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View Full Version : Some Galaxies Are Made Almost Entirely of Dark Matter



Fraser
2007-Feb-26, 05:57 PM
When we think of a galaxy, we think of our own Milky Way or perhaps Andromeda; a majestic spiral containing hundreds of billions of stars. Or maybe we think of an irregular galaxy, not so majestic-looking, but still made of regular stuff, like stars, planets... ...

Read the full blog entry (http://www.universetoday.com/2007/02/26/some-galaxies-are-made-almost-entirely-of-dark-matter/?1359)

Jerry
2007-Feb-26, 07:10 PM
"An international team of researchers has developed a simulation to explain how galaxies like this could form."

So we have observational results that are different from what is expected, and a simulation which may or may not explain the descrepancy.

01101001
2007-Feb-26, 11:21 PM
So we have observational results that are different from what is expected, and a simulation which may or may not explain the descrepancy.

Are we to understand that bothers you? Should it bother us? Why? What should we do?

PhantomRacer
2007-Feb-27, 01:38 AM
if dark matter has mass and gravity would it not form large bodies ? objects compossed of just dark matter , and we cannot see them .

John Mendenhall
2007-Feb-27, 03:41 PM
"An international team of researchers has developed a simulation to explain how galaxies like this could form."

So we have observational results that are different from what is expected, and a simulation which may or may not explain the discrepancy.


Are we to understand that bothers you? Should it bother us? Why? What should we do?

This is better result in favor of dark matter than most. I don't want to beat it to death, but I'm in the dim view of dark matter school. However . . .

In this case, the observation is that dwarf spheroidals appear to have way more mass than they should on the basis of the visible stars seen in them. The simulations indicate that it is possible, if they have large amounts of dark matter, to strip out their original gas, thus putting a near stop to star formation. And my guess is that MOND would have problems with this scenario. The relativists, maybe, but the math is beyond me. In summary, this is pretty good support for dark matter, much as I dislike the idea. If we keep turning up gravitational sources not associated with any visible objects, it’s going to be hard to argue against DM.

Oh, phooey, probably wrong again! Such is life. Keep an open mind, folks.


If dark matter has mass and gravity would it not form large bodies? Objects composed of just dark matter, and we cannot see them?

There are some good arguments against dark matter clumping, and I don't remember the references. As I recall, DM doesn't do the baryonic matter trick of simply sticking together; gravity is it.

antoniseb
2007-Feb-27, 04:10 PM
if dark matter has mass and gravity would it not form large bodies ? objects compossed of just dark matter , and we cannot see them .

Dark matter is not going to stick to other dark matter. It is plausible that you could get swarms of dark matter particles orbiting around a common center of mass, with a concentration toward the center.

Doodler
2007-Feb-27, 04:14 PM
Given some recent measurements, Andromeda just might be one of those galaxies composed mostly of dark matter...

John Mendenhall
2007-Feb-27, 05:01 PM
Given some recent measurements, Andromeda just might be one of those galaxies composed mostly of dark matter...

And the Milky Way is headed there. Why do I feel like Slim Pickens riding the bomb down?

antoniseb
2007-Feb-27, 06:34 PM
Doodler, most galaxies are composed mostly of dark matter, M31 and the Milky Way included. This thread is about galaxies that have a much higher proportion of dark matter to visible matter than the local galaxies have.

Doodler
2007-Feb-27, 07:32 PM
Doodler, most galaxies are composed mostly of dark matter, M31 and the Milky Way included. This thread is about galaxies that have a much higher proportion of dark matter to visible matter than the local galaxies have.

What I was suggesting is that every time we look at Andromeda lately, the measurement of its dark matter halo grows. So while I understand the reference here, my comment was to suggest that if we continue detecting more an more dark matter around this one particular galaxy, which is already noted to have a much higher ratio of dark matter to matter than our own, it might reach a similar ratio to these "dark matter" types, which I'm assuming based on what I'm reading, are not themselves matterless.

GBendt
2007-Feb-27, 08:36 PM
There is a lot of dark matter in galaxies and around them. What this matter is composed of, nobody knows. The matter is dark matter because we can´t see it. The fact that we can´t see it does not necessarily mean that it must be something strange. It is not strange that there are invisible things.

It is easy to say that light does not exist if you have been blind throughout your life: As a result, your view of the world is hampered. Whatever you get in touch with is "dark matter". Therefore I think, it is very necessary to train one´s eyesight carefully, even if almost everyone around imagines dark matter.

