Arp 227 is the central galaxy pair in the image. These galaxies, also known as NGC 474 (elliptical) and NGC 470 (spiral), are both located 100 million light years away in the constellation of Pisces at a declination of +3 degrees. These galaxies are each about 5 arc-minutes in diameter in our apparent view, which corresponds to a diameter of 150,000 light years.

Both galaxies have fascinating structures. The left galaxy, NGC 474, is a shell galaxy with tidal streams which suggest that a previous merger with a smaller galaxy may have taken place. 

Indeed, simulations seem to accurately model the complex tidal streams that we see in this case.  They are particularly well described in the excellent 2022 paper, “Origin of the spectacular tidal shells of galaxy NGC 474“ (Bílek et al).  The paper concludes that all of the tidal features seen here originated from one merger, an accretion of a smaller spiral galaxy by a large elliptical.  The merging galaxies first collided 1.3 billion years ago and then again on a rebound 400 million years later. The study concluded that the magnificence of this object is due to us observing it at the optimum “moment” in time. Earlier, the debris did not yet take up the shell morphology, and later the shells will lose contrast and disappear.  

In the figure below from the paper showing the results of the simulation over time, everything you see is the smaller spiral companion, except for the red dot which is the center of the larger elliptical:


The paper goes on to say that there is little evidence to suggest that the prominent companion spiral galaxy NGC 470 has been involved in this interaction at all.  But certainly it seems to be impacted by the presence of NGC 474, as seen by its bright blue star formation inner ring and its slight deformations of the extended spiral arms.

Interestingly, this paper observes that today’s simulation models together with the ability now to derive the star formation histories of the centers of shell galaxies will make the determination of formation history a fairly simple process from now on.  This is amazing to me, and great news! 

As much as I enjoyed NGC 474, it is truly incredible that there is another, yet larger (in real size) shell galaxy in this small field of view of 1/3 of a degree.  Take a close look at NGC 467, the shell galaxy in the lower right corner of the image.  Although it appears smaller than the two central galaxies in our view, it is 2.5 times further away. The actual diameter of this galaxy is huge, at over 200,000 light years.

I find NGC 467 even more fascinating than NGC 474, because it appears that the merger and shell formation is still in progress.  Its spiral companion, LEDA 1249151, is clearly interacting with it.  Is this the sole galaxy interacting with NGC 467, or has there been a previous smaller galaxy absorbed by NGC 467?  Sadly, I could find no simulations for this fascinating pair. 

I have included this image of 2 shell galaxies in my Astrobin Shell Galaxy Collection, seen here.

Although this object is not imaged that often, 2 superb Astrobin images of this object have been posted in the past week, both with 10x the integration time of my image.  The image by @Anis Abdul is here, and the image by @Sendhil Chinnasamy is here.