By definition, a peculiar galaxy is not a normal galaxy; for example, one that can be classified into one of the categories defined by Hubble. Astronomers have identified two types of peculiar galaxies, interacting galaxies and active galactic nuclei (AGN).
The first type of peculiar galaxy are the interacting galaxies, or those galaxies whose structures have been altered by the gravitational attraction of neighboring galaxies.
There is no general classification possible for interacting galaxies simply because of the innumerable ways in which two galaxies can affect each other. A small galaxy will have little effect on a larger one (though it can stimulate a grand design two‐armed spiral pattern), but the smaller galaxy may have its whole structure distorted or even be completely pulled apart by the tidal gravitational field of the larger galaxy (as is occurring for the Magellanic Clouds). Bridges and tails of luminous material may be produced. Totally disrupted, the stars and gas of a small galaxy may orbit a large galaxy perpendicular to the plane of its disk, forming a polar ring galaxy. These phenomena are short‐lived by astronomical time scales, and ultimately the mass of the destroyed galaxy becomes part of the larger system, a case of galactic cannibalism.
A special case of a small galaxy dramatically but temporarily changing the appearance of a large galaxy occurs when the smaller galaxy's path is by chance nearly along the rotational axis of a large disk galaxy. As the small galaxy passes through the disk, its extra contribution to gravitation pulls in stars and interstellar material from the outer disk of the large galaxy. As the small galaxy departs, this material moves back outward. The momentary compression of the interstellar gas, however, triggers major star formation and the appearance of a ring galaxy.
Gravitational interaction that disturbs the mass distribution in a galaxy may also trigger major events of nearly simultaneous star formation over the whole disk of a galaxy. Such activity explains starburst galaxies such as M82. Many nearly simultaneous supernovae would greatly disrupt the remaining interstellar matter pattern, producing what looks like an exploding galaxy.
Mergers and cannibalism
More dramatic is the strong interaction that can occur when two large galaxies of similar masses interact. Even if the galaxies are originally in orbit about each other, energy exchange between orbital motion and the motions of the stars in each galaxy will cause the two to approach each other. This will result in the two galaxies ultimately merging (which will likely occur for the Milky Way and the Andromeda galaxies). If such a merger is relatively gentle and the galaxies are oriented properly with respect to each other, the final object will most likely resemble a spiral galaxy of some type. If the collision is more dramatic (head on; galaxies rotating in opposite directions; and/or the planes of rotation perpendicular to each other), little trace of the structures of the original colliding galaxies may remain. Energy exchange will randomize stellar orbits, but stellar collisions would be rare, because stars are so small compared to their average separations. The resulting distribution in space of the stars in the surviving single galaxy will not have an identifiable plane, nor will there be circular symmetry.
The effect on the interstellar gas when two galaxies merge would be more dramatic than the effect on the stars. Even with typical densities of one particle per cubic centimeter in each galaxy, the gas particles will collide, kinetic energy going into internal energy that then will be radiated away. Strong shock fronts will develop and, contrary to the normal inefficiency of conversion of interstellar material into stars, there can be complete conversion of the gas into stars. Within a few tens of millions of years of the merger, the bright blue stars will have evolved and disappeared, leaving behind a redder, stellar population of stars. Such an object would have all the characteristics of what Hubble classified as an elliptical galaxy. If, indeed, ellipticals are the result of mergers of disk galaxies, then observation of the population of galaxies early in the universe should show a smaller fraction of ellipticals than are present in today's universe. Studies with the Hubble Space Telescope have confirmed that this does appear to be the case.
Active Galactic Nuclei (AGN)
The second group of peculiar galaxies are those whose appearances and characteristics have been modified by internal processes, energetic events that appear to occur in the very nucleus of the galaxies. These are termed active galactic nuclei objects, or AGNs for short.
