The landmark Arp Atlas of Peculiar Galaxies is a catalog of 338 peculiar galaxies produced by Dr. Halton Arp in 1966.  The original images were taken in black and white by the largest telescope in the world at the time. A good link to his original work is here.

I have spent the past 5 years capturing these beautiful Arp objects in color.  Although each of these 338 objects can be seen individually in my Astrobin Arp Collection,  the real power of Arp’s work becomes apparent when comparing objects side-by-side on the same page. That is the purpose of this post.  Attached Revisions A through I show the Arp Catalog in each of the original 8 Arp categories:

1. Spiral Galaxies (1-36)
2. Companions on Arms (37-101)
3. Elliptical Galaxies (102-145)
4. Galaxies (146-268]
5. Double Galaxies (269-310)
6. Groups of Galaxies (311-321)
7. Chains of Galaxies (322-332)
8. Miscellaneous (333-338]

Within each category, Arp grouped the galaxies according to one of over 30 traits, such as Loops or Countertails.  On each poster, I have grouped galaxies with the same trait on the same row.  When the number of galaxies with the same trait exceeds 10, I have split the group into multiple rows.

Following the posters are spreadsheets shown in Revisions J through R.  These spreadsheets contain the name and location of each object.  All of this information will be included in my upcoming free 200 page eBook on deep sky objects.

Below is a brief description of the highlights of each category from my perspective.  Note that the galaxies shown on the posters are sometimes a bit blurry because many are very small and the images are a zoomed-in view.  Some of these objects are smaller in our apparent view than the planet Jupiter.  

Spiral Galaxies [1-36] 

This first Arp category shows individual isolated spiral galaxies which have some deformation but no nearby companions.  The category consists of 7 traits, one shown in each row of the poster.

The 1st trait is "Low Surface Brightness", or LSB.  The 6 galaxies here are inconsistent in their LSB trait.  Galaxies 2, 3 and 4 represent the classic LSB type, but 1 and 6 appear to be normal brightness galaxies.  My broader collection of LSB galaxies is shown here.

The "Split Arm" trait of row 2 consists of all barred galaxies, most with inner pseudorings.  The split arm trait is often a function of the trailing spiral arm falling back into the pseudoring.  This phenomenon typically occurs with galaxies which have been undisturbed by companions for long periods, referred to as secular growth.  My broader collection of pseudoring galaxies is shown here.

The galaxies with the traits of "Detached Segments" and "Three-Armed" appear similar to each other (compare 17 and 20, for example) and are likely the result of some galaxy disturbance.  

The "One-Armed" trait is second only to the Integral Sign as the most interesting and distinctive in this category.  These galaxies appear to be Magellanic Galaxies, which typically have the following characteristics - asymmetric,  an off-center white stellar bar, 1 dominant arm, blue star formation areas, and a diameter between 15,000 and 40,000 light years.  My broader collection of Magellanic galaxies is shown here.

Like the LSB trait described above, the "One Heavy Arm" trait appears to be inconsistent, and more illustrative of the nature of VV Rows than of arm heaviness.  The only truly heavy arm galaxy that I see is 28.  30 appears to be 2 superimposed galaxies, with the one heavy arm actually being a separate edge-on galaxy..  

The final trait here, "Integral Sign", is my favorite on the page.  4 of the objects appear as regular integral signs while 2 (34 and 36) appear as reversed.  If we could travel to the backside of these galaxies, 34 and 36 would appear as a regular integral sign.  Most of these objects are barred, with the bar likely the cause of the strong arm appearance.  Note that many of the other galaxies on this poster (e.g., 12, 16, 19) also have an integral sign appearance if their disks were cleaner away from the bars, as these are.  My broader collection of Integral Sign galaxies is shown here.

Companions on Arms [37-101]

The second Arp category, shown on the poster of Revision B, contains individual spiral galaxies which each have a companion on their arm.   I have oriented each object so that the companion is towards the top of the poster.

