Atlas of Peculiar Galaxies

The Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced by Halton Arp. A total of 338 galaxies are presented in the atlas, which was originally published in 1966 by the California Institute of Technology.

The primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among nearby galaxies. Arp realized that the reason why galaxies formed into spiral or elliptical 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).

Today, the physical processes that lead to the peculiarities seen in the Arp atlas are now well understood. A large number of the objects are interacting galaxies, including M51 (Arp 85), Arp 220, and the Antennae Galaxies (NGC 4038/NGC 4039, or Arp 244). A few of the galaxies are simply dwarf galaxies that do not have enough mass to produce enough gravity 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. These objects contain active galactic nuclei that produce powerful jets of gas called radio jets. The atlas includes the nearby radio galaxies M87 (Arp 152) and Centaurus A (Arp 153).

Notable Arp galaxies

Galaxies with detached segments

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.

piral galaxies with 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) 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.cite web
title=NASA/IPAC Extragalactic Database
work=Results for NGC 6365
url=http://nedwww.ipac.caltech.edu/
accessdate=2006-07-12 ]

piral galaxies with 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.

Elliptical and elliptical-like galaxies

Elliptical 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.

Material emanating from elliptical galaxies

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,cite web
title=NASA/IPAC Extragalactic Database
work=Results for NGC 2914
url=http://nedwww.ipac.caltech.edu/
accessdate=2006-07-20 ] and NGC 4015 (Arp 138) is an interacting pair of galaxies where one galaxy is an edge-on spiral galaxy.cite web
title=NASA/IPAC Extragalactic Database
work=Results for NGC 4015
url=http://nedwww.ipac.caltech.edu/
accessdate=2006-07-20 ] 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.cite journal
author= R. Lynds, A. Toomre
title=On the interpretation of ring galaxies : the binary ring system II Hz4
journal=Astrophysical Journal
year=1976
volume=209
pages=328–388
url=http://adsabs.harvard.edu/abs/1976ApJ...209..382L
doi=10.1086/154730 ]

Disturbed galaxies with 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 together. 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
title=NASA/IPAC Extragalactic Database
work=Results for NGC 4747
url=http://nedwww.ipac.caltech.edu/
accessdate=2006-07-24 ]

Galaxies with 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.

Galaxies with 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.

Galaxies with concentric rings

These are galaxies with shell-like structures. Some shell structures have been identified as the results of recent mergers.Fact|date=March 2007 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.

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.

Galaxies with connected arms

All of these galaxies are interacting pairs of galaxies except for NGC 5679 (Arp 274), which may be an interacting galaxy triplet.cite web
title=NASA/IPAC Extragalactic Database
work=Results for NGC 5679
url=http://nedwww.ipac.caltech.edu/
accessdate=2006-08-10 ] The connected arms described here are tidal bridge features that form between interacting galaxies. These bridges form early during galaxy interactions.

Galaxies with 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.

Brightest Arp galaxies for amateur astronomers

Maynard Pittendreigh, 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:
*Arp 26, also known as M 101
*Arp 37, also known as M 77
*Arp 76, also known as M 90
*Arp 77
*Arp 85, also known as M 51
*Arp 116, also known as M 60
*Arp 120
*Arp 152, also known as M 87
*Arp 153
*Arp 168, also known as M 32
*Arp 244
*Arp 269
*Arp 270
*Arp 271
*Arp 281
*Arp 286
*Arp 317, also known as M 65
*Arp 313
*Arp 337, also known as M 82

ee also

*Messier object (M)
*New General Catalogue (NGC)
*Index Catalogue (IC)

External links

* [http://www.arpgalaxies.com Amateur observations of the galaxies]
* [http://www.3towers.com/arptable.htm Grasslands Observatory Arp Galaxies]
* [http://nedwww.ipac.caltech.edu/level5/Arp/Arp_contents.html NASA/IPAC Extragalactic Database Electronic Copy of the "'Atlas of Peculiar Galaxies"]
* [http://www.paulandliz.org/Galaxies/Galaxies_ARP.htm Earthlings - Astronomy web site] Paul and Liz Downing
* [http://starrymirror.com/1031arp87hubble.htm Hubble Images Peculiar Galaxy Pair Arp 87]

References

* [http://nedwww.ipac.caltech.edu/level5/Arp/Arp_contents.html 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.
* [http://nedwww.ipac.caltech.edu/ 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.

Notes