The Angry Physicist makes a good spot
Press conference next week on a "ring of dark matter" discovered by the Hubble Space Telescope, Advanced Camera for Surveys (RIP).
This is almost certainly this result reported at the AAS
The authors took strong and weak lensing data from a moderate redshift cluster and see a ring mass density enhancement at about 1.5 core radii.
The interpret it, with supporting collisionless numerical models, as a "turnaround ripple" from a near head-on collision of two clusters 1-2 Gyrs ago.
I am skeptical but intrigued. Not having seen the actual paper, I'd worry about systematics in stitching the strong and weak lensing data to an absolute mass surface density.
The "ripple" effect was shown by Quinn and Hernquist ~ 25 years ago (for colliding galaxies), but it is hard to get a lot of mass excess into the ripples for plausible collisions of collisionless particles.
Be intriguing if it is correct.
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James's result is a curious but sketchy, S/N~3-5, one. He says you can get it from a line of sight merger, but I have heard others say that that result is artificial (maybe it happens in dark matter only simulations?).
As for the absolute mass density, I am not sure that is a problem. I think that this shows up in the shear data alone, but I could misremember, it has been a long time since I have seen a draft of this paper.
A ring of dark matter? Try using a detergent with bleach.