Galaxy Formation

My research interests centre around galaxy formation. My specific interests are:

Lyα in Emission and Absorption

Luminosity function of faint lyman alpha emitters.

Galaxy formation is one of the central problems of Physical Cosmology. Neutral hydrogen plays an important role, linking the collapse of cooling gas into haloes with the formation of stars. Lyman alpha, hydrogen's strongest spectral line, can directly probe neutral hydrogen in the high redshift Universe. Lyman alpha can be observed in absorption in Damped Lyman Alpha systems (DLAs): high HI column density regions that dominate the neutral gas content of the Universe between z ~ 0-5. Lyman alpha in emission is an important signature of early, star-forming galaxies. Both populations, however, present significant theoretical challenges.

Rauch et al. (2008) performed an ultra-deep spectroscopic survey and discovered a new population of very faint, spatially extended Lyman alpha emitters, which they claimed to be the long-sought host galaxies of DLAs at z ~ 3. My work has tested and supported this claim. Ultra-faint observations of Lyman alpha emission have exceptional potential to directly probe the spatial distribution and kinematics of neutral hydrogen in early galaxies.

I have used my Lyman alpha code to perform detailed 1D radiative transfer calculations, investigating the spatial and spectral distribution of Lyman alpha emission due to star formation at the centre of DLAs, and its dependence on the spatial and velocity structure of the gas. The modelling reproduces the observed properties of both DLAs and the faint Lyman alpha emitters, including the velocity width and column density distribution of DLAs and the large observed spatial extent of the faint emitters. In the model, haloes hosting DLAs retain up to 20% of the cosmic baryon fraction in the form of neutral hydrogen. The scattering of Lyman alpha photons at the observed radii, which can be as large as 50 kpc, requires the bulk velocity of the gas at the centre of the haloes to be moderate. The model suggests that galaxies in haloes with virial velocity ~ 100-150 km/s account for the majority of DLA host galaxies, and that these galaxies at z ~ 3 are the building blocks of typical present-day galaxies like our Milky Way. These results are published in Barnes and Haehnelt, 2008 and 2009, and in my thesis.

3D Lyα Radiative Transfer

Column density of neutral gas. Lyman alpha emission. 2D lyman alpha spectra.

I have also performed 3D Lyman alpha radiative transfer simulations, building on numerical simulations of galaxy formation that include galactic winds and gas infall. The Lyman alpha emission region is larger and smoother than the cross-section for damped absorption by ~ 50%, with Lyman alpha photons scattered effectively by gas with column densities > 1017 cm-2. The spectra typically show two peaks, with the relative strength of the red (blue) peak being a reflection of the relative contribution of outflow (inflow) in the velocity profile. There is considerable variation in the observed line profile and spectral intensity with viewing angle. These more realistic models support many of the simplifying assumptions of my previous models, and have the potential to probe the important role of galactic winds in protogalaxies. The results have been published, and can be found in my thesis.

The images to the right show the column density (top panel), Lyman alpha surface brightness (middle panel), and a 2D spectra for a slit placed vertically through the centre of the halo (bottom panel).

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