<>Adapted from The Astronomy and Astophysics Encyclopedia and T.M. Heckman

The most unusual property of the stars and gas in the nuclei of galaxies is the chemical composition. Astronomers refer to all the chemical elements heavier than hydrogen and helium as metals. The sun has about 2% of its mass in the form of metals, and this solar metal abundance is typical of the chemical composition of the bulk of the stars and gases in bright galaxies like our Milky Way. In contrast, the metal abundance in the nuclei of bright galaxies is apparently two or three times higher than that of the sun. Such high metal abundances are essentially unique to the nuclei of galaxies. Indeed, many galaxies show a steady decrease in metal abundance from the nucleus outward. The reason for the high content of metals in galactic nuclei is not entirely clear. Because metals are formed by nuclear reactions inside stars, the high metal abundance in galactic nuclei means that the material we see today has been extensively processed by previous generations of stars. One possibility is that the strong gravitational field in the nucleus has enabled it to retain a relatively larger fraction of the metals expelled by dying stars in the form of winds or supernova explosions than was possible in the outer parts of the galaxy.

The most conspicuous form of interstellar gas in the nuclei of galaxies is ionized gas (gas consisting of free electrons and the corresponding ions - atoms with one or more of their normal complement of electrons missing). In the nuclei of late-type galaxies, emission from this gas can be very bright, and its properties imply that the gas is kept in its ionized state by energetic photons emitted from hot young stars. Such regions of gas ionized by young stars are commonly found throughout late-type galaxies and not just in the nucleus.

The emission from ionized gas in the nuclei of early-type galaxies is usually weak. Surprisingly, the strength of this emission does not appear to be related to the relative numbers of young stars (which are scarce in such nuclei in any case). Moreover, the nature of the gas is inconsistent with ionization by normal young stars. The detailed properties of these so-called LINERs (low ionization nuclear emission-line regions) can be explained if either they are ionized by a source of photons that is much hotter than ordinary stars or ionized by shock waves resulting from high speed collisions between gas clouds or from explosions. LINERs are often taken as evidence that the nuclei of early-type galaxies are commonly in a state of low-level activity: a dormant, but still living monster may lurk at the heart of most such galactic nuclei.

The recent birth of the field of extragalactic millimeter-wave astronomy has led to the discovery that most of the interstellar matter in the nuclei of many spiral galaxies is in the form of molecular gas. Throughout the spiral disk of our own Milk Way galaxy, molecular gas is intimately related to the process of star formation. Thus, it is not surprising that the nuclei of galaxies that are actively forming stars are rich in molecular gas. Indeed, some starburst nuclei may contain as much molecular gas as an entire normal spiral galaxy.

Compiled by G.T.Petrov, 2004