What a difference a display range makes

For those not following this regularly, I'm simulating what would happen if a dark galaxy (gas disc with dark matter but no stars) fell into a galaxy cluster. Last time we saw that very little happens to most of the discs, most of the time.

But that's not the whole story. Most of the discs do survive just fine, but some material get ripped off by other galaxies passing by. Some of this material gets pulled out into extremely long features. This gif is exactly the same data set as the last one, just with each galaxy shown at a much larger field of view and with the low density material enhanced with my shiny new method. That's important because although these streams are enormous - some of them are more than 3 million light years long - they have incredibly low density. If they exist in reality, there's no way to detect them.

But looking on this larger scale does give some clues as to features we could detect. For example, the famous VIRGOHI21 "dark galaxy" has been shown to possibly just be some sort of unusual tidal debris. VIRGOHI21 is a dense blob of hydrogen in the middle of a long stream originating from a spiral galaxy. Previously, it was thought impossible to produce long one-sided tails like this through tidal interactions until one now famous paper by Duc & Bouranud showed it was possible.

What this quick and dirty little analysis shows is that such features are indeed possible but they are very rare and not long-lived. Unfortunately, whenever any starless clouds of hydrogen are discovered there's a tendency to say, "it could be tidal debris, look at the Duc paper". However the Duc paper only shows that such things are possible - not that they are likely. From this small effort it appears that the setup has to be quite precise to produce something even close to VIRGOHI21. That other similar features exist may indicate that hand-waving about "tidal debris" is not an adequate explanation.

On the other hand this model doesn't use a very realistic galaxy - it's far too small and the density profile is all wrong. So this is just a preliminary examination, pointing the way to a proper study and indicating that it should be worthwhile. Fortunately, that's now a matter of some minor code adjustments and pressing "go". With any luck I'll soon have a nice controversial result to publish.