B Rutgers finds a transparent Einstein Cross - apparently a dark matter "halo"

[.Scott]

TL;DR Summary

Using NOEMA and ALMA in Chile, astronomers at Rutgers, New Brunswick find five images of galaxy Hers-3 in an Einstein Cross pattern, but with the central image still visible. So, apparently, gravitational lensing of something very heavy and very transparent.

The formal paper is here.

The Rutgers University news has published a story about an image being closely examined at their New Brunswick campus.
Here is an excerpt:

Computer modeling of the gravitational lens by Keeton and Eid showed that the four visible foreground galaxies causing the gravitational bending couldn’t explain the details of the five-image pattern. Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations. “We tried every reasonable configuration using just the visible galaxies, and none of them worked,” said Keeton, also a professor in the Department of Physics and Astronomy and a co-author of the study. “The only way to make the math and the physics line up was to add a dark matter halo. That’s the power of modeling. It helps reveal what you can’t see.”

I find it interesting because this single observation eliminates some of the wilder "theories" about the nature of dark matter.
And I wouldn't be surprised if it eventually allows us to further characterize dark matter.

 

[Jaime Rudas]

I find it interesting because this single observation eliminates some of the wilder "theories" about the nature of dark matter.

I think you underestimate the enormous retrospective adaptability that these "theories" have always demonstrated.

 

[timmdeeg]

Here is an excerpt:

The only way to make the math and the physics line up was to add a dark matter halo. That’s the power of modeling. It helps reveal what you can’t see.”

I find it interesting because this single observation eliminates some of the wilder "theories" about the nature of dark matter.
And I wouldn't be surprised if it eventually allows us to further characterize dark matter.

As I understand it, many observations show that dark matter is gravitationally bound to visible matter. Supposed this dark matter halo has existed initially together with visible matter, are there mechanism thinkable wich could separate both types of matter?

 

[.Scott]

As I understand it, many observations show that dark matter is gravitationally bound to visible matter. Supposed this dark matter halo has existed initially together with visible matter, are there mechanism thinkable which could separate both types of matter?

For clarity: Many observations show dark matter coexisting with visible matter. The dark matter conversation started when inventories of visible galaxies failed to tally enough mass (gravitation) to keep spinning galaxies intact.

Separable? Yes, absolutely. Any electromagnetic force that pushes visible matter hard enough would be expected to separate it from dark matter. Truly dark matter would not "collide" with the surface of a planet, star, or anything else short of an event horizon.
And if it is dark even to other dark matter, it could not form an effective dark matter accretion disk either - and so it would last in orbit longer.

 

[timmdeeg]

Separable? Yes, absolutely. Any electromagnetic force that pushes visible matter hard enough would be expected to separate it from dark matter.

The main mass of a galaxy is located in its bulge. In this case separation is hard to imagine, isn't it?

 

[.Scott]

The main mass of a galaxy is located in its bulge. In this case separation is hard to imagine, isn't it?

Not at all. The bulge itself is the result of visible matter colliding with visible matter. The dark matter "halo" would be the result of the dark matter's failure to participate in those collisions.
Basically (as the theory goes), in a galaxy such as the Milky Way, both dark and visible matter start out in a variety of orbits around the center of gravity. For visible matter, this results in a lot of collisions that force a lot of the mass to "line up" in orbits that lie on a flat disk. Meanwhile, dark matter retains in its original wide variety of orbits - forming a sphere-like "halo".

 

[timmdeeg]

Meanwhile, dark matter retains in its original wide variety of orbits - forming a sphere-like "halo".

... around the bulge, yes. From here we start. The Bullet Cluster shows that even after collisions of Galaxies dark matter is still around visible matter.

So, how can visible matter, e.g. the bulge of a galaxy, get rid off its halo of dark matter?

 

[.Scott]

So, how can visible matter, e.g. the bulge of a galaxy, get rid off its halo of dark matter?

There's two questions here:

There are galaxies that are light on dark matter (NASA article). The explanations provided in that article is either that the dark matter was stripped away by tidal force when it collided with another galaxy - or that the galaxy in question is closer than we think.

The other question - and more to the point of the observation in the OP - is how does a dark matter halo get rid of its visible bulge? I would think that the answer is 1) arrange to be looking side-long at the galaxy and 2) let the bulge do its visible-matter flattening out process.

 

[timmdeeg]

Thanks, to me its hard to see though how 1) and 2) result in a dark matter halo without visible matter.