Dark Matter, Cluster Dynamics and The Bullet

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Everyone points to the Bullet Cluster as the best proof of Dark Matter. But I have a few questions about the rationale. There are aspects of the proof I don't understand.

1) What is the mechanism which makes the interstellar gas visible at such a distance in this particular case? Is all intergalactic gas visible in the same way and at the same brightness levels?

2) How do we know that all or most of the intergalactic gas collided? How do we know that it wasn't the case that most of the gas stayed with the galaxies and only some collided which was subsequently made much brighter because of the heating due to the collision? So in effect, how do we know that the part that forms the bullet wasn't merely just a visible small fraction of the intergalactic gas that collided while the bulk of the gas passed through the other cluster with the visible galaxies themselves?

3) What would have caused the gas to slow down so much as it passed through the other gas? At the densities of intergalactic gas, it seems odd that much of it would have actually collided at the atomic level. What am I missing?

Thanks in advance.
 

Answers and Replies

  • #2
DevilsAvocado
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I’m going to try to answer your questions, but I suspect the pros to come up with better answers.

The Bullet Cluster:
hs-2006-39-a-web.jpg


1) What is the mechanism which makes the interstellar gas visible at such a distance in this particular case? Is all intergalactic gas visible in the same way and at the same brightness levels?
Intergalactic gas is normally invisible to the 'naked eye' (telescopes), except if it’s illuminated by (newborn) stars etc. In this case we are talking about X-rays (from the very hot gas) detected by the satellite Chandra X-ray Observatory. The pink color is applied to show where the gas is located.

2) How do we know that all or most of the intergalactic gas collided?
Gravity tends (by time) to concentrate matter more and more. Planet and stars forms out off concentrated gas, to build galaxies. Dark matter makes 23% and ordinary matter (gas/stars) only makes 4%, therefore we should expect the much heavier Dark matter to attract the gas pretty easy.

3) What would have caused the gas to slow down so much as it passed through the other gas?
Think of normal colliding air in strong wind, or water.

The Dark Matter Mystery:
https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>
 
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  • #3
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I’m going to try to answer your questions, but I suspect the pros to come up with better answers.

Intergalactic gas is normally invisible to the 'naked eye' (telescopes), except if it’s illuminated by (newborn) stars etc. In this case we are talking about X-rays (from the very hot gas) detected by the satellite Chandra X-ray Observatory. The pink color is applied to show where the gas is located.
I'm familiar with the fact that the X-rays are the only way we can "see" them. I suppose I should have used a different word than visible as that implies human wavelengths. I really meant detectable via photon emission. Is the intergalactic gas always hot enough to emit enough photons so we can see it with devices like Chandra?

Gravity tends (by time) to concentrate matter more and more. Planet and stars forms out off concentrated gas, to build galaxies. Dark matter makes 23% and ordinary matter (gas/stars) only makes 4%, therefore we should expect the much heavier Dark matter to attract the gas pretty easy.
This is a description of their attraction through gravity, not the likelihood of their collision. The same could be said of the galaxies and stars but they didn't seem to collide. In fact, the highest concentrations of dark matter would have to be in the galaxies themselves, if one is to believe the models, I think.

With all due respect, I don't see how this answers my question at all.

Think of normal colliding air in strong wind, or water.
Really? We are talking about 10s of orders of magnitude lower density here, are we not?

The density of intergalactic space is reported to be 10 to 100 hydrogen atoms of hydrogen per cubic meter. In a cubic meter of air you have about 10^25 molecules. The potential for a collision must go up as the square of the number of molecules, so we are talking about hundreds of orders of magnitude less likelihood of a collision for intergalactic molecules when compared with air at STP, unless I'm missing something.
 
  • #4
DevilsAvocado
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... Is the intergalactic gas always hot enough to emit enough photons so we can see it with devices like Chandra?
I’m only a layman, but my guess is definitely no. Or maybe better – there is no other (known) intergalactic gas that extremely hot.
http://en.wikipedia.org/wiki/Bullet_cluster" [Broken]
"The Bullet cluster is one of the hottest known clusters of galaxies ... a bow-shaped shock wave located near the right side of the cluster formed as 70 million degree Celsius gas in the sub-cluster plowed through 100 million degree Celsius gas in the main cluster at a speed of about 6 million miles per hour."
I can’t tell you at what temperature intergalactic gas starts to emit photons (X-rays).

