Theoretical Radiation Transfer
Kenneth Wood
Large ground- and space-based telescopes provide a wealth of data spanning high energy X-rays through the optical spectrum, to radio waves. How this light forms the spectral lines and complex nebular patterns we observe depends on how it is produced and how it interacts with matter along its path to Earth. Theoretical radiation transfer modelling at St Andrews seeks to unite theories with high resolution observations to determine the structure and dynamics of circumstellar and interstellar matter. We achieve this with detailed Monte Carlo computer codes that simulate the emission, absorption and scattering of light within three dimensional density structures.
Our Monte Carlo codes produce synthetic spectra and multiwavelength images of collapsing gas clouds and accretion discs. Comparing these models with data enables us to test theories of how protostellar clouds evolve into dense discs from which planets will form. The left frame in the above figure is a Hubble Space Telescope image of the protoplanetary disc HH30 IRS, and on the right is a simulation produced by our code. Modelling multiwavelength data from such objects enables us to determine not only the geometry, but also the chemical composition and dynamics of discs.
These radiation transfer codes can be adapted to any size scale and we are also using them to study the formation and appearance of the extended ionised gas in our Milky Way and other galaxies. The clumping of interstellar gas and dust plays a major role in the diffusion of high-energy photons from stars in the Galactic plane to gas in the Galactic halo. We are extending this work to even larger scales, to model the leakage of ionising photons from galaxies into the intergalactic medium early in the Universe's history.