Atlas of Peculiar Galaxies
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.
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.
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 220, and the Antennae Galaxies. 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 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 and Centaurus A.
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.
Notable Arp galaxies
| Arp Number | Name | Magnitude | Notes |
| 26 | Pinwheel Galaxy | +7.5 | spiral galaxy |
| 37 | Messier 77 | +8.9 | radio galaxy |
| 41 | NGC 1232 | +9.8 | spiral galaxy |
| 76 | Messier 90 | +9.5 | spiral galaxy |
| 77 | NGC 1097 | +9.5 | galaxy interacting with its satellite |
| 85 | Whirlpool Galaxy | +8.4 | galaxy interacting with its satellite |
| 116 | Messier 60 | +8.8 | colliding galaxies |
| 152 | Virgo A | +8.6 | elliptical galaxy |
| 153 | Centaurus A | +6.6 | radio galaxy in a collision? |
| 188 | Tadpole Galaxy | +14.4 | galaxy finishing merging |
| 242 | Mice Galaxies | +14.7 | colliding galaxies |
| 244 | Antennae Galaxies | +10.3 | colliding galaxies |
| 317 | Messier 65 | +9.2 | spiral galaxy |
| 319 | NGC 7320 | +15 | galaxy in colliding group |
| 337 | Cigar Galaxy | +8.6 | starburst galaxy |
Catalog list
Spiral galaxies
Low surface brightness
These are mostly dwarf galaxies or poorly defined spiral galaxies that have low surface brightnesses. Low surface brightness galaxies are actually quite common. The exception is NGC 2857, which is an Sc spiral galaxy.| Arp number | Common name | Notes |
| 1 | NGC 2857 | Sc spiral galaxy |
| 2 | UGC 10310 | |
| 3 | Arp 3 | |
| 4 | Arp 4 | |
| 5 | NGC 3664 | |
| 6 | NGC 2537 |
Split arms
This category contains spiral galaxies with arms that split into two separate parts.| Arp number | Common name | Notes |
| 7 | Arp 7 | |
| 8 | NGC 497 | |
| 9 | NGC 2523 | |
| 10 | UGC 1775 | Contains an off-center nucleus |
| 11 | UGC 717 | |
| 12 | NGC 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 in the spiral arms.| Arp number | Common name | Notes |
| 13 | NGC 7448 | |
| 14 | NGC 7314 | |
| 15 | NGC 7393 | |
| 16 | M66 | |
| 17 | UGC 3972 | |
| 18 | NGC 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 | Notes |
| 19 | NGC 145 | |
| 20 | UGC 3014 | |
| 21 | Arp 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.| Arp number | Common name | Notes |
| 22 | NGC 4027 | |
| 23 | NGC 4618 | Interacting with NGC 4625 |
| 24 | NGC 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 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 | Notes |
| 25 | NGC 2276 | |
| 26 | M101 | Face-on spiral galaxy with five notable companion galaxies |
| 27 | NGC 3631 | |
| 28 | NGC 7678 | |
| 29 | NGC 6946 | |
| 30 | NGC 6365 | Interacting pair of galaxies, with one galaxy viewed edge-on |
Integral sign
These are galaxies that look like a stretched-out S shape. 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 | Notes |
| 31 | IC 167 | |
| 32 | UGC 10770 | |
| 33 | UGC 8613 | |
| 34 | NGC 4615 | |
| 35 | UGC 212 | |
| 36 | UGC 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 | Notes |
| 37 | M77 | |
| 38 | NGC 6412 | |
| 39 | NGC 1347 | |
| 40 | IC 4271 | |
| 41 | NGC 1232 | |
| 42 | NGC 5829 | |
| 43 | IC 607 | |
| 44 | IC 609 | |
| 45 | UGC 9178 | Galaxy triplet |
| 46 | UGC 12665 | |
| 47 | Arp 47 | |
| 48 | Arp 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 | Notes |
| 49 | NGC 5665 | |
| 50 | IC 1520 | |
| 51 | Arp 51 | |
| 52 | Arp 52 | |
| 53 | NGC 3290 | |
| 54 | Arp 54 | |
| 55 | UGC 4881 | |
| 56 | UGC 1432 | |
| 57 | Arp 57 | |
| 58 | UGC 4457 | |
| 59 | NGC 341 | |
| 60 | Arp 60 | |
| 61 | UGC 3104 | |
| 62 | UGC 6865 | |
| 63 | NGC 2944 | |
| 64 | UGC 9503 | |
| 65 | NGC 90 | |
| 66 | UGC 10396 | |
| 67 | UGC 892 | |
| 68 | NGC 7757 | |
| 69 | NGC 5579 | |
| 70 | UGC 934 | |
| 71 | NGC 6045 | |
| 72 | NGC 5994, NGC 5996 | |
| 73 | IC 1222 | |
| 74 | UGC 1626 | |
| 75 | NGC 702 | |
| 76 | M90 | |
| 77 | NGC 1097 | |
| 78 | NGC 772 |