This is a description of their attraction through gravity, not the likelihood of their collision. The same could be said of the galaxies and stars but they didn't seem to collide. In fact, the highest concentrations of dark matter would have to be in the galaxies themselves, if one is to believe the models, I think.
Let’s start from scratch. Dark Matter is the main gravity attractor for ordinary matter (i.e. intergalactic gas etc). After the Big Bang when the universe expanded there were 'lumps' of Dark Matter that was crucial for the formation of stars and galaxies. The Dark Matter attracted and concentrated the gas, and concentrated gas gave birth to stars etc. This is crucial – this evolution could not take place without Dark Matter – there just isn’t enough ordinary matter in the universe to overcome the expansion of the universe.

Now we know that the intergalactic gas and the galaxies are 'inside' the Dark Matter, 'caught' by gravity, a long time ago, right? And if two galaxy clusters collide – there’s 100% chance that the gas that actually 'made' these galaxies also collide, right?

(To be even more specific – the Dark Matter is so dominant, that it actually surrounds the whole galaxy cluster.)

The reason the galaxies didn’t collide, is the same as why Andromeda and Milky Way don’t collide at present.

Really? We are talking about 10s of orders of magnitude lower density here, are we not?
I have no numbers on the gas density. But I do know that the vast majority of the (ordinary) mass is in the intergalactic gas, not the galaxies (as you might have guessed). AND the speed of the collision was of about 6 million miles per hour (2,682,240 m/s ≈ 1% speed of light)!!

I hope you realize what this means? If not: Imagine driving the hottest open sports car in extremely light rain – at 200 mph... Now, imagine doing the same thing but 30,000 times faster!

Yes right, big 'problems'...

(And you must not think of gas particles as 'nice' raindrops – these little 'creatures' moves around, rapidly. And the hotter they get, the more they move = more heat...)

I can guarantee you that the collision created this enormous heat.
EDIT: Extremely bad formulation. As a layman I can’t guarantee you anything. Read the "popular press" (=Wikipedia??) and everything else you can find on the subject, and make your own decision.

Don’t agree? Well, then you better come up with a logical explanation for the gas at 100 million degree Celsius...
 
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  • #5
Vanadium 50
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I can guarantee you that the collision created this enormous heat.
How then do you explain the (many) other x-ray emitting gas regions outside the Bullet Cluster?
 
  • #6
turbo
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How then do you explain the (many) other x-ray emitting gas regions outside the Bullet Cluster?
Thank you. The simplistic summaries in the popular press, and popularized versions of professional publications are often wildly inaccurate, especially in the the omission of relevant facts. We do not adequately understand the excitation/emissions of some materials, nor do we adequately understand the mechanics behind the formation of jets emerging from very active objects such as quasars and AGNs.

Once we start integrating our views of our local universe in LOTS of wavelengths, the universe seems a lot more interacting and a lot less discontinuous. Browse through here:

http://www.nrao.edu/astrores/HIrogues/webGallery/index.html
 
  • #7
DevilsAvocado
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How then do you explain the (many) other x-ray emitting gas regions outside the Bullet Cluster?
Help! I’m haunted by a PF Mentor!? :smile:

Seriously, if the universal standard for intergalactic gas is 100 million degree Celsius – I give up.

If not – you come up with a better 'natural' explanation for this 'behavior' and formation.

(I asked first, and you are the expert... :wink:)

(Gas 'dragged' into Black Holes at extreme speed emits x-ray, right?)
 
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  • #8
DevilsAvocado
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... popularized versions of professional publications are often wildly inaccurate, especially in the the omission of relevant facts.
Is this a joke?

So, when cosmologist Max Tegmark (MIT) says that the Bullet Cluster probably is going to prove DM, or when Sean Carroll (Caltech) writes about the Bullet Cluster as the proof for DM – this "popularized versions of professional publications are often wildly inaccurate" ...??

And Wikipedia is classified as "popular press" ...!?

And NASA is wildly inaccurate? Promoting unscientific LIES on their official HubbleSite ...!?!? :surprised

Good night everybody.

http://hubblesite.org/newscenter/archive/releases/2006/39/image/a" [Broken]

Visible-Light and X-Ray Composite Image of Galaxy Cluster 1E 0657-556

This composite image shows the galaxy cluster 1E 0657-556, also known as the "bullet cluster." This cluster was formed after the collision of two large clusters of galaxies, the most energetic event known in the universe since the Big Bang.

Hot gas detected by Chandra in X-rays is seen as two pink clumps in the image and contains most of the "normal," or baryonic, matter in the two clusters. The bullet-shaped clump on the right is the hot gas from one cluster, which passed through the hot gas from the other larger cluster during the collision. An optical image from Magellan and the Hubble Space Telescope shows the galaxies in orange and white. The blue areas in this image depict where astronomers find most of the mass in the clusters. The concentration of mass is determined by analyzing the effect of so-called gravitational lensing, where light from the distant objects is distorted by intervening matter. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark.