The companion brightness distinctions between the first 3 traits - low, small and high, and large and high - are fairly accurate but not always consistent.   Generally, the companions evolve from dim and fuzzy in the 1st row to bright and sharp in the 5th row.  

The companions shown on this page fall into 3 types - star clouds, fake companions, and true companions:

1.  Star clouds are seen as tiny, dim blue regions with little surrounding disturbance (e.g., 37, 38, 49, 79, and 80).  It is hard to imagine how these were believed to be individual galaxy companions.

2. Fake companions are simply distant superimposed galaxies.  The majority of the objects on this page fall into this category.  Sometimes the redshift data shows that the companion is distant, but more often this data is not available.  Instead, look closely at the deformation of each object.  Often you will see that the companion is neither deformed or causing deformation in the main object, both of which would be expected if they were true companions.  Also, companion brightness and sharpness are keys as well - fake companions look clear and distinct.  Examples of fake companions include the sequential group of 39-48, as well as the sequential group of 82-86. Many more examples of fake companions can be seen on the poster. 

3.  True companions typically look diffuse with little structure – often just fuzzy blobs.  Deformation should be seen in the vicinity of the companion, particularly of the companion but often of the main galaxy as well.  Some of the examples that Arp has chosen to include here are spectacular.  I particularly like the bridging star streams and local deformation that accompany these true pairs.  Examples include 55, 58, 75, 81, and 87. 

A number of ellipticals in the poster appear to be “sucking” a companion spiral arm towards their cores.  These include 90, 91, 92, 94, 97 and 98.  I don’t recall seeing such behavior on any simulation, so I am not convinced that this is true behavior.  I have always felt that these alignments were just superposition, but it is seen on this poster so often that perhaps it is a true aspect of the interaction.

Finally, towards the end of the category and especially in the elliptical section, there appear to be a number of objects which are double galaxies (objects of fairly equal size) instead of companions.  These include 99, 100 and 101.  I am not sure why these weren’t included in the later elliptical galaxy or double galaxy categories instead of this "companions on arm" category.

Elliptical Galaxies [102-145] 

This category, shown on the poster of Revision C, contains objects which are pairs of galaxies where one of the galaxies is a spiral galaxy and the other galaxy is an elliptical galaxy.  The exceptions are the 4 galaxies (133-136) which have the "With Nearby Fragments" trait.  These are each a single elliptical galaxy which show signs in their outer extents of star clouds that are likely remnants of a previous merger.

Overall, the trend from top to bottom of the page is from widely spaced pairs with narrow star stream bridges at the top (e.g., 102-105), to closely spaced pairs with broad, bright short connecting bridges in the bottom row.  Through the middle of the page are many pairs with no star streams between them, indicating that interaction is minimal or perhaps even non-existent.  Many of these objects are likely simply superimposed galaxies (114, 115, 122, 127).  However , some of the objects are clearly interacting and are spectacular.  

The last row, with the “Material Emanating From” trait, is where the Arp traits start to become really interesting.  Most of these objects look like galaxy mergers, but Arp did not agree with that idea and believed that such objects were examples of "ejections" - the opposite of mergers.  He writes in his controversial book Seeing Red [1998] that "for me, the whole lesson of the Atlas of Peculiar Galaxies was that galaxies are generally ejected material."  Arp was a controversial figure - he was a vocal critic of the Big Bang theory (through which he lost his access to the Palomar Observatory) and advocated a different interpretation of cosmic redshift. Despite the controversies, they is no doubt that the galaxies of Arp's collection are interesting and magnificent, no matter if these pairs are merging or ejecting.  In particular, the last 6 galaxies of this poster (140-145) are incredible and worthy of close examination.  In these 6, both galaxies of each pair are clearly disturbed.  The spirals of each pair have prominent blue regions of new star formation which have been initiated by the gravitational interaction.

