Proof of Dark Matter Existence from a Collision of Galaxy Clusters

[Borodog]

In an attempt to inject some actual science into SMP, I thought I would post a trip report from the UNC physics department colloquium I attended today, given by Maxim Markevitch, one of the team that showed undisputably that dark matter must exist. This is prepared from notes that I took during the lecture, so it may be slightly disjointed. I will attempt to elucidate where necessary, but it is inevitable that non-physicists may be left with questions.

The Necessity of the Existence of Dark Matter

Dark Matter was postulated as far back as the 1930s. If you look at the rotation curves of galaxies, as well as the orbital velocities of globular clusters, as well as the orbital velocities of galaxies within clusters, you discover that they are moving far too fast for the amount of mass that is visible; i.e. they should not be gravitationally bound and the systems should "fly apart."

The amount of mass tied up in stars in a typical cluster is about 1-3%, while the mass tied up in intergalactic gas (which we can see because it has some temperature and hence emits radiation) is about 10-20%. This leaves around 80-90% of the mass not visible at all, except in it's gravitational effects on the visible matter. Hence, Dark Matter.

Modify Gravitation?
The idea that the universe is dominated by matter that we can't see is an uncomfortable thought, and attempts have been made to modify the laws of gravity to account for the gravitational effects without invoking Dark Matter. Luckily, such theories are falsifiable. All we have to do is locate an object whose visible mass is displaced from it's total mass in space, and voila, you've disproved simple modification of the law of gravity and hence proved the existence of Dark Matter.

That's exactly what Markevitch and the rest of the team did, using colliding galaxy clusters.

Colliding Galaxy Clusters

Galaxy clusters are the highest energy events in the history of the universe subsequent to the big bang, with kinetic energies of ~10^63.5 ergs.

Galaxy clusters are composed of pointlike (collisionless) galaxies embedded in clouds of intergalactic gas. Because the gas is a fluid (in the hydrodynamic sense; not a liquid, but a fluid, which both liquids and gases are), it is not collionless. So the galaxies will pass right through during the collision, but the gas will not. It will "pancake" in the middle and form nice things like shock waves. Depending on how interactive the dark matter is, we might expect it to pass right through with the galaxies (collisionless), or pancake in the middle with the gas (collisional).

The team looked at the collision of two galactic clusters that have formed the cluster 1E 0657-56, at a distance of about z = 0.3. The Chandra x-ray image shows two bright regions where the intergalactic gas has been heated by shocks, including a gorgeous textbook conical bowshock on one side with a bullet shaped core of hot gas. Such a configuration might form when a smaller denser cloud of gas plowed through the center of a larger, thinner cloud.

The Chandra image shows us exactly where the visible mass is, because it has been heated by shocks and is emitting.

Meanwhile, Hubble optical images were also examined for evidence of gravitational lensing. Gravitational lensing occurs when the light from a distant gallaxy on the far side of a cluster of galaxies is bent by the mass of the cluster and redirected towards the Earth, resulting in distorted images of the subject galaxy around the rim of the lens, usually smeared into arcs and lines. There will often be multiple images of the same subject galaxy, which can be identified with each other spectrographically. You can always tell the background (lensed) galaxies from the foreground (lens) galaxies because the former have much higher z's (redshift).

The Hubble optical image is essentially deconvolved to produce a map of the distribution of the total mass in the lens (the intervening cluster).

So they did this, and lo and behold, there were again two centers of mass, but they were not coincident with the two centers of mass of the gas from the X-ray image; they bracketed the gas by a good amount. In fact the centers of mass of the total mass were coincident with the galaxies from the two subclusters that had collided.

So the two mass peaks from the total mass were NOT coincident with the two peaks of the visible mass. This disproves the theory that we can just modify the law of gravity, and proves conclusively the existence of dark matter.

The fact that the total mass peaks were coincident with the galaxies also proves that the Dark Matter is collisionless, or at least has such low interaction that it is astrophysically insignificant and uninteresting (although it might not be uninteresting to particle physicists, though!). However, totally collisionless calculations seem to not match other observations, like they predict too sharp of a mass peak at the centers of dwarf galaxies, and the existence of galactic dark matter "halos", neither of which seem to happen. So there may be some interaction cross section, it would just have to be between half a square centimeter per gram to 5 square centimeters per gram.

In the galactic cluster collision, there is a textbook bow shock, ~1Mpc across. The Mach number of the shock is M = 3.0 +/- 0.4, and the shock velocity v_shock = 4700 km/sec. Analysis of the velocities and the separations show that the collision happened approximately 100 million years ago, which is a blink of the eye in cosmic terms.

A minor note is that there are two types of gravitational lensing that can be used, so called "strong lensing" and "weak lensing." I am unclear on the difference and didn't get a chance to ask for clarification. An interesting result is that the two methods produce total masses that are different by a factor of 2; the strong lensing results show a total mass twice that of the weak lensing results. This is an unresolved mystery, but it is consistent across for different cluster collisions they have analyzed.

