henk21cm wrote: . . . Next, ask yourself the question why not all -or a large percentage of- galaxies show these faint trails like NGC 5907, perpendicular to the main rotational plane.
While I'm not a proponent of the "tumbling" hypothesis, let me proffer the following (in the spirit of Billy Ockham).
It does not seem appropriate to me to consider many of the things we see as singular, "one of" type of events. If one accepts the explanation in the caption as credible, is it not reasonable to expect that similar circumstances (i.e., satellite galaxy ingestion) will result in similar manifestations in other galaxies and to consider that "trails" of this type are more-or-less "normal" when smaller galaxies are ingested? It does not seem unreasonable to me that the fact that we don't observe them ubiquitously is related to things like time since ingestion, different ingestion geometries, different galactic mass ratios, different illumination angles, etc., etc. It seems as if that might be a more fruitful approach rather than to consider NGC 5907 as a completely unique, one-of-a-kind galaxy.
Just like the rings of Saturn, which, many years ago, were taught to be a singular and unexplained planetary anomaly, have now been explained and found to be a "normal" characteristic based on our knowledge of physical laws. In fact, every planet has a set of rings, although most are so tenuous that they are difficult to discern, or they have been dissipated by other phenomena. It took until 1979 for the Jovian rings to be found by Voyager, using observational techniques not feasible until that time, and, several years later, the rings of Uranus and Neptune.
