This is either very interesting, not interesting at all, or I've made a mistake somewhere...
Three galaxies falling into a cluster and being pummelled by the gravity of 400 other galaxies (not shown). The field of view is the same size in each case, tracking the centre of the galaxy as it falls through the cluster. The orbit of each galaxy should be very similar (though I need to check this to be sure), so each one should be experiencing the same gravitational field from the other galaxies.
On the left we have a small, lightweight galaxy, let's call it galaxy A. It gets fairly heavily disrupted, losing ~50% of its initial mass. Not terribly surprising, actually it did better than I thought it would.
On the middle and right we have a much larger galaxy that was designed to be similar to a specific galaxy. Not all of its properties are very well known though. The middle panel (let's call it B) uses properties similar to that of an earlier simulation on the same object, which has a particularly massive and extended gas disc but not so much dark matter. The one on the right (C) uses more realistic parameters, with a slightly smaller, less massive gas disc but quite a bit more dark matter.
Galaxy C suffers by far the least disruption, losing only around ~10% or so of its gas. Hurrah ! It's something like 40 times more massive than the one on the left, so that's not too surprising.
What is surprising is galaxy B. It's 20 times more massive than galaxy A, but suffers just as badly - if not worse. Half its gas gets ripped off, but when you look at the disc close-up you see it's in far worse shape. So the factor of 20 in mass difference hasn't made much difference, but apparently a further increase in a factor of 2 to galaxy C makes a huge difference. That doesn't make a lot of sense to me. OK, B also has a bit more gas which is a bit more extended, but still...
There's a couple of possibilities. Possibly the difference in mass has put the galaxies on different orbits and B has been unlucky and collided with a particularly massive galaxy. Or, perhaps the difference in rotation speed might be responsible - B might have had a resonant encounter with a galaxy moving past it as roughly the same speed as it's rotating, which maximises the time the other galaxy has to pull on the gas. But neither of these seem like good explanations for why there's such a dramatic difference. Hmmm....
It looks like a freakishly strong encounter may be responsible. The other simulations running (which are not yet complete but are should be, touch wood, complete enough to judge) show strong distortions, but not anything like this strong. I won't know for sure until Monday when probably ~50% will be complete. They're all identical except for the initial position in the cluster, for the very reason that it was unclear how much difference it would make. For the previous simulations (A and C) it made a difference to the details, but not really all that much to the major parameters. It seems that B might be different, but I'll have to wait and see to know for sure.
ReplyDeleteSo far my hypothesis is standing up to scrutiny. It seems that B happens to spend a very long time interacting slowly with one of the other galaxies. Normally the galaxy falls so quickly through the cluster that it doesn't spend much time in close proximity to any individual object (maybe a few tens of megayears, less than one rotation period). High velocity encounters aren't very damaging because there's not much time for the gravitational acceleration to change the particle's velocity or position. With low velocity encounters, the mass of the other object can be the same (so the force is just as strong) but it has a longer time to do damage.
ReplyDeleteIn the case of B it so happens that it spends more than a billion years interacting with one of the galaxies - its velocity is only just barely high enough to escape. By the time it does so it's lost a full half of its dark matter content. So it ends up being only ten times more massive than A (instead of its initial twenty), but with a much more extended gas disc. The escape velocity is only 50% larger than A. C, in comparison, has an escape velocity four times larger than A. I suspect there's also something of a runaway effect going on : the more material is removed, the easier it is to remove the remaining material.
Well, that works well enough so far, but now I should check what's happening to C. If C doesn't have a similar encounter and doesn't lose so much mass, that would pretty much confirm it. If it does, that might need a re-think.