Dark Matter Interaction Observational Evidence:
• Offset between gas and dark matter
• No offset between dark matter and galaxies
• Subcluster’s velocity not less than free-fall velocity (would be lower if Dark Matter were interactive)
• Subcluster’s M/L ratio (mass to light) close to universal (would be different if dark matter particles were collisional, subcluster (smaller one, from the bullet shaped gas cloud) should lose particles in the collision (for esoteric physical reasons I won't get into). But it isn’t).

Therefore sigma/m < 0.7 cm^2/g, which excludes most of the possible region (sigma is the interaction cross section). Whatever it is, it must be small (unless it is velocity dependent).

What is Dark Matter made of?
Sterile neutrinos as Warm DM?

So-called sterile neutrinos are one possibility.
• Do not interact with ordinary matter
• m_s ~ 0.3 – 100 keV
• Helps explain cores in dwarf galaxies and deficit of small halos (m_s ~ 1 – 2 keV)
• Radiative decay E_gamma = m_s/2 -> x-ray constraint (gobble gobble wha? I missed this part)

So sterile neutrinos are still alive as a possible source of Dark Matter; however, these results and others have constrianed them into a pretty small box of parameter space.

If that last bit didn't make much sense to you, it didn't make much sense to me either. All I know is that neutrinos are weakly interactive but have mass, so if you have a [censored] load of them you have essentially "Dark Matter."

There was some more esoteric stuff involving "mainstream" physics that interested me, but probably wouldn't interest you. They identified ultrarelativistic particles (gamma ~ 10^4) from synchrotron radiation, indicating high energy electrons, most likely accelerated by converging magnetic fields during the cluster collision (bounce a ball on the ground and the lower a book on top of it; the ball will bounce faster and faster between the floor and the book; charged particles can do this between colliding "magnetic mirrors"), there was some interesting stuff about the shape of the bowshock, which seems to be wrong (specifically the opening angle of the Mach cone), but can be accounted for by the fact that the subcluster is changing size over time since the collision, also the Mach cone has a funny deflection in it, which may be due to a compressed layer of magnetic field (which also might be inhibiting the Kelvin-Helmholtz instability).

Lastly, there was a prediction that the bowshock should break apart in the next hundred million years due to the onset of the Rayleigh-Taylor instability, which is cool. Essentially, the dense gas behind the bowshock is now suspended "over" the lower density gas ahead of it, since the center of mass (the dark matter) is beyond the bowshock. Thus is like a heavier fluid suspended over a lighter fluid, and it is unstable; it will break up into "fingers" of gas that will extend beyond the bowshock toward's the Dark Matter center.

Altogether nifty [censored].

[Borodog]

*Galaxy cluster collisions are the highest energy events in the history of the universe subsequent to the big bang, with kinetic energies of ~10^63.5 ergs.

[Nielsio]

cliff?

[Prodigy54321]

very interesting even though I don't understand it [img]/images/graemlins/tongue.gif[/img]

[Borodog]

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cliff?

[/ QUOTE ]

The existence of dark matter was conclusively proven by identifying two objects (colliding galactic clusters) whose visible centers of mass are in different places from their total centers of mass as determined from gravitational lensing measurements.

[Borodog]

Here's an article on the "sterile neutrino" theory that went over my head (never heard the term until this talk, actually):

http://www.universetoday.com/am/publ...r.html?2332006

[FortunaMaximus]

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Altogether nifty [censored].

[/ QUOTE ]

Away from home for a couple days, surfin' the usual. Nice. Nowhere nearly able to chew on the technical details as you well know. Still gonna be fun to read and reread for awhile yet.

[img]/images/graemlins/tongue.gif[/img]

K.

[John21]

First we're no longer at the center of the universe, now we're not even made of the same stuff as most of the universe. Great.

Its still a big leap to make the assumption that all the ordinary matter in galaxies is in easily-visible, bright, stars. They seem to be saying that the gravitating mass is much greater than the mass in easily-visible stars, and conclude that since there is no observable matter, the missing mass must be dark matter.

Out of curiosity, did they pose options other than neutrinos for DM?

FWIW: An alternate theory to the big bang: Wiki Plasma Cosmology

Overview:
The basic assumptions of plasma cosmology which differ from standard cosmology are:

1) Since the universe is nearly all plasma, electromagnetic forces are equal in importance with gravitation on all scales.[3]
2) An origin in time for the universe is rejected due to causality arguments and rejection of ex nihilo models as a stealth form of creationism.
3) Since every part of the universe we observe is evolving, it assumes that the universe itself is evolving as well, though a scalar expansion as predicted from the FRW metric is not accepted as part of this evolution (.

[carlo]

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The existence of dark matter was conclusively proven by identifying two objects (colliding galactic clusters) whose visible centers of mass are in different places from their total centers of mass as determined from GRAVITATIONAL lensing measurements.


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Are there any astrophysicists who deny that gravity as on earth is different than outer space as in this post? Does anyone state that the earthly laws are only good for earth/our solar system?

[Borodog]

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Out of curiosity, did they pose options other than neutrinos for DM?

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Nope, just the one theory.