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UBC Theses and Dissertations

Structure of disk dominated galaxies MacArthur, Lauren Anne

Abstract

A robust analysis of the structural properties of late-type spiral galaxies, based on the modeling of bulge and disk brightnesses, is presented. The resulting structural parameters provide important constraints to predictions of structure formation models for the bulges and disks of spiral galaxies. Two independent programs were developed which exploit 1D profiles and 2D images. These algorithms use a non-linear least-squared fitting technique to reduce the profiles and images into bulge and disk components described by smooth mathematical functions. The reliability of the decomposition algorithms was thoroughly tested with ~ 250,000 simulated profiles and images. The simulations include testing for sensitivity to initial estimates and seeing and sky measurement errors, and cover the full range of late-type disk and bulge sizes, as well as the bulge shape. These tests provide us with a set of guidelines for the reliability of our decompositions. Among others, we find that an additional grid search must be applied in order to recover the bulge shape parameter reliably. Another important conclusion from these simulations is that the 2D decomposition technique does not provide a significant advantage over the 1D algorithm for recovering axi-symmetric galaxian parameters to warrant the extra computational time and effort. This is likely only true for the small bulges considered here. Hence, our results for the actual data reductions are based solely on the 1D decomposition technique. Decomposition results are presented for the largest multi-band data set of late-type galaxy images to date. We use a sub-sample which includes a total of 523 images in the BVRH passbands for 123 nearby late-type spiral galaxies with face-on and intermediate inclinations. The galaxies are divided into three major types according to the shape of their luminosity profiles in the optical and infrared passbands; type variations are ultimately linked to the presence of dust. For each galaxy brightness profile a total set of 1080 decompositions were reduced and analyzed according to the selection criteria provided by the simulations. We have used repeat observations to test the reliability of our decompositions. The typical parameter errors are ~20% for the bulge and ~5% for the disk components. We also point out that some types of disk profiles, dubbed "Type II," cannot be properly described by a simple two-component (bulge + disk) model. A new approach which includes, but may not be limited to, the effects of dust extinction is required. The final set of galaxy parameters is studied for parameter variations and correlations both in the context of profile type differences and wavelength dependence. The bulge shape parameter shows a clear range but has a mean value corresponding to an exponential profile. The size ratios of bulges and disks are also tightly coupled with (re/h) = 0.23 ± 0.10. A natural interpretation of this remarkable observation is that the bulge formed via viscous transport of material from the disk, in agreement with current models of secular evolution. The distribution of disk scale lengths shows a clear decreasing trend as a function of wavelength. This is interpreted as being due to dust extinction (and probably to some degree by stellar population effects). Detailed modeling of colour gradients in these galaxies will be addressed in a subsequent analysis.

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