There is a vast range in the energetics of nuclear violence in galaxies. At the lower end of the energy spectrum is the phenomenon occurring in the nucleus of the Milky Way Galaxy. At higher energy levels are hot jets of material streaming out of galactic nuclei at nearly the speed of light (a so‐called relativistic velocity as the rules of the theory of special relativity must be used to describe phenomena at these exceedingly high velocities). Jets are observed in visible light and in the radio region of the spectrum, for example, in the giant elliptical galaxy M87 in the Virgo Cluster of galaxies. The characteristics of the radiation identify it as synchrotron radiation, emitted by charged particles under the influence of a weak magnetic field. The appearance of a single bright jet is an illusion, however. When luminous material moves nearly at the speed of light, it preferentially emits its light in the direction of travel. The M87 jet is roughly pointed in the direction of Earth, but there is evidence for a faint counterjet on the opposite side of the galaxy nucleus. So there are actually two jets—a bipolar gas outflow, moving in opposite directions from a central source. The exact nature of this central source is unknown, but observation of normal stars very near the center of this galaxy shows large motions, which would imply (using Kepler's Third Law) an immense mass concentrated in a very small volume. The argument for a large mass is now believed to be evidence for the existence of a massive, nonstellar black hole that presumably formed at or near the time of the origin of the galaxy.
If the same phenomenon were to occur in the center of a spiral galaxy, however, the external appearance of the event would be very different. Unlike an elliptical galaxy, a spiral galaxy has significant interstellar material. Jets emanating from the center would immediately collide with the interstellar material, their energy stirring up the gas and heating it. Such spiral galaxies, identified by having exceptionally bright central regions (especially in the infrared), variable central luminosity, strong emission lines, and random gas motions up to thousands of kilometers per second, are known as Seyfert galaxies.
Given sufficient time, if there is no interstellar material to collide with or block them, jets can move outward to distances that are large with respect to the spatial distribution of the stars in the galaxy. The gas in the jets will cool and expand into volumes that may be several times the size of the central elliptical galaxy. Such gas still emits radio radiation and may be observed in large blobs (the radio astronomers call these lobes) on either side of a galaxy. If a radio lobe galaxy is drifting through space, the lobes may be trailed behind, forming butterfly‐like structures with a size up to 1,000,000 pc in length, the largest individual objects known in the universe.
At the most extreme energies are the quasi‐stellar objects (so named because they look star‐like), shortened to quasars, or QSOs. The widely diverse nature of QSOs has led them to be identified under a number of terms including designation as BL Lac objects and blazars, although fundamentally these are believed to be the same type of object. They are characterized by high variability of their light output, with luminosities equivalent to that of a whole galaxy but coming from within a region of the size of the solar system. Careful photography reveals the faint outer luminosity surrounding the quasars showing that these actually are an occurrence in the nuclei of galaxies.
Quasars do not exist in the present‐day universe, but were instead a phenomenon of the past, a fact revealed by their great velocities of recession, which places them at large cosmological distances. Given estimated quasar lifetimes of tens to hundreds of millions of years, statistical studies suggest that the number of ancient quasars is roughly equal to the number of present‐day galaxies.
Most mechanisms for producing energy (gravitation, thermonuclear reactions, as in stars) require a great deal of mass, which in the case of quasars is prohibitive because of the constraint on the volume of space from which this energy must come. The most efficient energy source is the conversion of matter into energy via infall into a black hole. Any falling mass gains kinetic energy as it drops, but mass falling toward a small black hole, close to which the gravitational field becomes immense, gains a tremendous amount of kinetic energy—in fact an amount that is equal to the energy equivalence of the mass itself as given by the Einstein formula E = mc 2 (in contrast, the fusion of hydrogen to helium releases only 0.7 percent of the mass in the form of energy). In the right circumstance, this energy can be released as electromagnetic radiation. Thus quasars are thought to represent the era of formation of large central black holes in galaxies, from tens of millions to perhaps as much as a billion solar masses. Such masses would affect only the very central conditions of a galaxy, and would have little effect on the structure and motions in the rest of the galaxies, thus they could be “hidden” in the centers of present‐day galaxies, essentially invisible if no longer actively gaining new mass. The re‐excitement of such a “central engine” by renewed mass infall (due to gravitational interaction with neighboring galaxies) could explain the lower energy phenomena seen in the Milky Way and other present‐day galaxies (AGNs).