full spectral fitting of stellar population models for studies of galaxy evolution
Student thesis: Doctoral Thesis
In this work we present a new full spectral fitting code called FIREFLY. It is a c2-minimisationcode that obtains thousands of spectral fits in order to build probability distribution functions of stellar population properties, and includes an innovative method to treat dust attenuation.We use the code to determine galaxy properties, including age, metallicity, stellar mass and dust extinction, of over 2 million galaxy spectra, both from point-source and from resolved galaxy surveys, using modern high-resolution stellar population models. We analyse the results to assess the red shift evolution of galaxy properties, and the importance of their internal processes. We test a set of stellar population models based on three stellar libraries to assess the systematic effects of changing model ingredients and provide a detailed assessment of degeneracies in the models, in all stages of the thesis. After introducing the central concepts in galaxy evolution and astrophysics, we describe the advancements of stellar population models and their use in the derivation of galaxy properties.We then give a detailed overview of the landscape of full spectral fitting and its application to observational data. We describe the motivation, features and function of FIREFLY, performing careful calibration on a set of mock galaxies and globular clusters. We also very carefully assess the degeneracies in model spectra and measure the uncertainties from applying a full spectral fitting approach to optical data. We apply FIREFLY to two observation point-source surveys with millions of galaxy members:the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7), and SDSS Data Release9, the galaxy data of which is called the Baryon Oscillation Spectroscopic Survey (BOSS). We present the full star formation histories of all of the galaxies in the surveys and important subsamples within them. We use the derived galaxy properties as the calibrator for combining these surveys into one large survey across red shifts 0.0 < z < 0.8. This enables us to assess the red shift evolution of the most luminous and passive galaxies across both samples. Significantly, we use FIREFLY in the first scientific publication of SDSS-IV, for the MaNGA Integral Field Unit survey. We retrieve stellar population maps and radial profiles from high spatial resolution prototype observations of 18 galaxies, encompassing thousands of individual spectra. Our analysis gives detailed measurements of the precision to which one can recover stellar population gradients and resolved maps as a function of observational conditions and stellar population model ingredients, paving the way for future work both in MaNGA and other spatial galaxy surveys.
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