Hubble Interacting Galaxy NGC 6621 (2008-04-24)

Hubble Interacting Galaxy IC 883 (2008-04-24)

The ''Atlas of Peculiar Galaxies'' is a

catalog Catalog or catalogue may refer to: *Cataloging **'emmy on the 'og **in science and technology ***Library catalog, a catalog of books and other media ****Union catalog, a combined library catalog describing the collections of a number of libraries ...

peculiar galaxies A peculiar galaxy is a galaxy of unusual size, shape, or composition. Between five and ten percent of known galaxies are categorized as peculiar. Astronomers have identified two types of peculiar galaxies: ''interacting galaxies'' and ''active ga ...

produced by

Halton Arp Halton Christian "Chip" Arp (March 21, 1927 – December 28, 2013) was an American astronomer. He was known for his 1966 ''Atlas of Peculiar Galaxies'', which (it was later theorized) catalogues many examples of interacting and merging galaxie ...

in 1966. A total of 338 galaxies are presented in the atlas, which was originally published in 1966 by the

California Institute of Technology The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

. The primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among galaxies. (online version, including Arp's original tabular data, and PDF link)

Background

Arp realized that the reason why galaxies formed into

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:elliptical Elliptical may mean: * having the shape of an ellipse, or more broadly, any oval shape ** in botany, having an elliptic leaf shape ** of aircraft wings, having an elliptical planform * characterised by ellipsis (the omission of words), or by conc ...

shapes was not well understood. He perceived peculiar galaxies as small "experiments" that astronomers could use to understand the physical processes that distort spiral or elliptical galaxies. With this atlas, astronomers had a sample of peculiar galaxies that they could study in more detail. The atlas does not present a complete overview of every peculiar galaxy in the sky but instead provides examples of the different phenomena as observed in nearby galaxies. Because little was known at the time of publication about the physical processes that caused the different shapes, the galaxies in the atlas are sorted based on their appearance. Objects 1–101 are individual peculiar spiral galaxies or spiral galaxies that apparently have small companions. Objects 102–145 are elliptical and elliptical-like galaxies. Individual or groups of galaxies with neither elliptical nor spiral shapes are listed as objects 146–268. Objects 269–327 are double galaxies. Finally, objects that simply do not fit into any of the above categories are listed as objects 332–338. Most objects are best known by their other designations, but a few galaxies are best known by their Arp numbers (such as

Arp 220 Arp 220 is the result of a collision between two galaxies which are now in the process of merging. It is the 220th object in Halton Arp's ''Atlas of Peculiar Galaxies''. Features Arp 220 is the closest Ultraluminous Infrared Galaxy (ULIRG) to ...

). Today, the physical processes that lead to the peculiarities seen in the Arp atlas are thought to be well understood. A large number of the objects have been interpreted as

interacting galaxies Interacting galaxies (''colliding galaxies'') are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a ...

, including M51 (Arp 85),

, and the Antennae Galaxies (NGC 4038/NGC 4039, or Arp 244). A few of the galaxies are simply

dwarf galaxies A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is s ...

that do not have enough

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

to produce enough

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

to allow the galaxies to form any cohesive structure. NGC 1569 (Arp 210) is an example of one of the dwarf galaxies in the atlas. A few other galaxies are

radio galaxies A radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus. They have luminosities up to 1039 W at radio wav ...

. These objects contain

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...

that produce powerful jets of gas called

radio jets An astrophysical jet is an astronomical Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest includ ...

. The atlas includes the nearby radio galaxies M87 (Arp 152) and Centaurus A (Arp 153). The peculiar associations present in the catalogue are now interpreted as

galaxy merger Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies i ...

s or non-interacting line-of-sight overlap, though Arp disputed that idea, claiming that apparent associations were examples of ejections.Halton Arp, ''Seeing Red: Redshift, Cosmology and Academic Science'', Aperion, Montreal (August 1998), pp. 14, 61-62, 72, 104-105

Notable Arp galaxies

Catalog list

Spiral galaxies

Low surface brightness

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).

Split arms

This category contains

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae'' This category contains spiral galaxies with arms that appear to be segmented. Some spiral arm segments may appear detached because dust lanes in the spiral arms obscure the arms' starlight. Other spiral arms may appear segmented because of the presence of bright star clusters (or discontinuous chains of bright star clusters) in the spiral arms.

Three-armed

Usually, most spiral galaxies contain two clearly defined spiral arms, or they contain only fuzzy filamentary spiral structures. Galaxies with three well-defined spiral arms are rare.

