Master Thesis

Title: Lyman α Resonant Scattering in Young Galaxies
Supervisors: Jesper Sommer-Larsen & Johan Peter Uldall Fynbo
Period of work: January 2006 – January 2007

Short explanation: I have developed a code (a computer program) that simulates how galaxies look.

Motivation

The Lyα line (λ = 1216 Å, corresponding to the energy difference between the ground and the first excited state of neutral hydrogen) is a very important diagnostic in a wide range of fields of astrophysics, not least in galaxy formation, providing us with extensive information on redshift, dynamics, kinematics, morphology, etc.

When observed in Lyα, young galaxies and proto-galaxies (i.e. at redshifts greater than z ∼ 3) are often observed to be significantly more extended on the sky than when observed in continuum bands. To gain a better understanding of the processes of galaxy formation and the physical conditions governing these epochs, we wanted to investigate whether the resonant scattering of the emitted Lyα in the surrounding gas can explain these observations.

Project

Toward these ends, I have developed a 3D, high resolution numerical code, capable of treating an arbitrary distribution of source Lyα emission, neutral hydrogen density, temperature, and peculiar velocity field of the interstellar medium. The code is a "Monte Carlo" code, i.e. it follows the indivual paths of a high number of photons as they scatter stochastically in real and frequency space until they escape the galaxy and can travel freely toward a virtual observer, providing a detailed surface brightness map and spectrum of the radiation.

All relevant quantum physics concerning the scattering processes is taken into account. The code has been tested thoroughly on various idealized configurations for which analytical solutions are applicable.

Results

The code was applied to a simulated Lyman Break Galaxy at z = 3.6 of representative physical properties (it evolves into a Milky Way-like galaxy at z = 0), resulting from a fully cosmological smoothed particle hydrodynamics code. It is found that proper treatment of the radiative transfer can reproduce the observations beautifully. The impact on the observations of the angle from which the galaxies are viewed is investigated, as well as the possibility of deriving intrinsic physical properties of such systems from observed surface brightness and spectra. Moreover, various numerical techniques are discussed.

For a more thorough description of the code and the results, see Laursen & Sommer-Larsen (2007). You can also download my master thesis here:
          MScThesis.pdf
          MScThesis.ps.gz