This poster is the 43rd in my series of compilation posters.  All of these posters can be seen in my Astrobin Poster Collection.

This poster is a collection of flocculent galaxies.  The word "flocculent" comes from the Latin word floccus, which means “flock of wool”. Flocculent galaxies have discontinuous spiral arms and do not look like your classic spiral galaxy. Spiral arms are very difficult to trace in the flocculent galaxies, often starting and stopping seemingly at random.  

It is easiest to describe flocculent galaxies by what they do not have: a clear spiral structure. It is believed that flocculent galaxies form as a result of the presence of weak spiral density waves.  Instead of star formation occurring in the spiral arms, the star formation in a flocculent galaxy occurs more randomly on a local basis (mainly from supernovae explosions), forming only small-scale structures that slowly spread throughout the disk.  This effect has been compared to a forest fire, with the flames jumping from tree to tree.

The obvious question then is why flocculent galaxies have weak spiral density waves?  I don't remember seeing an explanation.  I find it curious that none of these galaxies have bars.  Perhaps something is inhibiting bar formation and thereby resulting in weak spiral density waves.

To my surprise, the flocculent characteristic of a galaxy is not part of a standard classification system, which makes them a bit harder to identify as a group.  This poster includes only those galaxies which I could confirm as described as flocculent by various sources.

In compiling this poster, I did not include some galaxies which were described as flocculent.  These fell in 4 broad categories:
1. Small Magellanic spirals (such as NGC 178)
2. Faint low surface brightness (LSB) galaxies (such as NGC 45)
3. Galaxies with a strong spiral arm structure in the outer disk (such as NGC 5033)
4. Galaxies with a strong concentric ring structure (such as NGC 2775)

Although the above 4 examples are considered by some to be flocculent galaxies, they appear to me to be different than the other, more classic flocculent galaxies on this poster and so I did not include them here.

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 Flocculent Galaxies Collection.