Galaxies [146-268]

Although the simple title “Galaxies” does not do it justice, this category represents a transition in focus, from galaxies as a whole to specific elements of galaxies such as arms, dust bands and tails.  This is the largest section, taking 3 posters (D, E, and F) to cover and containing 13 traits.  It is also my favorite section and where I think that Arp did his best work.

This category starts with a bang, with 3 galaxies that have the trait of "With Associated Rings".  We now know that these are collisional ring galaxies.  Collisional ring galaxies occur when a larger galaxy (typically 30,000 to 50,000 light years in diameter) is impacted head-on along its polar axis by a smaller dwarf galaxy (typically 5,000 to 10,000 light years in diameter). The collision creates an expanding density wave of star formation in the larger galaxy, leading to the formation of a hole in the larger galaxy which usually develops into a ring. Surprisingly, the small galaxy often remains relatively intact.  Many more collisional ring galaxies are shown here.

The next trait is an odd one – "With Jets".  There is only one true jet seen in these 4 galaxies, and that is in the blue jet emanated from the core of 152 (M87).  The other 3 features are also fascinating but are not jets – they are simply long star streams resulting from interacting galaxies.

The trait of the 2nd row is a challenging one for us to understand Arp's intention – “Disturbed with Interior Absorption”.  My guess is that the title identifies objects that have a strong central dust lane across the core, since all 8 objects are the same in that regard.  I love this trait.  We have since learned that such a trait is usually the result of a merger, with at least one of the original galaxies being a spiral.  I have captured these types of objects in 2 dust band collections -  elliptical dust bands (shown here) and merger dust bands (shown here).

The rest of poster D addresses star streams.  Arp uses the terms “countertails” and “filaments” to describe star streams, which at first glance appear to be describing similar tail-type objects.  But I believe that he used the term "filament" to describe a tail emanating from a single object (or from a closely spaced pair of cores), while the term "countertail" is used to described a tail emanating from 2 more widely spaced (i.e., more than 1 diameter apart) galaxies.  All of the objects described as “countertails” [167-178] involve pairs of galaxies, although at first glance 168 may not appear that way.  But 168 is galaxy M32, a satellite galaxy of M31 which appears behind it in the image.

The next poster, poster E, continues this category with perhaps the most controversial trait – “Material Ejected from Nuclei”.  As I stated earlier, Arp believed that these galaxies are separating, not merging.  Today we believe that these objects are merging simply because the underlying gravitational processes seem more consistent with that explanation.  The “material ejected” seen here is typically star streams bridging 2 galaxies.  But, like other pairs seen earlier, some of these pairs are likely not interacting and are just line of sight partners, most notably 198, 199, and 208.  Another galaxy, 200, is simply a single galaxy which has one arm with a line of bright star producing regions. 

The next trait seems to be a bit of a catch all – “Irregularities, Absorption and Resolution”.  Most of these objects appear to be in a late merger stage, although I don’t see enough commonality amongst these 6 objects to say that they have a common trait and should be together.

Across the middle of the page, the next trait finally brings us back to an easy-to-understand title – “Adjacent Loops”.  The use of “adjacent” is an interesting one – I would have simply said “loops”.  Most of the objects appear again to be late stage mergers, although 215 and 217 have less disturbed inner regions and appear to be at an earlier stage with no obvious disruptor present. My broader collection of galaxy loops is shown here.

More loops are seen in the next row, although Arp has called this trait "Amorphous Spiral Arms".  The nature of these galaxies in each case seems to be a late stage merger.

"Concentric Rings" is a trait which is easy to understand but hard to see.  They typically are the result of past small mergers in elliptical galaxies and appear as faint concentric arcs in the outer region of the galaxy.   Today these features are called shells - my broader collection of shells is here.

The top 4 rows of the next poster, poster F, have yet another intriguing trait title – “Appearance of Fission”.  Again, Arp believed that these galaxies were all in the process of separating.  Most of the top rows contains late stage mergers where the 2 former cores are not distinct.  The next 3 rows show more separation between the galaxies.  In today’s nomenclature, these galaxies are in various merger stages, with the top row being at the M5 stage and the other rows generally being at the M2S (short bridge) stage.

