galaxies, formation and evolution

galaxies, formation and evolution

Galaxies must have condensed out of the gases expanding from the Big Bang, beginning at a time when the average density of the Universe was roughly the same as the current mean density of a galaxy. Details of the formation of galaxies are still highly uncertain, as is their subsequent evolution. Astronomers are not agreed, for example, on the extent to which the different types of galaxy have been determined by conditions at formation or by later evolution. The current picture of galaxy formation assumes a cosmic power spectrum of fluctuations in the cold dark matter. Gravitational instability causes the overdense regions to collapse and the gas falls into these developing gravitational potential wells. This may lead to dissipation of gravitational energy heating the gas. That gas which can cool rapidly forms the visible stars of the galaxy; as time progresses more and more fluctuations become unstable and the potential wells of galaxies merge and cluster. This is known as hierarchical clustering. Although the details of the evolution of the cold dark matter can be calculated fairly precisely, the behavior of the gas is much more complicated, with energy and metals injected by supernovae playing a major role in the visible appearance of the final galaxies.

Elliptical galaxies probably formed in the densest regions of the original fluctuations. Rapid star formation converted almost all the available gas to stars in less than a thousand million years. The most distant ellipticals should contain a proportion of younger bluer stars, and observations of distant radio galaxies display a bluer continuum that may be consistent with intense star formation. Spiral galaxies formed by the slower accumulation of fragments or collapse of larger clouds in less dense regions, and where turbulence caused the protogalaxy to rotate. Fairly rapid star birth during formation produced the old stars of the halo and central regions; the remaining gas settled into a disk, where stars continued to form much more slowly and interstellar gas remains to the present day.

The most dramatic examples of galaxy evolution are caused by external factors. For example, galaxies may collide and merge, or the hot intracluster medium may strip gas from a rapidly moving galaxy.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006