The study of galaxies, their formation, evolution and structure, is one of the main research interests at Leiden Observatory.
Galaxies are where stars live and die, and whenever we look deep into space, galaxies are what we see everywhere. Their centres often harbour supermassive black holes that can give rise to quasars and active galactic nuclei, which are some of the brightest objects in the universe. Their stars contain a record of the past history of the galaxy and by studying galaxies at different distances we look back in time and can reconstruct how they evolve.
Galaxies form through the gravitational collapse of clouds of intergalactic matter. Small galaxies merge together to form larger ones, building up a rich hierarchy of structures in the process. The stars and active nuclei in galaxies interact with their environment by pervading it with radiation, by polluting it with heavy elements, and by driving supersonic winds into intergalactic space. These feedback processes are not well understood, but are thought to have profound consequences for the formation of subsequent generations of galaxies. Theoretical work on the formation of galaxies at Leiden includes both analytic modelling and large-scale computer simulations.
By observing stars in galaxies, researchers in Leiden learn about their ages, their heavy element content and the star formation history of the galaxies. But stars not only provide information about themselves: by studying the motion of stars we can infer the mass content of galaxies and this provides some of the most convincing evidence for the presence of dark matter in the Universe. Leiden astronomers study the dynamics of distant galaxies using the largest telescopes on Earth, while closer to us the GAIA satellite will provide velocity vectors for a billion stars in our Galaxy over the coming decade.
A galaxy emits radiation over a wide range of wavelengths, and this encodes a wealth of information about the baryonic content of the galaxies, such as the amount of stars, the gas content - both atomic, ionised and molecular - the star formation activity and the physical conditions in the interstellar medium, and the presence and activity of black holes.
For all these reasons, a multi-wavelength approach to studying galaxies is fundamental and this is a cornerstone of the observational studies of galaxies in Leiden - from the very nearby galaxies in the Local Group, through large samples of galaxies in what we call the relatively nearby Universe using large surveys such as the Sloan Digital Sky Survey and those taking place with the new VISTA survey telescope in Chile, out to the most distant galaxies known using ultra-deep images from the Hubble Space Telescope.
Much of the energy emitted by stars is absorbed by dust, and re-radiated in the far infrared. Leiden astronomers study these processes using the Herschel space telescope and a range of sub-mm and mm telescopes around the world. This is complemented by studies in the radio wavelength range where one can also study energetic processes taking place close to the supermassive black holes in the centres of most galaxies. Leiden is strongly involved in the LOFAR project which will provide a vast amount of information on the radio Universe.
The outer parts of galaxies are usually too dilute to emit much radiation, but if there are bright sources, such as quasars, in the background, then they can be studied in great detail through the light it absorb from bright background sources such as quasars. Quasar absorption line studies currently provide some of the best constraints on the distribution and composition of matter in and around galaxies, particularly in the early universe.