Why does dark matter separate from baryons in cluster mergers?

[Samuel.J]

I was looking at some recent Chandra data on galaxy cluster collisions, and I’m stuck on how the inferred dark matter distribution can appear so different from the baryonic matter after the interaction. The separation seems so pronounced in these major mergers, but I’m having trouble visualizing the actual gravitational dynamics at play during the event itself.

[DonaldSJ]

I think from looking at Chandra data and the lensing maps, it works like this: the halos of mass mostly cruise through each other while the hot gas cores collide and get ram‑pressure pushed aside. The gas shines in X rays and heats up, but the invisible stuff mostly keeps going along its original path. So after core passage you see the X‑ray peaks offset from the mass peaks seen in lensing, sometimes by a hundred kiloparsecs or more. It feels like the gas gets stuck in the drag, while the dark matter component behaves almost like a couple of bullets that don’t slow much. This makes the distribution look really different between baryons and what’s driving the gravity.

[Aaron53]

I tried to draw a simple visual: imagine two ice skaters wearing invisible capes that don't feel drag, gliding through a fog bank that represents the gas. The fog slows and shifts, the skaters don't. When the two skaters pass through, the fog is left behind in a different shape than the skaters’ trails, so the bright X-ray cloud and the lensing‑mapped mass are not coincident anymore. The offset can be a big clue, but you have to keep track of projection and timing because a single snapshot only catches a moment in a very dynamic process.

[LarryL]

Could the issue be more about projection or timing of the snapshot than about the gravity itself?

[OliverH]

I once wrestled with a toy model where I treated the collision like two clouds plowing through each other. I kept chasing whether the shocks would align with the peak of the mass map, and I kept running into the fact that the gas path is shaped by ram pressure and the halos by gravity. It helped to watch the observed X-ray fronts and the lensing maps side by side, but I still felt the picture was missing a piece, and I’m not sure what it was supposed to show next.