THE FORMATION AND EVOLUTION OF GALAXIES

Abstract

The past decade has witnessed the establishment of a “standard paradigm” for structure formation in the Universe. It is now universally accepted that the dominant matter component of the Universe is in some form of non-baryonic, weaklyinteracting dark matter. Structure in the dark matter originated from inhomogeneities that were generated shortly after the Big Bang during a period of accelerated expansion, termed inflation. These early inhomogeneities were gravitationally amplified as the Universe expanded. Eventually, material contained in initially overdense regions began to collapse. Small objects were the first to form and these later merged together to form larger and larger structures. This picture has received spectacular confirmation from a series of experiments designed to probe anisotropies in the cosmic microwave background radiation. As a result of these experiments, cosmologists now believe they know the values of most of the basic parameters of the Universe (for example the density parameter R, the value of the Hubble and cosmological constants and the amplitude of the power spectrum of initial fluctuations) to better than 10%. The development of structure in the dark matter component of the Universe is also extremely well understood, thanks to a program of detailed numerical simulations that have elucidated how structures such as clusters form from the merging of smaller lumps as they stream in along filaments of dark matter. In spite of these advances, the formation and evolution of galaxies remains poorly understood. In the standard picture, a galaxy will form when gas is able to reach high enough densities to cool, sink to the centre of a high density lump of dark matter (called a “halo”) and form stars. What happens to the galaxy after that de9e:ids cn the interplay between a host of complex physical processes. The most massive stars quickly run out of fuel and end their lives as supernovae. These supernovae may be responsible for reheating gas and expelling heavy elements from the galaxy, thereby altering its structure and slowing down the rate at which it can form stars. Galaxies will also merge with each other as their surrounding dark matter halos coalesce. During these mergers gas is compressed and the star