I love extended star streams.  They have a certain elegant beauty and grace to them which bely the tremendously destructive gravitational forces that have led to their creation. 

They are as mysterious as they are beautiful.  Many astronomers in the mid 20th century still weren’t convinced that spatially gradual tidal forces could create such thin filament-like features.  But subsequent excellent theoretical work (by Toomre and many others) as well as advances in computer galaxy collision simulations have shown us that these streams are generated by gravitational tidal forces between galaxies. 

Although we now have a better understanding of the formation mechanism for star streams, the understanding of star stream loops is still evolving.  I had always assumed that loops were formed when 2 galaxies of similar size approached each other, developed long opposing tails, and then danced around each other over millions of years,  twisting the long streams into loops.  But that last step apparently is not correct. 

It turns out, through learning from simulations, that approaching galaxies of similar size don't "dance around" much because their gas essentially puts the brakes on such a dance.  Instead, the galaxy cores coalesce and merge in a surprisingly quick manner.

So how are the loops formed?  Scientists believe that these loops are formed when the long opposing star streams (tails) lose their outward tidal momentum and then start sliding back towards (and eventually past) the coalesced galaxy core. 

Compared to other star stream features such as tails and plumes, star stream loops are rare.   Why are they rare?  I think it boils down to 3 factors:

1. The colliding galaxies have to approach each other with the right mass, offset distance, and rotational axis orientation to create the long star streams which are needed to start the loop formation process.
2. The loop result we see is likely very transient, existing for a relatively short cosmic time like the dust settling from a car crash.
3. Our viewing angle is also critically important – from another view, all of these loops would look different, possibly like plumes or simple streams.

I still have so many questions about loop formation.  If you have knowledge about them, please share in the comments section below.  I am particularly puzzled by the apparent "double loop" structures seen in the objects in the third column of my poster.

If you would like to read more about any of these objects, each of the objects in the poster has previously been uploaded and described individually on Astrobin. They all reside in my Astrobin Loops Collection.

If you would like to access all my DSO compilation posters, please click here.