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.

[1] Arp realized that the reason why galaxies formed into spiral or elliptical shapes was not well understood.

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.

Individual or groups of galaxies with neither elliptical nor spiral shapes are listed as objects 146–268.

Today, the physical processes that lead to the peculiarities seen in the Arp atlas are thought to be well understood.

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).

In the photographic plates produced by Arp, the bright arm would look dark or "heavy".

Some objects, such as IC 167,[6] 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.

Simulations have shown that such features can be formed through gravitational interactions alone; no repelling forces are needed.

Arp thought that the elliptical galaxies in this category were ejecting material from their nuclei.

This passage causes a gravitational wave in which gas first falls inward and then propagates outward to form the ring structure.

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.

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.

NGC 4747 (Arp 159) may be nothing more than an edge-on spiral galaxy with a significantly dark dust lanes.

However, NGC 3414 (Arp 162) appears to be merely an unusual S0 galaxy with a very small disk relative to its bulge size.

[4] NGC 4670 (Arp 163) is a blue compact dwarf galaxy with extremely strong star formation activity;[31] 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.

NGC 3712 (Arp 203) is an exception; it is merely a low surface brightness spiral galaxy.

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.

[citation needed] In other cases, however, the shell structure may represent the outer disk of an S0 galaxy.

Many of the objects have very pronounced tidal tails and bridges that have formed as a consequence of the interaction.

[113] The connected arms described here are tidal bridge features that form between interacting galaxies.

The "wind effects" refer to the appearance, not the actual detection of high-velocity gas (such as is found in M82).

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.

[4] 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.

The galaxies on the list can be observed visually and do not require special photographic or imaging equipment.