One-armed

One-armed spiral galaxies, including Magellanic spirals, are also rare. In this case, the single spiral arm may actually be formed by a gravitational interaction with another galaxy (as with the

Large Magellanic Cloud The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...

itself, although it is not a member of the catalog).

One heavy arm

The spiral arms in these galaxies have an asymmetric appearance. One spiral arm may appear to be considerably brighter than the other. In the photographic plates produced by Arp, the bright arm would look dark or "heavy". While most of these galaxies (such as M101 and

NGC 6946 NGC 6946, sometimes referred to as the Fireworks Galaxy, is a face-on intermediate spiral galaxy with a small bright nucleus, whose location in the sky straddles the boundary between the northern constellations of Cepheus and Cygnus. Its dista ...

) are simply asymmetric spiral galaxies, NGC 6365 is an interacting pair of galaxies where one of the two galaxies is viewed edge-on and just happens to lie where the spiral arm for the other face-on galaxy would be visible.

Integral sign

These are galaxies that look like a stretched-out S shape (or like the integral sign used in

calculus Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizations of arithm ...

). Some objects, such as

IC 167 IC 167 is a barred spiral galaxy in the constellation of Aries. It was first reported by Bigourdan in 1891 and included in Dreyer's first ''Index Catalogue''. Galaxy group information IC 167 is a member of the NGC 697 group. There is some evid ...

, are simply ordinary spiral galaxies viewed from an unusual angle. Other objects, such as UGC 10770, are interacting pairs of galaxies with tidal tails that look similar to spiral arms.

Low surface brightness companions

Many of these spiral galaxies are probably interacting with the low surface brightness galaxies in the field of view. In some cases, however, it may be difficult to determine whether the companion is physically near the spiral galaxy or whether the companion is a foreground/background source or a source on the edge of the spiral galaxy.

Small, high surface brightness companions

Again, many of these spiral galaxies are probably interacting with companion galaxies, although some of the identified companion galaxies may be foreground/background sources or even bright star clusters within the individual galaxies.

Large, high surface brightness companions

Galaxies in this category are almost always clearly interacting sources. The most famous of these objects is the

Whirlpool Galaxy The Whirlpool Galaxy, also known as Messier 51a, M51a, and NGC 5194, is an interacting grand-design spiral galaxy with a Seyfert 2 active galactic nucleus. It lies in the constellation Canes Venatici, and was the first galaxy to be classifie ...

(M51; Arp 85), which is composed of a spiral galaxy NGC 5194 that is interacting with a smaller elliptical galaxy

NGC 5195 NGC 5195 (also known as Messier 51b or M51b) is a dwarf galaxy that is interacting with the Whirlpool Galaxy (also known as M51a or NGC 5194). Both galaxies are located approximately 25 million light-years away in the constellation Canes Venatic ...

. The interaction has distorted the shape of both galaxies; the spiral arm pattern has been enhanced in the larger spiral galaxy, and a bridge of stars and gas has formed between the two galaxies. Many of the other galaxies in this category are also connected by bridges.

Elliptical companions

Like the spiral galaxies with high surface brightness companions, most of these spiral galaxies are clearly interacting systems. Tidal tails and bridges are visible in many of the images.

Elliptical and elliptical-like galaxies

Connected to spiral galaxies

These objects are very similar to the spiral galaxies with elliptical companions. All of the galaxies have features such as tidal tails and tidal bridges that have formed through gravitational interaction.

Repelling spiral arms

Based on the description of these objects, it appears that Arp originally thought that the elliptical galaxies were pushing away spiral arms in companion galaxies. However, the tidal spiral arms may actually look distorted because of the interaction. Some of these "repelled" spiral arms are on the opposite side of the spiral galaxy from the elliptical galaxy. Simulations have shown that such features can be formed through gravitational interactions alone; no repelling forces are needed.

Close to and perturbing spiral galaxies

This is another category in which the majority of objects are interacting galaxies. As noted in the category name, the spiral galaxies look perturbed. Arp originally described some of the elliptical galaxies as repelling.

Nearby fragments

Emanating material

Arp thought that the elliptical galaxies in this category were ejecting material from their nuclei. Many of the pictures could be interpreted that way. However, these objects are actually a mixture of 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. Heic1311a

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 Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy with several trillion stars in the constellation Virgo. One of the largest and most massive galaxies in the local uni ...

