The Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced by Halton Arp 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 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 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 better understood than in the 1960s when Arp's book was published. A large number of the objects have been interpreted as 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).
The peculiar associations present in the catalogue are now interpreted as galaxy mergers 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
Arp Number Name Magnitude Notes26Pinwheel Galaxy (M101) +7.5spiral galaxy37Messier 77 +8.9radio galaxy41NGC 1232 +9.8spiral galaxy76Messier 90 +9.5spiral galaxy77NGC 1097 +9.5galaxy interacting with its satellite85Whirlpool Galaxy (M51) +8.4galaxy interacting with its satellite116Messier 60 +8.8colliding galaxies152Virgo A (M87) +8.6elliptical galaxy153Centaurus A (NGC 5128) +6.6radio galaxy in a collision?188Tadpole Galaxy +14.4galaxy finishing merging242Mice Galaxies +14.7colliding galaxies244Antennae Galaxies +10.3colliding galaxies317Messier 65 +9.2spiral galaxy319NGC 7320 +15galaxy in colliding group337Cigar Galaxy (M82) +8.6starburst galaxy
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).
Arp number Common name Notes1NGC 2857Sc spiral galaxy2UGC 103103Arp 34Arp 45NGC 36646NGC 2537
Split arms This category contains spiral galaxies with arms that split into two separate parts.
Arp number Common name Notes7Arp 78NGC 4979NGC 252310UGC 1775Contains an off-center nucleus11UGC 71712NGC 2608
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.
Arp number Common name Notes13NGC 744814NGC 731415NGC 739316M6617UGC 397218NGC 4088
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.
Arp number Common name Notes19NGC 14520UGC 301421Arp 21
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 itself, although it is not a member of the catalog).
Arp number Common name Notes22NGC 402723NGC 4618Interacting with NGC 462524NGC 3445
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.
Arp number Common name Notes25NGC 227626M101Face-on spiral galaxy with five notable companion galaxies27NGC 363128NGC 767829NGC 694630NGC 6365Interacting pair of galaxies, with one galaxy viewed edge-on
Integral sign These are galaxies that look like a stretched-out S shape (or like the integral sign used in calculus). Some objects, such as IC 167, 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.
Arp number Common name Notes31IC 16732UGC 1077033UGC 861334NGC 461535UGC 21236UGC 8548
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.
Arp number Common name Notes37M7738NGC 641239NGC 134740IC 427141NGC 123242NGC 582943IC 60744IC 60945UGC 9178Galaxy triplet46UGC 1266547Arp 4748Arp 48
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.
Arp number Common name Notes49NGC 566550IC 152051Arp 5152Arp 5253NGC 329054Arp 5455UGC 488156UGC 143257Arp 5758UGC 445759NGC 34160Arp 60 (PGC 1762846)61UGC 310462UGC 686563NGC 294464UGC 950365NGC 9066UGC 1039667UGC 89268NGC 775769NGC 557970UGC 93471NGC 604572NGC 5994, NGC 599673IC 122274UGC 162675NGC 70276M9077NGC 109778NGC 772
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 (M51; Arp 85), which is composed of a spiral galaxy NGC 5194 that is interacting with a smaller elliptical galaxy NGC 5195. 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. Arp number Common name Notes79NGC 5490C80NGC 263381NGC 6621, UGC 11175, NGC 662282NGC 2535, NGC 253683NGC 3799, NGC 380084NGC 5394, NGC 539585Whirlpool Galaxy (M51)86NGC 7752, NGC 775387NGC 3808A, NGC 3808B88Arp 8889NGC 264890NGC 5929, NGC 593091NGC 5953, NGC 5954
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.
Arp number Common name Notes92NGC 760393NGC 7284, NGC 728594NGC 3226, NGC 322795IC 4461, IC 446296UGC 352897UGC 7085A98UGC 109599NGC 7547, NGC 7549, NGC 7550Galaxy triplet100IC 18, IC 19101UGC 10164, UGC 10169
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.
Arp number Common name Notes102Arp 102103UGC 10586Galaxy triplet104NGC 5216, NGC 5218105NGC 3561106NGC 4211107UGC 5984108Arp 108
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.
Arp number Common name Notes109UGC 10053110Arp 110111NGC 5421Galaxy group112NGC 7805, NGC 7806
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.
Arp number Common name Notes113NGC 70Part of a group of galaxies114NGC 2276, NGC 2300115UGC 6678Galaxy triplet116Messier 60, NGC 4647117IC 982, IC 983118NGC 1141, NGC 1142119Arp 119120NGC 4435, NGC 4438121Arp 121122NGC 6040123NGC 1888, NGC 1889124NGC 6361125UGC 10491126UGC 1449127NGC 191Actually interacting S0 galaxy and spiral galaxy128UGC 827129UGC 5146130IC 5378131Arp 131132Arp 132
Nearby fragments
Arp number Common name Notes133NGC 541134Messier 49135NGC 1023136NGC 5820
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.
Arp number Common name Notes137NGC 2914Spiral galaxy with faint spiral arms138NGC 4015Interacting pair of galaxies139Arp 139Interacting pair of galaxies140NGC 274, NGC 275Interacting pair of galaxies141UGC 3730Ring galaxy system142NGC 2936, NGC 2937, PGC 1237172Galaxy triplet143NGC 2444, NGC 2445Ring galaxy system144NGC 7828, NGC 7829Ring galaxy system145UGC 1840Ring galaxy system
Amorphous galaxies