The large Galaxy category concludes with the last 2 rows of poster F.  These were some of the hardest objects for me to understand until I saw them all on the same page.  The trait description of “Irregular Clumps” is not very helpful. I wish he would have just called this one "irregular Galaxies". Each of these objects shows characteristics that are common for an irregular galaxy - small size (<50,000 ly), blue color, strong whitish bar and absence of spiral structure.  They look like so much more than clumps to me.  

Double Galaxies [269-310] 

The 5th Arp category covers 1-1/2 posters, starting on poster G.  Each object of this category consist of 2 large distinct galaxies, usually both spiral.  Like the companion series, many of these galaxies do not appear to be interacting.  Examples include 276, 279, 285, and almost all of the double galaxies seen on the top half of poster H.

I see little difference between the "Connected Arms" and "Interacting" traits of the first 2 rows of the poster. 

The 3rd row, "Infall and Attraction", really appeals to me for the first 2 objects (281, 282).  But, the remaining objects in the row are not interacting in the same interesting manner.  

I like the "Wind Effects" trait of row 4, which captures galaxies where the entire disk appears to be more broadly affected by the gravitational influence of a nearby galaxy. Note that only a single galaxy of each pair is shown for each of these 7 objects .  For this row, I decided to focus only on the most impacted galaxy of each pair in order to more clearly see close-up these interesting gravitational smearing effects, which are lost if the wide view of both galaxies is displayed.

I also really like the "Long Filaments" trait of the last row.  Unfortunately, this trait is a bit short on content with 4 galaxies, only 2 of which have long filaments.  My broader superlong star stream collection is shown here.

The Double Galaxies category concludes with the top half of poster H.  There is no specific trait associated with these galaxies, and as I said above, most of the pairs do not even look like they are interacting.

Groups of Galaxies [311-321]

The bottom half of poster H shows groups of galaxies.  Several of these are well known, such as the Leo Triplet (317), Stephan’s Quintet (319), Copeland’s Septet (320) and Hickson 40 (321).  These are all small galaxy groups, not larger galaxy clusters.   The groups are interesting but do not contain as much peculiar content as the previous categories.  My broader collection of Hickson small galaxy groups is shown here. 

Chains of Galaxies [322-332]

The top half of poster I shows galaxy chains.  As in the Groups of Galaxies of the above section, these objects are interesting but not particularly peculiar.  However, I love galaxy chains and I am glad that Arp included them.  Galaxy chains are fascinating to me - are they lined up through gravitational dynamics, point of origin, or simply by chance?  My broader galaxy chain collection is here.

Miscellaneous [333-338] 

Arp saved both the best and worst for last.  The final section, Miscellaneous, is shown in the bottom half of Poster I.  The highlights are Arp 335-337 – 3 galaxies which are amongst the most interesting in the collection.  335 has a single long arm which has been bent backward by an unseen force.  336 is one of the best polar ring galaxies in the heavens.  And 337, better known as M82 and the Cigar Galaxy, has a bright core which is highlighted by dramatic dust lanes and perhaps the strongest HII emission region in any galaxy.  

The list concludes with 338, a real clunker.  Not only is it tiny, but its feature (the bright white line) is simply a distant superimposed edge-on galaxy.

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I close with selected quotes from Dr. Halton Arp's introduction to his atlas, published over 50 years ago:

"Not all galaxies fit the schematic idealization of the Hubble sequence of nebular forms.  In fact, when looked at closely enough, every galaxy is peculiar.  Appreciation of these peculiarities is important in order to build a realistic picture of what galaxies are really like.  The overall aim of the Atlas is to present a number of examples of various kinds of peculiar galaxies.  They are displayed in groupings that appear roughly similar.  It is hoped that this investigative procedure will not only clarify the workings of galaxies themselves, but reveal physical processes and how they operate in galaxies, and ultimately furnish a better understanding of the workings of the universe as a whole."