The hot gas in each cluster was slowed by a drag force, similar to air resistance, during the collision. In contrast, the dark matter was not slowed by the impact because it does not interact directly with itself or the gas except through gravity. Therefore, during the collision the dark matter clumps from the two clusters moved ahead of the hot gas, producing the separation of the dark and normal matter seen in the image. If hot gas was the most massive component in the clusters, as proposed by alternative theories of gravity, such an effect would not be seen. Instead, this result shows that dark matter is required.

Comparing the optical image with the blue emission shows that the most of the galaxies in each cluster are located near the two dark matter clumps. This shows that the galaxies in each cluster did not slow down because of the collision, unlike the hot gas.

Object Name: 1ES 0657-55.8

Image Type: Astronomical
Credit: X-ray: NASA/CXC/M.Markevitch et al.
Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.
Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

http://hubblesite.org/newscenter/archive/releases/2006/39/image/a
 
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  • #9
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1) What is the mechanism which makes the interstellar gas visible at such a distance in this particular case? Is all intergalactic gas visible in the same way and at the same brightness levels?
For starters, so we don't get confused with nomenclature here, while the gas within clusters is a subset of the intergalactic medium, it is generally referred to as the intracluster medium (ICM). It is certainly different from the interstellar medium (although it sort of is an interstellar medium).

The primary emission mechanism for a large cluster is Bremsstrahlung emission -- Bremsstrahlung literally means "braking radiation" and it is so named because it is caused by charged particles which have been accelerated by the electric field of another charged particle. In the case of the ICM, which is so hot that the gas is ionised and emits in X-rays, it is a fast moving electron which is accelerated by a slower moving proton, or positively charged ion. All clusters emit X-rays via Bremsstrahlung, and particularly large clusters have their X-ray emission dominated by Bremsstrahlung. Smaller clusters may have a larger contribution of their X-ray emission from emission due to collisional excitation of the ions.

Both emission mechanisms are caused by collisions, thus the brightness of the X-ray emission depends on the square of the gas density.

2) How do we know that all or most of the intergalactic gas collided?
How do we know that it wasn't the case that most of the gas stayed with the galaxies and only some collided which was subsequently made much brighter because of the heating due to the collision? So in effect, how do we know that the part that forms the bullet wasn't merely just a visible small fraction of the intergalactic gas that collided while the bulk of the gas passed through the other cluster with the visible galaxies themselves?
Well, we know that most of the subcluster gas is with the bright Bullet part because the brightness of the X-ray emission is proportional to the square of the electron density integrated along the line of sight, so it is fairly easy to estimate the gas mass from the ICM, assuming some geometry for the gas. Of the baryonic content of clusters, the ICM clearly dominates making up around 80% of the baryonic mass. Were there a portion of the ICM significant enough to cause the lensing signal coincident with the subcluster galaxies, it would definitely be visible in X-rays. It is not.

3) What would have caused the gas to slow down so much as it passed through the other gas? At the densities of intergalactic gas, it seems odd that much of it would have actually collided at the atomic level. What am I missing?
The gas is slowed by ram pressure, which is similar to the pressure felt when you stick your head out of a car window while driving. Your question regarding whether the ICM is collisional is understandable since the ICM is less dense than the best vacuums we can create on Earth (electron densities ~.01-.0001 per cm cubed) so it is hard to see how they will collide. The answer comes by comparing the scale of the system with the mean free path of the particles making up the ICM. When the mean free path is very small in comparison to the scale of the system, the system acts like a collisional fluid. For the ICM, the mean free path is ~1000 parsecs, compared to the size of the system which is of the order of 1 000 000 parsecs. Thus the ICM can be treated as a collisional fluid and we see things like ram pressure stripping of gas from galaxies and subclusters.
 
  • #10
DevilsAvocado
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Thanks matt.o! Brilliant!!

You are the first one here who seems to know what he’s talking about (including me :wink:).

Just one question:
... The gas is slowed by ram pressure, which is similar to the pressure felt when you stick your head out of a car window while driving.
Doesn’t the high velocity (~2,600,000 m/s) of the gas make some difference? If I stick my head out of a car window while driving 50 km/h, or 150 km/h, makes a pretty big difference?
 
  • #11
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Thanks matt.o! Brilliant!!

You are the first one here who seems to know what he’s talking about (including me :wink:).

Just one question:

Doesn’t the high velocity (~2,600,00 m/s) of the gas make some difference? If I stick my head out of a car window while driving 50 km/h, or 150 km/h, makes a pretty big difference?
Yes, it sure does. In fact, the ram pressure [tex]\propto \rho v^2[/tex] where [tex]\rho[/tex] is the density and [tex]v[/tex] is the velocity.
 
