving population whose stars formed at higher redshift than I observe. This is a clear result of downsizing, and sets tough restrictions on future models of galaxy formation and evolution
Date of AwardJan 2015
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorDaniel Thomas (Supervisor) & Claudia Maraston (Supervisor)

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