Dynamical Studies of Galaxy Evolution

Galaxies are gravitationally-bound collections of billions of stars, together with gas and the mysterious dark matter that pervades the Universe. The larger-scale structure that we see in the Universe, comprising clusters, superclusters, filaments and voids, is outlined by the galaxies that lie within it, so galaxies can truly be thought of as the 'building bocks' of the Universe. The aim of this proposal is to obtain a deeper understanding of how these building blocks form and evolve. To-date, most studies have concentrated on reproducing the spatial properties of galaxies - how the individual stars are distributed within them. However, a galaxy is fundamentally a dynamical entity: if its constituent stars were not all moving around on their individual orbits, then it would rapidly collapse under its own gravity. Thus, the motions of the stars are as important as their spatial distribution: a theory of galaxy formation that predicts the wrong velocities for the stars fails as categorically as one that predicts the wrong spatial arrangement. Although galaxies are generally too distant to see individual stars move within an astronomer's lifetime, we can still measure their velocities along the line of sight by the Doppler shift: just as the change in pitch of a fir engine's siren tells us whether it is approaching or receding, so the shift in wavelength of light from stars tells us their speed. Thus, but splitting the light from the stars in a galaxy into a spectrum and studying whether the light has been shifted toward the red or blue, one can study the stars' motions. This technique has been used for many years to measure basic properties of galaxies such as how fast they rotate, but here I am proposing to make measurements of some of the subtler properties. For example, studies of the motions of stars in fuzzy elliptical galaxies have mainly been restricted to the inner parts of these systems, where they are bright enough to obtain a reasonable spectrum. However, we have built a new instrument that can measure the velocities of stars in the faint outer parts of these systems. It does this by picking out stars at the end of their lives called 'planetary nebulae'. These stars emit most of their light at a single colour, making it easy to see how this light is shifted in wavelength even in the faintest parts of galaxies. When we started using this instrument, we were surprised by how slowly these stars were moving: we had expected to find that the gravitational pull of halos of dark matter around the galaxies should keep the stars travelling at high speed. Since they are moving slowly, it appears that, unlike the rest of the Universe, these galaxies do not contain much of the mysterious dark matter. To try and understand where the dark matter has gone, we are extending this study to more galaxies. We are also developing other new techniques to study the masses and other properties of galaxies. For example, light escaping from the centre of a galaxy uses up energy in escaping its gravitational pull, and the amount of energy it loses depends on the amount of mass from which the light has to escape. When light loses energy in this way, its wavelength becomes shifted toward the red end of the spectrum. Thus, we can find the masses of the central parts of galaxies by seeing how much the light from the centre of a galaxy has been 'gravitationally redshifted' by this effect. By comparing the amount of mass near the centres of galaxies to the amount of light produced there, we will be able to figure out what kind of stars the galaxy is made of even in these regions where the light is all blurred together so we cannot see the individual stars. By comparing these properties that we observe in nearby galaxies to the predictions from theories of how galaxies form, we will be able to see which theories succeed because they produce 'end products' that look like the galaxies around us, and which fail because