Timothy M. Heckman
Unlike the nucleus of a cell, the nucleus of a galaxy is not a precisely defined entity or distinct subcomponent. Rather, it is simply the central-most part of a galaxy. The region referred to as "the nucleus" is roughly the innermost 1% of a galaxy. Most galaxies are rather symmetric in form with the density of stars decreasing smoothly from the center (nucleus) outward. Thus, the nucleus is not only the center of the galaxy, it is also the region of highest density. As such, the nucleus can also be thought of as the "bottom" of the galaxy: gas clouds or stars that move too slowly within the galaxy can be pulled inward by gravity toward the nucleus. This may have some interesting consequences, as described below.
The nuclei of galaxies have been extensively investigated by astronomers for at least two reasons. The first, more prosaic reason is that the nucleus is usually the brightest part of a galaxy (because the density of stars is highest there). This means that the nucleus is the most easily studied part of a galaxy. The second and more exciting reason is that the nuclei of galaxies are often the sites of qualitatively unusual energetic phenomena that are observed nowhere else. These are the so-called active nuclei.
Galactic nuclei (like galaxies themselves) are composed of stars and interstellar matter (mostly gas, plus small dust grains). To explain active galactic nuclei, some additional object must be present. Because the fundamental nature of this object remains mysterious, it is often referred to by deliberately vague and fanciful terms like "the monster" or "central engine."
Galaxies can be broadly classified into early-type galaxies, which consist predominantly of old stars, and late-type galaxies, which contain old stars, young stars, and cool interstellar gas clouds out of which young stars are formed. This pattern is repeated in the nuclei of early- and late-type galaxies. The nuclei of early-type galaxies apparently formed the great majority of their stars billions of years ago, and in so doing depleted the raw material needed to make new stars (cool, dense gas clouds). The properties of the nuclei of late-type galaxies are consistent with a rate of forming new stars that has been almost constant since the time the galaxies themselves formed. A spectacular exception is the class of starburst nuclei, which are apparently undergoing short-lived episodes of star formation at rates much higher than the past average rate.
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.
As already noted, an active nucleus is one in which processes are observed that cannot be readily explained by the mere presence of normal stars and interstellar gas clouds. By this definition, a starburst nucleus is not a truly active nucleus, but we will discuss such objects in this section because they are rare and can be highly energetic.
LINERs are the most common type of active nucleus, and may in fact be present at a very low level in the nucleus of every early-type galaxy. Their spectra are characterized by weak emission lines that have been significantly broadened by the Doppler effect, indicating high speed gas motions (typically a few hundred to a few thousand kilometers per second). LINERs usually contain a compact source of radio synchrotron emission that is qualitatively similar to (but much weaker than) the radio sources seen in radio galaxies and quasars.
Seyfert galaxies are usually spiral galaxies whose nuclei are exceptionally bright. A few percent of spiral galaxies contain a Seyfert nucleus. The spectrum of the nucleus shows Doppler-broadened emission lines whose widths are similar to those in LINERs but whose strengths are much greater. The gas is in a highly ionized state, requiring the presence of a source of photons of much greater energies than can be produced by ordinary stars. Direct evidence for this ionization source is provided by the strong ultraviolet and/or x-ray continuum emission observed from Seyfert nuclei.
Radio galaxies are usually elliptical or elliptical-like galaxies that are strong sources of radio sychrotron emission (emission produced by electrons moving nearly at the speed of light while spiraling around magnetic field lines). A few percent of bright elliptical galaxies are classified as radio galaxies. Although most of the radio emission arises from twin radio "lobes" located far outside the radio galaxy, there is convincing evidence that the lobes are powered by matter that has been expelled from the active nucleus of the galaxy: Narrow radio-emitting channels or jets link the distant radio lobes to a compact radio source in the nucleus.
Quasars were originally defined to be star-like (quasistellar) objects with large redshifts. Today they are believed by the great majority of astronomers to be the highly powerful nuclei of distant active galaxies. Quasars share many properties in common with Seyfert nuclei (strong, broad emission lines and powerful ultraviolet and x-ray emission). The subclass of "radio-loud" quasars, which are strong radio emitters, is closely related to radio galaxies in their observed properties.
Starburst nuclei can rival Seyfert nuclei or even some of the less powerful quasars in terms of their total power output. However, unlike Seyferts or quasars, the properties of starburst nuclei can be adequately explained by young stars (albeit a highly unusual number of such stars). Starburst nuclei often radiate most strongly in the infrared portion of the electromagnetic spectrum. This infrared emission comes from dust grains that have been heated to temperatures of several tens to several hundreds of degrees Kelvin by the ultraviolet light produced by the hot young stars. The presence of the dust is not surprising, because dust is found to be closely associated with cool, dense molecular clouds of the kind that are apparently present with great abundance in starburst nuclei.
Currently, the most popular theory holds that the monster that powers active galactic nuclei is a supermassive black hole, a region of high density within which the escape velocity exceeds the speed of light. The mass of the black hole must be at least several million times the mass of the sun for a Seyfert nucleus, and several billion times the mass of the sun for a powerful quasar. Energy would be produced by the supermassive black hole as its powerful gravitational field compresses and heats infalling gas, causing the gas to emit highly energetic photons before it falls into the hole and vanishes. Recall that galactic nuclei are at the bottom of the galaxy, a favorable location for accreting material to "feed" the monster. Although there are a variety of plausible lines of indirect evidence that favor this model, the case is by no means clear.
The difficulty in proving this model rests largely with the fact that powerful active nuclei are located so far away that the direct presence of the supermassive black hole cannot be unambiguously detected. However, there are at least two pieces of evidence that suggest that dormant supermassive black holes may reside at the centers of many nuclei that are not presently in a highly active state. The first is the LINER phenomenon, which may be the result of a "starved monster" - a supermassive black hole that is producing very little energy because it is receiving only a slow trickle of food in the form of infalling gas. The second is the fact that the most powerful active nuclei were evidently much more common in the distant past than they are today. Many more galactic nuclei may contain the essential equipment for producing a quasar (i.e., supermassive black hole) than is evident from the scarcity of highly active nuclei in the present universe.
Motivated by such ideas, astronomers have recently searched the nuclei of nearby galaxies (including the Milky Way) for the presence of a "dead quasar" (a supermassive black hole that is presently producing little or no light). The basic technique is to measure the motions of stars in the nucleus, and then to use Newton's laws of motion and law of gravity to "weigh" the mass contained in the center of the nucleus. The presence of a supermassive black hole would be revealed by stellar velocities that increase rapidly toward the center of the nucleus (because of the strong gravitational field of the black hole) and by a calculated mass that is far in excess of the mass that could be contained in normal stars. The interpretation of such observations is a subtle and difficult task. Nevertheless, there is now good enough evidence to strongly suggest that the nuclei of several of the nearest galaxies may well contain supermassive black holes with masses that are several millions to several tens of millions times that of the sun.
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Adapted from The Astronomy and Astophysics Encyclopedia, ed. Stephen P. Maran