- #1
Halc
Gold Member
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- TL;DR Summary
- Galaxy collisions seem to involve considerable friction which seems absent in simulations.
There are countless time lapse simulations of galactic collisions, squashing say 20 billion years into a few minutes or even seconds. Many of these involve the upcoming collision with Andromeda, but my question is more general.
No, I don't have a specific video in mind.
Most of these show a pair of fairly neat spiral galaxies with defined arms. The near arms distort first and the symmetry is lost. The cores of the galaxies represent considerable mass and their close pass causes at one point a sort of explosion of stars being flung away by gravitational acceleration (which robs especially the central black holes of much momentum). This part is fine and expected. Most of that material was already close to one of the central masses. Point is, those simulated flung objects tend to exit the maelstrom at speed without slowing, and I question this.
Compare this to observations of some recent galactic collisions. You have this disorganized mess shortly after what is concluded was a fairly high speed collision. What stands out with some of these is that each carried a cloud of dark matter which for the most part kept going and exited the combined galaxy whose rotation curve is now far lower than it would be with the DM. The baryonic matter coming in at those speeds all managed to slow down by interaction with matter from the other galaxy, when the dark matter did not. That demonstrates significant friction preventing these galaxies from just parting again like the dark matter did. Sure, some stuff was flung away, but if there was that much friction and it came from the combined core, it probably lost much of its speed to friction on the way out, dropping much of it back below escape velocity. The simulation don't seem to account for this.
I know they can't separately compute the effect between each planet, star, rock, dust, whatever, but to ignore average friction seems to be an oversight, and it would significantly alter the videos created. I'm not sure if there's a way to measure the actual baryonic mass ejected at escape velocity from a collision and compare it to the simulation.
Irrelevant to the question:
I presume the dark matter will eventually find its way back, but will always have this larger 'orbit' which prevents it from clustering significantly around the new galaxy. Its cloud will remain more dispersed that ones from lower speed collisions. I don't thing Andromeda is considered to be particularly high speed. In peculiar velocity terms, we're actually moving away from it and it is overtaking us from behind.
No, I don't have a specific video in mind.
Most of these show a pair of fairly neat spiral galaxies with defined arms. The near arms distort first and the symmetry is lost. The cores of the galaxies represent considerable mass and their close pass causes at one point a sort of explosion of stars being flung away by gravitational acceleration (which robs especially the central black holes of much momentum). This part is fine and expected. Most of that material was already close to one of the central masses. Point is, those simulated flung objects tend to exit the maelstrom at speed without slowing, and I question this.
Compare this to observations of some recent galactic collisions. You have this disorganized mess shortly after what is concluded was a fairly high speed collision. What stands out with some of these is that each carried a cloud of dark matter which for the most part kept going and exited the combined galaxy whose rotation curve is now far lower than it would be with the DM. The baryonic matter coming in at those speeds all managed to slow down by interaction with matter from the other galaxy, when the dark matter did not. That demonstrates significant friction preventing these galaxies from just parting again like the dark matter did. Sure, some stuff was flung away, but if there was that much friction and it came from the combined core, it probably lost much of its speed to friction on the way out, dropping much of it back below escape velocity. The simulation don't seem to account for this.
I know they can't separately compute the effect between each planet, star, rock, dust, whatever, but to ignore average friction seems to be an oversight, and it would significantly alter the videos created. I'm not sure if there's a way to measure the actual baryonic mass ejected at escape velocity from a collision and compare it to the simulation.
Irrelevant to the question:
I presume the dark matter will eventually find its way back, but will always have this larger 'orbit' which prevents it from clustering significantly around the new galaxy. Its cloud will remain more dispersed that ones from lower speed collisions. I don't thing Andromeda is considered to be particularly high speed. In peculiar velocity terms, we're actually moving away from it and it is overtaking us from behind.
