The Morphology of the Milky Way

This thesis presents an investigation into the morphological features of the Milky Way, the exact structure of which is somewhat of an unknown. We begin with a discussion of the problem at hand, and a review of the literature and methodology associated with determining Galactic structure (Chapter 1). The methodology of the investigation is to use numerical simulations to reproduce the structure of the interstellar medium (ISM) gas under the effect of gravitational forces that represent possible morphologies of the Milky Way, such as spiral arms and inner bars. The ISM is simulated using smoothed particle hydrodynamics (SPH), which has been tailored to ISM scales by the inclusion of cooling, heating and a simple chemical network, discussed in Chapter 2. The Milky Way is first assumed to be grand design in nature, with analytic potentials representing the various arm and bar components. Simulations are then compared to longitude velocity CO emission observations to assess the quality of the reproduction of Galactic morphology. These results are shown in Chapter 3, where best fitting models have a bar pattern speed within 50-60km/s/kpc, an arm pattern speed of approximately 20km/s/kpc, a bar orientation of approximately 45 degrees,and arm pitch angle between 10-15 degrees. While nearly all observed emission features are reproducible, there is no model that reproduces all simultaneously. Using both bar and arm components together we find a better match to the data, but still no perfect reproduction. Models with two arms lack many of the observed features, but models with four arms produce too much local emission in the inner quadrants. Chapter 4 shows more sophisticated synthetic observations, created using a radiative transfer code. Resulting emission features are broadly in keeping with those seen in observations, the strength of which appears a strong function of gas surface density. The analytic potentials are then replaced by a set of discretised mass components that represent the stellar system, which is the subject of Chapter 5. Using a live N-body disc then allows for the dynamic creation of bar and arm features, from which further synthetic observations are produced. Transient arm and bar features are relatively easy to produce, though not necessarily simultaneously. Arm patterns showing two to five arms and some with an effectively flocculent structure are created, with pitch angles around 20 degrees. The pattern speed of which tends to decrease with radius, highlighting that the arms are material rather than wave-like in nature. Best fitting synthetic observations show that a four-armed spiral pattern provides good agreement with observations, more so than that of the fixed potentials, with clear reproduction of nearly all arm features. However, an inner bar appears necessary to remove excess emission seen towards the Galactic centre, which was not present in these models.