  • #12
DevilsAvocado
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Yes, it sure does. In fact, the ram pressure [tex]\propto \rho v^2[/tex] where [tex]\rho[/tex] is the density and [tex]v[/tex] is the velocity.
WOW! Haha!! And I used the popularized version of my unprofessional silly little brain to figure that one out completely by myself!! :biggrin: :biggrin: :biggrin:

Hehe! Victory is sweet! matt.o if you are a girl I want to marry you!! You are my hero!! (noo, just a silly joke, sorry) :rofl:

Thank you so very very much matt.o!!!


EDIT: Sorry for the 'madness'... It’s very late, and yes, I was somewhat 'frustrated'... But I’m cool now (and perfectly normal, I promise :smile:). Peace to everyone. Good night.
 
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  • #13
Chalnoth
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Everyone points to the Bullet Cluster as the best proof of Dark Matter. But I have a few questions about the rationale. There are aspects of the proof I don't understand.
Well, I definitely don't think it's the best proof of dark matter. The best proof of dark matter is the confluence of the multiple lines of converging evidence. The best single piece of evidence is debatable. But what the Bullet Cluster most definitely is is the most visceral piece of evidence, the one that most obviously jumps out and screams, "Dark Matter!"

There are other pieces of evidence that may, on balance, be better evidence (such as the CMB), but tend to be rather complicated and messy to explain. The Bullet Cluster has the virtue of being rather simple and obvious.

1) What is the mechanism which makes the interstellar gas visible at such a distance in this particular case? Is all intergalactic gas visible in the same way and at the same brightness levels?
It's really freakin' hot. The heat of the gas is driven by an incredibly simple process: as matter falls into the potential well of a galaxy cluster, it speeds up. The speeds of all these individual bits of matter make for a temperature of many millions of degrees, which makes for emission in the X-ray range. This hot cluster gas is a property of galaxy clusters, makes up most of the mass of galaxy clusters, and can be seen in at least nearly all of them, if not all (I suppose it's possible that collisions have stripped the gases out of some clusters).

I think matt.o did a good job with the rest of it.
 
  • #14
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matt.o and Chalnoth,

Thanks for the explanations. That clears some things up.
 
  • #15
DevilsAvocado
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... The best proof of dark matter is the confluence of the multiple lines of converging evidence.
I agree, and this lively "pink-hot-colliding-speedy-gas-discussion" is rather excessive. We really only need to know 3 basic facts:

1) Where is the gas located?
2) Where are the galaxies located?
3) Does the gas make up for ~80% of the baryonic (ordinary) mass?

I could be wrong, but to my understanding these questions can be answered with rather high precision by the cosmologist.

Now, we use http://en.wikipedia.org/wiki/Gravitational_lens" [Broken] to find where the majority of mass is located.

grav_lens_pic_1_large.jpg


And it’s not in the gas, where you would expect it to be.

One could argue that this doesn’t prove anything – this is only one strange phenomenon in the gigantic universe. It could be extremely fat green aliens in tremendously massive spaceships, hiding behind the galaxies, just as the photo is taken...

We can’t reject the "Massive-Alien-Theory" – but if this is the case, there must be a whole community of "massive aliens" out there, because now there’s another proof of "unknown massive objects" in http://en.wikipedia.org/wiki/MACS_J0025.4-1222" [Broken].

In the case of MACS J0025.4-1222, even a layman can see where the gravitational lensing is dominant:

macs.jpg


MACS J0025.4-1222 in 60 Seconds Plus
https://www.youtube.com/watch?v=<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/rn_CBHvq29k&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/rn_CBHvq29k&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>

If this was the only indication of Dark Matter, the discussion could be extensive whether it’s Aliens, or MOND 2eV hot neutrinos, or Dark Matter, or something else. But it isn’t:
  • http://en.wikipedia.org/wiki/LCDM" [Broken], the standard model of Big Bang cosmology, includes DM.
  • The http://www.sarahbridle.net/lectures/uclgrad07/lss_and_cmb.pdf" [Broken] shows clear indications of DM.
  • In computer simulations of the large-scale structure of the universe DM is needed, to make it work with the current theory of gravitation (to get a picture that fits real observations).
Large-scale structure of the Universe
https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/FFlzyxSQhTc&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/FFlzyxSQhTc&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>

etc...
 
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  • #16
Chalnoth
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I don't think anybody would ever seriously consider a "massive alien theory".
 
  • #17
DevilsAvocado
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I don't think anybody would ever seriously consider a "massive alien theory".
No, of course you’re right. It’s was more kind of a (obvious) 'joke'... :wink:
 
  • #18
I don't think anybody would ever seriously consider a "massive alien theory".
How about a "fat Americans from the future theory." That I could believe.
 

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