Dr. Meng Xiang-Gruess
University of Cambridge
A. Magnetic field and dark matter in disc galaxies: The most prominent theory for the description of grand-design spiral galaxies today is the density wave or Lin-Shu theory (Lin & Shu 1964). In the first part of my PhD work, we have extended the original DWT to a theory that is able to describe a more complete system. We studied a composition of a stellar and a gaseous disc embedded in an axisymmetric dark matter halo. In addition, the gaseous disc is crossed by a vertical magnetic field. For this galaxy model, the dispersion relation was derived analytically. For the special case of a stationary spiral structure, a relation between the dark matter amount and the magnetic field strength could be found. This relation was studied in detail for different cases, e.g. different surface mass density ratios between gaseous and stellar disc or pitch angles of spiral arms. The dispersion relation was derived analytically by hand. The study of the dependence of the dark matter amount on the magnetic field strength was performed with C++ and visualized with gnuplot.
B. Numerical simulations of disc galaxies: Numerical simulations in the past years have mainly concentrated on two features of disc galaxies. On the one hand, the response of gas discs to a fixed background grand-design spiral potential that is thought to be produced by the more massive stellar disc was studied. On the other hand, realistic isolated disc galaxies without any external potential have been studied. The next logical step in numerical simulations is the realization of classical density waves in the stellar discs that propagate on their own and the formation of shock waves in the gaseous disc due to their dissipative nature. In addition, possible influences of the gaseous phase or other mechanisms on the density wave should be studied. The main focus of the second part of my PhD work is therefore the numerical study of time-dependent evolution of grand-design spiral structures in realistic disc galaxies composed of gas, stars and dark matter. For the production of a grand-design spiral structure in the disc, we applied either a perturber or an external potential. The simulations were performed with the N-body / SPH (smoothed particle hydrodynamics) code GADGET-2 (Springel 2005) that is written in C and is massively parallelised. In addition to the original version of GADGET-2, several additional routines were written such as star formation, stellar feedback or external potential. For the analysis of e.g. star formation rate, appropriate output routines were also included. The snapshots were visualized and studied with the program SPLASH (Price 2007). Aditional parameters, e.g. star formation rate, were studied with gnuplot.
Satellite galaxies such as Segue 1 play an important role in understanding cosmological and galaxy formation processes. Recent discoveries of many dark matter dominated dwarf spheroidal galaxies, e.g. Segue 1, have led to several corrections of previous results (e.g. Xiang-Gruess et al. 2009) and a new understanding of the formation of galaxies.
Various observational missions can be used for the detection and study of exoplanets. For example, a recently proposed method (Lammer et al. 2010, 2011) for the study of so-called ENAs (from the stellar wind) can be used for the investigations of planet atmospheres.
In the past decades, the formation of extrasolar planets has been tackled by many astrophysicists. A research field which is closely linked to planet formation is the evolution of protoplanetary discs. Detailed studies are performed today for a better understanding of planet formation and the so-called planetary migration. Thereby, different combinations of planets of various masses and their interactions with the protoplanetary disc are studied. Our current work focuses on the interaction between planets and a circumstellar disc. For this project, we use a N-Body/SPH method in order to track the evolution of the disc and the planets in the 3D space.