The Evolution of Mass and Light

The fellowship programme is centred upon the Herschel Satellite's two largest imaging surveys. Using these, the programme will investigate two fundamental but poorly understood aspects of galaxy evolution. Herschel operates in an unexplored region of the electromagnetic spectrum (0.1-0.5 mm) where an incredible 50% of the Universe's radiation is emitted. The sources that dominate at these wavelengths are young dust enshrouded star forming galaxies that typically evade detection in the optical. A tantalisingly small fraction of these galaxies have been seen at other wavelengths from the ground comprising only a very bright distant subset. These rare galaxies are merely the tip of the iceberg, not representative of the whole population. Frustratingly, little is known about the evolution of these distant systems and their relationship with local galaxies because the underlying population bridging the huge void has not been detected. This is a vital missing piece of the galaxy formation puzzle that has proven a significant obstacle. Herschel is satisfying a long awaited demand for a sensitive, high resolution facility to rapidly survey the sky in the sub-millimetre and finally detect a significant proportion of this elusive population.

The first part of the programme exploits the fact that the sub-millimetre is extraordinarily well suited to detecting gravitational lenses. The Herschel survey data will contain around 500 new strong lens systems out to a distance looking back over two thirds of the Universe's age. The first milestone is to identify and characterise galaxies acting as lenses to produce the largest and most distant sample to date. Secondly, applying a powerful reconstruction technique developed by the author to the lenses to measure mass profiles, the morphology of the baryons and the dark matter will be measured (in some cases separately) to investigate their evolution over the last 70% of the Universe's history. This is an exceptional opportunity to constrain models of structure formation in a way that has not been feasible until now.

Lensing also magnifies the flux of background objects above the detection threshold. Therefore, in addition to studying the evolution of the lenses themselves, the sample will be used to probe the population of dusty sources fainter than the survey sensitivity. Furthermore, using the aforementioned reconstruction technique, highly magnified undistorted surface brightness maps of each source will be computed to analyse in detail the morphology of the faint (and therefore generally more distant) dusty sources. Finally, the lens sample will place the most restrictive lensing limits on the cosmological model to date in a way that is complementary to other methods.

The second part of the fellowship programme will apply a new method for optimally measuring the evolution in luminosity of the hundreds of thousands of new Herschel sources detected. This will be combined with the framework established over several years by the author to finally provide a definitive study of the link between local systems and the dusty star forming galaxies that prevailed when the Universe was only 40% of its current age. Specifically, this involves measuring the growth of stellar mass and evolution of star formation in sub-millimetre galaxies (using another new method developed for reconstructing star formation histories) and measuring their transformation into other galaxy types as they evolve.

In terms of scale and viability of the programme, the Herschel data account for an impressive 1500 hours of observations over 700 square degrees of sky. This reflects a large investment from the UK astronomical community and one which must be justified by maximising scientific exploitation and impact.