I find merging galaxies to be one of the most interesting types of deep sky objects. Classification systems for such a range of objects are useful tools for communication and scientific study.  Seeing a lack of such a classification system for these galaxy merger objects, I started my own classification system for them on Astrobin a few years ago.  

Now, however, I have come across a galaxy merger classification system that is starting to gain broader acceptance amongst astronomers.  It was first published in 2016 by Larson et al (10 scientists total) in the Astrophysical Journal under the unfriendly title of “MORPHOLOGY AND MOLECULAR GAS FRACTIONS OF LOCAL LUMINOUS INFRARED GALAXIES AS A FUNCTION OF INFRARED LUMINOSITY AND MERGER STAGE”.  But don’t let that long title scare you – the classification system is logical, simple and easy to use.  This classification system is referenced in the current June 2022 edition of Sky & Telescope magazine on page 37, but strangely the original 2016 paper mentioned above is not referenced at all in that article.

I have put together this description and poster so that I could better understand and interpret this classification system.  I am sharing it with you with the goal of accelerating acceptance and use of this tool by our community.  

The paper applied this classification system to 65 luminous infrared galaxy merger objects.  I have extended it to over 150 galaxy merger objects that I have collected over the past 5 years. 

Below is the description of the classification system from the paper.  Mergers are categorized into 5 buckets, from M1 to M5.  The intent, I believe, is to show the merger process progressing from a time perspective, starting at M1 and ending at M5. (The words in italics below are my summary of the detailed description of each category, since I can’t remember more than about 4 words at a time):

M1 - No Bridge:  
Galaxy pairs with line-of-sight velocities differing by less than 250 km/s and projected separations of less than 75 kpc, which have no prominent tidal features. These galaxies appear to be on their initial approach.

M2 – Bridged:
Interacting galaxy pairs with obvious tidal bridges and tails or other disturbances consistent with having already undergone a first close passage.

M3 – 2 Close Cores:
Merging galaxies with multiple nuclei. These systems have distinct nuclei in disturbed, overlapping disks, along with visible tidal tails.

M4 – 1 Core With Tails:
Galaxies with apparent single nuclei and obvious tidal tails. The galaxy nuclei have projected nuclear separations of less than 2 kpc.

M5 – 1 Core: 
Galaxies which appear to be evolved merger remnants. These galaxies have diffused envelopes which may exhibit shells or other fine structures and a single, possibly off-center nucleus. These merger remnants no longer have bright tidal tails.

A few objects contain three distinct galaxies.  These are classified using the above scheme depending on the earliest major merger interaction stage involved. 

In comparing the above system to my prior classification system, the only category that I think needs to be modified is the M2 – Bridged category.  There are just too many (85!) of those objects in that single category, more than half of the overall total.  Consequently, I have split M2 into 2 categories – M2L (Long Bridge) and M2S (Short Bridge).  The dividing line for bridge length, between long and short, is a 1 galaxy diameter length. 

In my poster, you will see 6 example objects from each of the following 6 categories, shown in columns on the poster progressing from left to right:

1.    M1 - No Bridge  (10)
2.    M2L – Long Bridge  (23)
3.    M2S – Short Bridge  (62)
4.    M3 – 2 Close Cores  (27)
5.    M4 – 1 Core with Tails  (19)
6.    M5 – 1 Core  (11)

Each of the 6 links above will take you to my corresponding Astrobin collection of these previous posted object images.  The number of objects in each collection is shown above in parentheses.

It should be emphasized that this is primarily a visual classification system.  From our limited visual perspective, the categorization for some objects is a difficult judgement and may be wrong.  For example, a short bridge (M2S) may actually be a long bridge (M2L) instead if our view perspective is primarily parallel to the bridge axis.  Another example is that the lack of bright tidal tails (M5) may be due to our viewing distance and not because of the density of the tails.

Since there is not yet a publicly available reference for this collection of galaxy merger objects, I have compiled an Excel spreadsheet of all of the over 150 galaxy merger objects listed above.  The spreadsheet includes object name, coordinates, notes, and classification. 3 screenshots of the full spreadsheet are shown above in the Revisions section.  The spreadsheet can be freely downloaded via this link to the file on my Google drive.