I for one think that NGC 4731 represents an early stage in the formation of a spiral galaxy. My basic idea is that you have two galaxies pass close to and disrupt each other (like the Antenna galaxies are doing). The result is that even their central cores get stretched out like silly putty. After they pass, there seems to be an effect whereby a lot of the mass gets pulled into the area between the galaxies, creating a gravitational field that halts their outward post-collision progression. It is as if their central objects are being drained to make a new central object. See the APOD for NGC 1300 (
http://antwrp.gsfc.nasa.gov/apod/ap060827.html).
My interpretation of NCG 1300 is that each of its arms are the remains of a galaxy. The bright areas at the transitions to the bar are the remains of the central objects of the old galaxy, and the bar is a newly forming central object and halo for the combined galaxy. NCG 4536 (seen in
http://www.atlasoftheuniverse.com/galgrps/virii.html) then is the next state of spiral galaxy formation, as the arms wrap around the new central object. Similarly, NGC 4731 is an earlier stage then NGC 1300, where the arms are still very loose and the new central object and halo are fairly young. (This is not to discount tidal effects from a nearby galaxy, which certainly are influencing the shape of the arms and the evolution of NCG 4731.)
It would be interesting to see if people can find pictures that represent transition stages between the initial collision and the formation of a strong bar. I also wonder what would cause such a phenomenon, and suspect that dark matter is the culprit. So maybe we can learn more given that a real-universe trajectory (or set of trajectories) for spiral galaxy formation from the remains of a collision can be identified.
Virgo Cluster Galaxy NGC 4731
