Mapping galaxy properties with large-scale galaxy surveys
Student thesis: Doctoral Thesis
Large scale observations of the Universe have highlighted that a galaxy’s mass, morphology, and environment are all key factors in a galaxy’s evolution. To what extent each of these contribute however, is still an openly debated question. In this thesis, I attempt to address the aforementioned question providing original research based on a sample of galaxies gathered from the MaNGA IFU survey. Stellar population properties are derived from the data using a newly developed full spectral ﬁtting code, FIREFLY, and state of the art stellar population models. A number of tests using mock galaxies, globular clusters and data from the SDSS DR7 are conducted with FIREFLY in an attempt to assess the codes ability to accurately recover stellar population properties and star formation histories. FIREFLY recovers galaxy properties reliably down to S/N ≥ 5 and S/N ≥ 10 for mock galaxies with both simple and complex star formation histories, respectively. The ages and metallicities derived for globular clusters and galaxies from the SDSS are in good agreement with determinations from colour-magnitude diagram ﬁtting, stellar spectroscopy and other full spectral ﬁtting codes. FIREFLY is then applied to MaNGA data enabling three scientiﬁc analyses to be conducted. First, I construct a value added catalogue based on the spatially resolved stellar population properties of MaNGA galaxies, derived from both FIREFLY and absorption line-strength indices. Secondly, I investigate the dependence of light- and mass-weighted stellar population properties, and their radial gradients, on galaxy mass and morphology. Full star formation and metal enrichment histories are reconstructed, and the impact of diﬀerent stellar population models and full spectral ﬁtting routines on the derived properties is quantiﬁed. Light-weighted age gradients are found to be ﬂat for early-type galaxies, and negative for late-type galaxies (∼− 0.11 dex/Re), suggesting an ‘inside-out’ formation of discs. Mass weighted age gradients of early-types are positive (∼0.09 dex/Re) pointing to an ‘outside-in’ progression of star formation. Negative metallicity gradients are detected for both morphological types, but these are signiﬁcantly steeper in late-types. Metallicity gradients correlate with galaxy mass, with negative gradients becoming steeper with increasing mass. The correlation is stronger for late-types, with a slope of d(∇[Z/H])/d(logM)∼−0.2±0.05, compared to d(∇[Z/H])/d(logM)∼− 0.05 ± 0.05 for early-types. Lastly, I study the eﬀect of galaxy environment on the derived stellar population gradients using three complementary measures of environment, namely the Nth nearest neighbour method, the tidal strength parameter, Q, and distinguishing between central and satellite galaxies. In all cases, no signiﬁcant correlation between the gradients and environment is found, both at ﬁxed galaxy mass, and for both morphologies. The scientiﬁc analysis presented in this thesis suggests that the cumulative merger history of galaxies plays a relatively small role in shaping their metallicity gradients and that internal processes, such as supernova and AGN feedback, matter most to the determination of stellar population gradients. These results set stringent constraints on future models of galaxy formation and evolution.
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