Any change in the properties of individual galaxies, or of populations of galaxies, with time. The properties of galaxies in the Universe today provide evidence that galaxy evolution does occur, and in more than one way. For example, elliptical galaxies today contain mostly old, low-mass stars which do not emit much light. At the time these stars formed, about 10 billion years ago, the galaxies must also have contained many short-lived, massive stars of high luminosity, so these galaxies must then have been much brighter than they are today. This change in the intrinsic luminosity of a galaxy with time is known as luminosity evolution. There is also a second type of evolution, still at work today. Some galaxies with close neighbours have distorted shapes, implying that the galaxy is being disrupted by the gravitational pull of its neighbour. The strong gravitational attraction may eventually cause them to merge into a single galaxy. The number of galaxies is therefore gradually changing as a result of such galaxy mergers; this is known as number-density evolution.
In the most popular models of galaxy formation, which are often called ‘bottom-up’ or hierarchical models, the first galaxies which formed from the gas that filled the early Universe are thought to have been small. Under the influence of gravity, these small galaxies gradually merged to form larger ones. A consequence of such a model is that in the Universe today the largest galaxies should be younger than the smallest galaxies, and also that at early times there should have been fewer large galaxies than there are today.
It is possible to watch galaxy evolution at work, thanks to the finite speed of light. For example, when we observe a galaxy 2 billion light years away we are looking back in time to see what that galaxy looked like 2 billion years ago. Of course, galaxies at such large distances are quite faint, and this kind of cosmic archaeology became possible only in the mid-1990s with the construction of new large telescopes and the installation of sensitive new instruments. The first big step in this research came in 1992–4 with the Canada–France Redshift Survey (see redshift survey), which discovered several hundred galaxies with redshifts of up to 1. A galaxy with a redshift of 1 is about 8 billion light years away, and so appears as it did 8 billion years ago, over half the present age of the Universe. Follow-up images of these galaxies made with the Hubble Space Telescope (HST), however, showed that the Universe at that time was not dramatically different from the Universe today—the same types of elliptical and spiral galaxies were present, although there were more irregular galaxies than we see today.
Shortly after this survey, Lyman-break galaxies, which have redshifts of approximately 3, were discovered. We see these galaxies as they appeared 12 billion years ago. This time, HST images did not reveal the familiar large spirals and ellipticals but instead rather smaller objects. These discoveries were at least roughly in agreement with expectations that large galaxies have gradually been assembled from smaller ones over the last 13 billion years or so. The most distant galaxies currently observable have redshifts of 6, which means that we are seeing them as they appeared less than a billion years after the Big Bang.
Subjects: Astronomy and Astrophysics.