I love imaging and examining interacting galaxies, with their disturbed arms and extended star streams.  Most of the time, the two (or sometimes three) interacting galaxies are easily identified, as seen in my collection here of merging galaxies with short bridges. 

Occasionally, in the hundreds of disturbed galaxies I have imaged, a galaxy appears to be disturbed in isolation, with no obvious companion or “intruder” causing the disturbance.   The poster contains the 20 galaxies which I have found to be most mysteriously disturbed.  All of these galaxies are asymmetric with distorted arms and no obvious companions. 

I have only included galaxies on the poster which are greater than 40,000 light years in diameter.  I have excluded smaller galaxies, typically categorized as irregular, dwarf, or Magellanic galaxies, since their commonly distorted appearances are not that surprising due to their low mass and rotational momentum.

I am fascinated with examining the galaxies on the poster and considering the many possibilities, confirmed and theoretical, of how they became so disturbed in apparent isolation:

Small Companions: 
Each of these objects have bright regions within their disks.  These bright regions are likely star clouds or Ha regions, but they could be small companions causing disturbance.  In each case I don't see much localized disturbance which would confirm that the bright region is a disrupting companion.  Also, such companions would be very small, on the order of 5000 light years or less in diameter, so a significant impact on the structure of the much larger disk is difficult to explain.

Past Companions: 
Perhaps the disturbance is from a long ago companion which has now been partially or fully absorbed. In the time in takes for a companion to be absorbed, I would think that the galaxy would have time to return to a more symmetric shape.

Galaxy Harassment:
We see evidence of galaxies which have likely been disturbed by a prior close pass of another similar-sized galaxy. M51 is an example of this.  Over time, the disrupting galaxy could now have moved far enough away to be difficult to determine that it was the culprit.

Galaxy Hit and Run:
We also see evidence of galaxies which have likely been disturbed by a direct impact from a smaller galaxy.  Collisional ring galaxies are examples of this.  If the relative velocities and directions of the galaxies are very different, the smaller intruder may be able to survive the collision somewhat intact and exit the scene in a hurry, leaving the disturbed galaxy in its wake.

Dark Galaxy Interaction (theoretical):
A dark galaxy has few visible stars but significant gravitational influence.  To date, these galaxies are hypothetical - none have yet to be confirmed.  If they exist, it is easy to imagine how they could interact with and disturb the disk of a visible galaxy.

Dark Matter Halo (theoretical):
Undetected dark matter within the galaxy is another hypothetical reason for disturbance.

Gas Accretion (theoretical):
Large gas accretion from intergalactic cosmic filaments has been identified as a possible reason for asymmetric disk development. I am not aware of any examples where this has been proven.

Other Effects (theoretical):
Could other phenomena, yet to be identified, be responsible for these disturbances?   Astronomy has seen many examples over the years where our current understanding has not been sufficient to correctly assess what is happening at the time.  

Hopefully some day we will have the answer.   But for now, I hope you enjoy examining these beautiful asymmetric galaxies as much as I do, regardless of the disturbance cause.

Each of the object images on this poster has been posted to my Astrobin account.  The links to each individual image are below:

NGC 812
Arp 335
NGC 6926
NGC 6570
NGC 6239
NGC 1359
Arp 005
NGC 3511
NGC 7303
Arp 020
Arp 012
NGC 150
NGC 5430
Arp 015
NGC 3020
NGC 662
Arp 049
UGC 06309
NGC 337
IC 1291

All of the above objects are contained in my Astrobin Mysteriously Disturbed Galaxy Collection.