# NASA pictures of dark matter collisions

1. Mar 27, 2015

### Staff: Mentor

http://www.nasa.gov/press/2015/marc...that-may-help-identify-dark-matter/index.html

The take away seems to be that some ideas about dark matter may have problems. The possibilities checklist of dark matter interactions has been shortened.

There are some really great pictures of galactic halos in colliding galaxies. Which make looking at the link a must. IMO.

2. Mar 27, 2015

### Greg Bernhardt

What about the hubble telescope makes the image blue?

3. Mar 27, 2015

### Chalnoth

The blue blobs are estimates of the mass of the galaxy cluster by examining the distortions of galaxies behind the cluster due to the cluster's gravity.

4. Mar 27, 2015

### marcus

http://arxiv.org/abs/1503.07675
The non-gravitational interactions of dark matter in colliding galaxy clusters
David Harvey, Richard Massey, Thomas Kitching, Andy Taylor, Eric Tittley
(Submitted on 26 Mar 2015)
Collisions between galaxy clusters provide a test of the non-gravitational forces acting on dark matter. Dark matter's lack of deceleration in the bullet cluster collision' constrained its self-interaction cross-section \sigma_DM/m < 1.25cm2/g (68% confidence limit) for long-ranged forces. Using the Chandra and Hubble Space Telescopes we have now observed 72 collisions, including both major' and `minor' mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6\sigma significance. The position of the dark mass has remained closely aligned within 5.8+/-8.2 kpc of associated stars: implying a self-interaction cross-section \sigma_DM/m < 0.47 cm2/g (95% CL) and disfavoring some proposed extensions to the standard model.
5 Pages, 4 Figures and 18 pages supplementary information
Science, Vol 347, Issue 6229 (2015)

5. Mar 27, 2015

### wabbit

6. Mar 27, 2015

### Drakkith

Staff Emeritus
What exactly does this mean?

7. Mar 27, 2015

### marcus

Hi! It's interesting how they map invisible mass concentrations using so called "weak lensing" of background shapes. I know you're familiar with this but someone new to it might not be.

Background shapes get "squashed" in the direction of increasing mass. Circles become ellipses elongated in the direction perpendicular to where the mass is. So the short axis of the ellipse, the "minor axis" will tend to be aligned along the mass gradient. So they can actually produce contour maps of the distribution of invisible mass.

Statistical methods are needed because the background shapes are not perfect circles. They are roughly circular galaxies but tilted randomly so that they appear ellipses oriented in random directions. As their light comes to us, passing the mass concentration, there is a further elongation or, to put it another way, squashing of the shapes.

Last edited: Mar 27, 2015
8. Mar 27, 2015

### Chalnoth

If you have one particle of dark matter passing through a cloud of dark matter particles, this cross section gives the expected distance before the dark matter particle is deflected.

The mass is a part of the calculation because the particle mass determines how many particles there are (we know how dense the dark matter is, but a dark matter particle with half the mass would require twice as many particles to make up that same density).

9. Mar 27, 2015

### marcus

If DM particles had a substantial collision cross section (non gravitational interaction) then two clouds could bump, and cancel each others momentum. So a larger cloud would remain at the site of the collision.
But for example in the "bullet cluster" collision where two clusters collided the two DM clouds basically just passed through each other and came out the other side. The ordinary matter galaxies did likewise because they were scattered so sparsely in the cluster that they had very little chance of colliding.

But the collision left a cloud of hot hydrogen gas in the middle, radiating Xray. Because the intergalactic medium hydrogen did have a substantial interaction cross section. Those clouds could collide and cancel each other's momentum and accumulate at the site of the collision.

When clusters collide the stars and dark matter particles pass through freely. Only the intergalactic medium, the ordinary (hot, partly ionized) gas, actually crashes.

Last edited: Mar 27, 2015
10. Mar 27, 2015

### wabbit

I was impressed by what seems to be very precise post-treatment of the image to correct for residual aberrations (off-axis astigmatism it seems from how they describe it) in Hubble's optics (figure S4, p.8, supplementary material). This might otherwise interfere with their interpretation of the image I presume, for smaller galaxy images.

Last edited: Mar 27, 2015
11. Mar 27, 2015

### wabbit

I was aware there was distortion but not really of its precise shape, thanks for the explanation.

12. Mar 29, 2015

### SpiderET

13. Mar 29, 2015

### wabbit

I would read it more as a measurement of dark matter properties (well, that's not much of a stretch given that this is how the experiment is constructed and reported). There aren't so many of these as far as I know, and more are needed to narrow down the search for what it might be. Excluding some of the proposed models sounds like excellent news for this search.

I see arstechnica titles "strongest case yet for dark matter". I need to read it now, not seeing yet why it adds so much to the case given that the evidence was already pretty strong. Edit : not sure the article really supports the title, but it's a well done piece, thanks for the link.

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14. Mar 29, 2015

### Chalnoth

15. Mar 29, 2015

### SpiderET

Somehow I cant copy text from the pdf, but in the preprint on page 1 and 2 is written, that interacting particles would solve some problems of incorrect predictions of Lambda CDM model. But this paper has shown that the particles dont interact.

But personally I would go even further. This dark matter has strange properties, it interacts gravitationally with normal matter but it doesnt interact gravitationally with other dark matter? Seems like another sign that we need new theory which will include some extended or modified gravity theory.

16. Mar 29, 2015

### Chalnoth

It hasn't shown that they don't interact. It's placed an upper limit on how strongly they can possibly interact. While there are models where dark matter particles have no interactions except through gravity, most models have dark matter that interacts weakly with itself and with normal matter.

Yes, dark matter interacts gravitationally with other dark matter. By "doesn't interact" they're talking about the fact that dark matter particles rarely collide with one another.

17. Mar 30, 2015

### PWiz

Very interesting. It would appear that even collisions between entire galaxies cannot induce dipole moments in the dark matter and "light it up". I think it's reasonable to suspect that dark matter does not have internal organization like atoms, which makes me suspect that the absence of any electromagnetic interaction is due to the presence of uncharged hadrons that are simply held together by the gravitational force. Anyone care to tell me how right/wrong I am?

18. Mar 30, 2015

### wabbit

Not an expert in any way here - but hadrons are standard model particles. As such, they are as far as I know pretty much excluded as a significant component of dark matter defined in a broad sense (which does include hadrons in "baryonic dark matter"), and I'm quite sure they're not a component of cold dark matter as currenly modelled in LCDM.

Also, hadrons are held together by the strong nuclear force, not by gravitation which affects them far more weakly. And they do interact with EM - free neutrons quicly decay to proton + electron + photon for instance (https://en.wikipedia.org/wiki/Neutron#Free_neutron_decay)

Last edited: Mar 30, 2015
19. Mar 30, 2015

### Jimster41

Would "uncharged Hadrons held together by g..." be little clumps of neutrons?