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Galaxy physical properties from population model fitting - Janine Pforr

In the last two decades astronomers carried out a large number of galaxy surveys tuned towards the study of galaxy formation and evolution. With the ever improving technology, increasing telescope sizes of ground-based telescopes and the development of space-based telescopes it has become possible to detect galaxies at a time when the Uni- verse was only a few hundred million years old. However, for the majority of galaxies a detailed spectroscopic analysis is not possible due to their distance and limited telescope time. Thus, many surveys rely on photometric data alone to help unveil the properties of galaxies. One of the most important areas of study within galaxy formation and evolution is the analysis of the galaxy stellar population parameters as these can provide us with information about the star formation histories of galaxies and when and possibly how they assembled their mass. A popular approach in the literature is the fitting of synthetic spectral energy distributions inferred from stellar population modelling to the multi-wavelength photometry of galaxies. However, this approach comes with a large number of fitting parameters all of which are essentially user-dependent and will bias the result in one way or another. The aim of this thesis is to investigate the accuracy and efficiency of spectral energy distribution fitting as derivation technique for the galaxy physical properties, such as age, stellar mass, dust reddening, etc., as a function of the fitting parameters, such as star formation histories, age grids, metallicity, initial mass function, dust reddening, reddening law, filter setup and wavelength coverage and stellar population model, and to find the setup of parameters that recovers the properties best. In particular, we investigate in detail the dependence of the derived properties on the assumed wavelength coverage and exact filter setups. Mock galaxies with known properties serve as test particles for this exercise. The synthetic spectral energy distributions used in this thesis are based on the Maraston (2005) stellar population models. Literature results which investigate similar problems are obtained using the models of Bruzual & Charlot (2003). Firstly, the fitting is carried out under the assumption that galaxy redshifts are known mimicking surveys for which galaxy redshifts are derived spectroscopically. Then we study the case in which the redshift is not known and needs to be determined alongside the galaxy physical properties which is the case for most photometric surveys. In general, we find that - using normal template star formation histories as widely used in the literature - ages and stellar masses of star-forming galaxies are underestimated, reddening and star formation rates are overestimated. This is due to a mismatch in star formation history and the overshining effect. The addition of the rest-frame near-IR appears to be crucial for the derivation of robust results. For aged galaxies with little or no on-going star formation we find that a setup covering a wide range of star formation histories and metallicities works best when the fit is carried out excluding dust reddening. For high redshift star-forming galaxies we find that a new type of star formation history (inverted-τ models which start forming stars at high redshift) recovers stellar masses and star formation rates best. The parameters of truly passive galaxies are much better determined. In order to ease the comparison of literature data that was analysed with different fitting parameter setups we provide scaling relations for the transformation of stellar masses between different setups. Our results concerning the importance of the wavelength coverage in the fitting are particularly useful for the planning of future surveys and observation proposals. We apply our findings from the study of mock galaxies to various samples of real galaxies which cover different redshift ranges and galaxy types. We derive the stellar population properties for a sample of star-forming galaxies at z ∼ 2 from the GOODS-S survey using inverted-τ models (with high formation redshifts) and show that the obtained dust reddening and star formation histories are in excellent agreement with those derived from other methods. We also show how the wrong set of fitting parameters can lead to unrealistically young ages, low stellar masses and high star formation rates which are a pure artefact from the fit. Furthermore, we study a sample of low redshift, predominantly passive galaxies from the SDSS-III/BOSS survey for which we use the spectral model of Maraston et al. (2009) that is tuned to the needs of this particular type of galaxies. We find that BOSS galaxies are mostly passive, old and massive at each redshift in the range 0 < z < 0.7. Finally, we complement the study of SDSS-III/BOSS galaxies by deriving stellar masses for the SDSS-I/II galaxies in a similar fashion. We conclude that the simultaneous derivation of stellar population properties of galaxies from spectral energy distribution fitting is difficult but that these properties can be very well derived provided the right setup and wavelength coverage are used in the fitting. We also conclude that more work is needed to better match star formation histories of aged galaxies with little on-going star formation in order to improve estimates of stellar population parameters.
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