(Arp 152), however, the jets are ionized gas that has been ejected from the environment around supermassive

black holes A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can def ...

in the galaxies'

. These jets, sometimes called

relativistic jet An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets bec ...

s or radio jets, are powerful sources of

synchrotron A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed p ...

radiation, especially at

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...

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 galaxies' disks have coalesced together but the nuclei have not.

Centaurus A Centaurus A (also known as NGC 5128 or Caldwell 77) is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable d ...

(Arp 153) and

NGC 1316 NGC 1316 (also known as ''Fornax A'') is a lenticular galaxy about 60 million light-years away in the constellation Fornax. It is a radio galaxy and at 1400 MHz is the fourth-brightest radio source in the sky. Structure and formation In the la ...

(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.

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 A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. ...

with a very small disk relative to its bulge size. NGC 4670 (Arp 163) is a

blue compact dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

with extremely strong star formation activity; it is clearly too small to be the merger remnant of two spiral galaxies like the other merger remnants in this sample, although it may have been involved in a much smaller interaction.

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 Messier 32 (also known as M32 and NGC 221) is a dwarf "early-type" galaxy about from the Solar System, appearing in the constellation Andromeda. M32 is a satellite galaxy of the Andromeda Galaxy (M31) and was discovered by Guillaume Le Gentil ...

(Arp 168), a

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is ...

interacting with the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

, is included in this category (although the "diffuse counter-tail" is very difficult to see in Arp's photograph).

Narrow counter-tails

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

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

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.

Irregularities, absorption, and resolution

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

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

, one of the best-studied ultraluminous infrared galaxies in the sky.

Amorphous spiral arms

Many of these galaxies are merger remnants. The "amorphous spiral arms" are the tidal debris that remains after the collision.

Concentric rings

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

. 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

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 The Antennae Galaxies (also known as NGC 4038/NGC 4039 or Caldwell 60/Caldwell 61) are a pair of interacting galaxy, interacting galaxies in the constellation Corvus (constellation), Corvus. They are currently going through a Starburst Galaxies, ...

(NGC 4038 and NGC 4039, Arp 244) and the

Mice Galaxies NGC 4676, or the Mice Galaxies, are two spiral galaxies in the constellation Coma Berenices. About 290 million light-years distant, they have begun the process of colliding and merging. Their name refers to the long tails produced by tidal acti ...

(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

These are objects that appear to be a series of irregular clumps with no coherent structure. Many of these objects are simply nearby

. 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

, 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

All of these galaxies are interacting pairs of galaxies except for NGC 5679 (Arp 274), which may be an interacting galaxy triplet. 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

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 NGC 3981 is an unbarred spiral galaxy located 62 million light-years away in the constellation of Crater. It was discovered on February 7, 1785 by William Herschel. NGC 3981 is a member of the NGC 4038 Group which is part of the Virgo Superclus ...

(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

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

Arp did not give a subclassification for objects 298–310 in his atlas. These objects are mostly interacting galaxy pairs.

Groups

Chains

Miscellaneous

Brightest Arp galaxies for amateur astronomers

Maynard Pittendreigh W. Maynard Pittendreigh is an astronomer, writer and an ordained minister in the Presbyterian Church (USA). As a minister, he has been a pioneer and leader in a movement toward multi-cultural/racial congregations, and in developing early Internet ...

, an amateur astronomer and occasional writer, has compiled a list of the brightest Arp Galaxies that are most easily viewed by typical amateur astronomers. The galaxies on the list can be observed visually and do not require special photographic or imaging equipment. These include:

References

''Atlas of Peculiar Galaxies'' courtesy of the NASA/IPAC Extragalactic Database
Retrieved on 2006-07-03, 2006-07-12, 2006-07-14, 2006-07-16, 2006-07-17, 2006-07-18, 2006-07-19, 2006-07-20, 2006-07-24, 2006-07-25, 2006-07-26, 2006-07-27, 2006-07-31, 2006-08-02, 2006-08-08, 2006-08-10, 2006-08-13.
NASA/IPAC Extragalactic Database
General information retrieval on individual Arp objects. Retrieved on 2006-07-03, 2006-07-12, 2006-07-14, 2006-07-16, 2006-07-17, 2006-07-18, 2006-07-19, 2006-07-20, 2006-07-24, 2006-07-25, 2006-07-26, 2006-07-27, 2006-07-31, 2006-08-02, 2006-08-08, 2006-08-10, 2006-08-13.

External links

Arp's ''Catalog of Peculiar Galaxies'' website

Notes

{{DEFAULTSORT:Atlas Of Peculiar Galaxies Astronomical catalogues of galaxies *