other phenomena. For example, NGC 2914 (Arp 137) is merely a spiral galaxy with faint spiral arms, and NGC 4015 (Arp 138) is an interacting pair of galaxies where one galaxy is an edge-on spiral galaxy. Some objects, such as NGC 2444 and NGC 2445 (Arp 143), are systems that contain "ring galaxies", which are created when one galaxy (the elliptical galaxies in these examples) passes through the disk of another. This passage causes a gravitational wave in which gas first falls inward and then propagates outward to form the ring structure. Amorphous galaxies Galaxies in this category are referred to by Arp as galaxies that are neither spiral nor elliptical in shape. Although he does not use the term "amorphous" to describe these galaxies, it is the best description of these galaxies. Many of these galaxies are either interacting galaxies or galaxies that are the remnants of the merger of two smaller galaxies. The interaction process will produce various tidal features, such as tidal tails and tidal bridges, that may last well after the progenitor galaxies' disks and nuclei have merged. Although the tidal tails are described as several different visual phenomena ("counter-tails", "filaments", "loops"), they are all manifestations of the same phenomena. Associated rings As noted above, these ring galaxies may have formed when a companion galaxy passed through the ring galaxy. The interaction would produce a wave effect that would first draw matter into the center and then cause it to propagate outward in a ring. Jets These are galaxies that appear to be ejecting material outwards from their nuclei. The "jets" themselves look similar to water spraying out of a hose. In the case of IC 803 (Arp 149) and NGC 7609 (Arp 150), the jets are simply part of the amorphous structure produced by the interacting galaxies. In Arp 151 and Messier 87 (Arp 152), however, the jets are ionized gas that has been ejected from the environment around supermassive black holes in the galaxies' active galactic nuclei. These jets, sometimes called relativistic jets or radio jets, are powerful sources of synchrotron radiation, especially at radio wavelengths. Interior absorption Galaxies in this category feature dark dust lanes that obscure part of the disk of the galaxy. All of these galaxies are the products of two galaxies merging. NGC 520 (Arp 157) is one of the best examples of an intermediate-stage merger, where the two progenitor