Spiral galaxies Low surface brightness image of
NGC 2537 (Arp 6) These are mostly dwarf galaxies or poorly defined spiral galaxies (with the designation Sm) that have low surface brightnesses (i.e. they emit little light per unit area). Low surface brightness galaxies are actually quite common. The exception is
NGC 2857 (Arp 1), which is an Sc spiral galaxy (which means that it has a definite structure with loosely wound spiral arms and a faint but well-defined nucleus). , once a standard spiral galaxy (center), and
NGC 2937, a smaller elliptical (bottom).
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 (Arp 152) 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.{{cite web
Interior absorption (Arp 157).
NGC 1316. 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 galaxies' disks have coalesced together but the nuclei have not.
Centaurus A (Arp 153) and
NGC 1316 (Arp 154) are both effectively elliptical galaxies with unusual dust lanes; their kinematics and structure indicate that they have undergone merging events recently.
NGC 4747 (Arp 159) may be nothing more than an edge-on spiral galaxy with a significantly dark dust lanes.{{cite web
Diffuse filaments The filaments in these objects may represent tidal tails from galaxy interactions. Many of the galaxies are the remnants of the mergers of two spiral galaxies to form a single elliptical galaxy. However,
NGC 3414 (Arp 162) appears to be merely an unusual
S0 galaxy with a very small disk relative to its bulge size.
NGC 4670 (Arp 163) is a
blue compact dwarf galaxy with extremely strong
star formation activity;{{cite web
Diffuse counter-tails All of these objects are galaxies involved in gravitational interactions. These counter-tails are tidal features caused by the gravitational interactions between two galaxies, just like similar features described in the Arp catalog.
Messier 32 (Arp 168), a
dwarf galaxy interacting with the
Andromeda Galaxy, is included in this category (although the "diffuse counter-tail" is very difficult to see in Arp's photograph).
Narrow counter-tails (Arp 174) This is another category containing galaxies with tidal tails produced by gravitational interactions. These tidal tails are narrower and better defined than the tidal tails in objects 167–172.
Narrow filaments (Arp 186). (UGC 10214; Arp 188). The "narrow filament", which appears to be tidal feature caused by a gravitational interaction, can be seen extending across this image. This category contains a mixture of different types of objects. Like the galaxies with diffuse filaments or galaxies with counter-tails, some of the galaxies in this category have been involved in interactions, and the filaments are tidal features created by those interactions. Other sources, however, are simply individual spiral galaxies with faint spiral arms that are described as "filaments" by Arp.
Material ejected from nuclei , about 747 million light-years from Earth The ejecta in many of these objects appear to be tidal features created by gravitational interactions. In some cases (such as for
NGC 5544 and
NGC 5545 in
Arp 199), the "ejecta" are clearly a spiral galaxy viewed edge-on that happens to line up with another galaxy's nucleus. Almost all of the objects in this category are interacting or have recently undergone interactions.
NGC 3712 (Arp 203) is an exception; it is merely a low surface brightness spiral galaxy.{{cite web
Irregularities, absorption, and resolution activity in nearby
dwarf galaxy NGC 1569 (Arp 210). Galaxies in this category have either irregular structures (irregularities), notable dust lanes (absorption), or a grainy appearance (resolution). This category contains a mix of interacting galaxies distorted by tidal interactions, nearby dwarf irregular galaxies, and spiral galaxies with unusual large amounts of gas.
Adjacent loops spiral galaxy NGC 3310 (Arp 217) These adjacent loops are another manifestation of the structures formed by gravitational interactions between galaxies. Some of these sources consist of galaxies that have nearly completed the merger process; the "adjacent loops" are merely the remnants of the interaction. Among the objects in this category is
Arp 220, one of the best-studied
ultraluminous infrared galaxies in the sky.
Amorphous spiral arms (Arp 226) Many of these galaxies are merger remnants. The "amorphous spiral arms" are the tidal debris that remains after the collision.
Concentric rings , Arp 227 These are galaxies with shell-like structures. Some shell structures have been identified as the results of recent mergers. In other cases, however, the shell structure may represent the outer disk of an
S0 galaxy. In some complicated cases, the galaxy with the rings or shells is an S0 galaxy interacting with another galaxy; the origins of the shells in such systems can be difficult to determine.
Appearance of fission ). and
NGC 5258 (Arp 240), interacting pair of spiral galaxies. pair in the early stages of colliding and merging. Although the description of the objects in this category implies that the galaxies are separating, most of these galaxies are merging. Many of the objects have very pronounced tidal tails and bridges that have formed as a consequence of the interaction. Most objects are in the early stages of the merging process, where the galaxies still appear to have distinct nuclei and distinct (albeit distorted) disks. Among the most notable galaxies in this category are the
Antennae Galaxies (NGC 4038 and NGC 4039, Arp 244) and the
Mice Galaxies (NGC 4676, Arp 242). However, not all of these objects are interacting galaxies. A few of these galaxies are simply nearby dwarf galaxies with irregular structure.
Irregular clumps /
UGC 4305,
dwarf galaxy (Arp 268) These are objects that appear to be a series of irregular clumps with no coherent structure. Many of these objects are simply nearby
dwarf galaxies. Some of these objects are interacting galaxies, while others are small
groups of galaxies. In both cases, many of the constituent galaxies are irregular galaxies. The superposition of two or more such irregular galaxies can easily look like a single larger irregular galaxy, which is why the Atlas of Peculiar Galaxies (and other catalogs) often classify these pairs and groups as single objects.
Double and multiple galaxies Arp originally referred to these galaxies as "double galaxies", but many of these sources are more than two galaxies. Some of the objects consist of
interacting galaxies, whereas other sources are actually
groups of galaxies. The difference is that interacting galaxies will be distorted, whereas galaxies in groups are simply gravitationally bound to each other but not necessarily close enough to each other to induce major structural changes.
Connected arms and
IC 1179, interacting spiral galaxies. All of these galaxies are interacting pairs of galaxies except for
NGC 5679 (Arp 274), which may be an interacting galaxy triplet.{{cite web The connected arms described here are tidal bridge features that form between interacting galaxies. These bridges form early during galaxy interactions.
Interacting galaxies Unlike many of the objects listed in the
amorphous galaxies section, the interacting galaxies that comprise these objects are still distinguishable from each other.
Infall and attraction NGC 4631 and
dwarf elliptical NGC 4627 (below) comprise the Arp 281 pair This category contains an odd mixture of objects. Two of the objects are edge-on disk galaxies with smaller companion galaxies nearby. Two of the objects are connected by tidal bridges. The last two objects may simply be interacting with each other over long distance.
Wind effects Although included in the double galaxies category, many of these objects are individual galaxies. The "wind effects" refer to the appearance, not the actual detection of high-velocity gas (such as is found in
M82). In some cases, the appearance may be the result of interaction. In other cases, particularly
NGC 3981 (Arp 289), the faint, extended emission may be related to the intrinsic nature of the galaxy itself and not interactions with other objects.
Long filaments (
NGC 5754) /
irregular galaxy (
NGC 5752) pair The long filaments in these systems are probably tidal tails or bridges that have been produced as the result of the gravitational interaction between the galaxies.
Unclassified objects and
NGC 3690 (Arp 299), interacting galaxy pair (Arp 302), a pair of a face-on and an edge-on
spiral galaxy Arp did not give a subclassification for objects 298–310 in his atlas. These objects are mostly interacting galaxy pairs.
Groups Chains Miscellaneous image of
Messier 82 ==Brightest Arp galaxies for amateur astronomers==