Dark matter is sensational. It is amazing. It keeps being invisible, and that fires our imagination. The danger is, that we replace investigation by imagination.
Of course, it is less cool to go through thousands of astronomical and astrophysical research findings of the last four decades to get a complete picture. Nobody wants to spend his or her professional career on that, everyone is inclined to find something NEW. But if you don´t know enough, you are more likely take a wrong conclusion. Sometimes I have the impression that a finding is more valued for the thrill it provides than for its truth.

Hundred years ago, people thought that there was a matter called "ether" that filled the whole universe. They thought that it must necessarily be there, because light is a wave, and something oscillating was required to carry it. Finally we learned that light waves travel very fine without needing anything like "ether". "Dark matter" might some day turn out to be the "ether" of today. Our look on the matter might still be an improper one. We shall find out some day.

The stars: They don´t mind.

Regards

Günther

John Mendenhall
2007-Feb-28, 09:42 PM
Of course, it is less cool to go through thousands of astronomical and astrophysical research findings of the last four decades to get a complete picture. Nobody wants to spend his or her professional career on that, everyone is inclined to find something NEW.

"Dark matter" might some day turn out to be the "ether" of today. Our look on the matter might still be an improper one. We shall find out some day.

Günther

GBendt, the DM people have a good case with this one. What would you look for in existing data that might contravene the DM idea?

Regards, John M.

GBendt
2007-Mar-08, 01:05 AM
Hi John,

there is a lot of invisible matter or transparent matter in galaxies and within galaxy clusters. This is a well known fact. Usually this matter is called "dark matter", because it looks like that it does not emit or absorb any type of electromagnetic radiation. As this is the case, the name "dark matter" is not correct, as a 'dark' body is dark because it absorbs light, which is electromagnatic radiation. Such, 'Transparent matter' would be a more proper designation for the so-called "dark matter".

The fact that this matter is invisible or transparent does not necessarily mean that this matter must be something strange. Up to now, various sophisticated experiments were carried out to spot that matter, as the general expectation is that this matter is not composed of atoms or charged particles, and that it does not interact with electromagnetic fields. Some friends an collegues of mine are involved in one of these experiments, they try to spot a neutral cold particle of 200 proton masses, the neutralino, a proper candidate for DM. Hitherto, the neutralino has not yet shown up, although the design of their detector is great, and although that detector is amazingly sensitive.
None of the other experiments that were carried out to find out what DM really is has given a better result. All we know is that our experimental designs and approaches have proven to be unable to detect a "dark matter" particle -yet. No reason to give up the quest.

On the other hand, I went through a lot of scientific articles and reports to find out from that first-hand information source what was (is) known and was detected about interstellar and intergalactic matter. If you want to know what the universe is made of, this is a useful information to obtain.

I expected to find thorough and sound information on the distribution of neutral gas, molecule clouds, hot and cold dust in our galaxy, and of the distribution of its mass. But what I met were guesses and carefully weighted assumptions. Clues, but not a picture.
From that I would say that we have a set of ideas, but that we do not know the distribution of matter in our galaxy, or what it is made of. We do not even know the size of its mass. Some authors say that it contains 100 billion solar masses and 250 billion stars, some say that it has 1400 billion solar masses and 800 billion stars.
In many articles you read about dark matter (or better: transparent matter) and its impact on the rotation and dynamics of stars in the milky way galaxy, or in any other galaxy. The authors say that they don´t know what it is and call it "dark matter", and then "close the book". This is honest, but it does not give any evidence that this matter is anything strange.

Matter that interacts with electromagnetism may do so without us having any chance to get a clue on it, because this radiation may not reach us, or because we do not have the appropriate detectors, or because we look at the wrong place, or use the wrong means, or apply an improper theory, or have too much interfering signal on the data. We have powerful instruments, and we have powerful means to hamper their effectiveness. Any mobile phone gives a radio signal a thousand times stronger than any celestial source, and there are millions of them. 98% of the electromagnetic spectrum up to 9 Ghz are occupied by human-caused transmissions, and in the 2% left, 99,999% of the spectral contents is man-made. Under such circumstances, radioastronomy is, let´s say "difficult" to yield reliable results... Just to tell one of the problems.

If everyone expects to find strange "dark matter", then there may be more likeliness that less people keep on looking for real matter in the universe. Such matter is difficult to find, not because it may not be there, but because it is very distant, very cold, very transparent, and very dispersed.
But every now and then, you find a report on new findings. The diameter of a hydrogen gas halo around a galaxy was detected. High velocity hydrogen gas clouds were found plunging from intergalactic space into the andromeda galaxy, each with a mass of several million solar masses. The gas halo interaction of colliding galaxy clusters was observed and the mass of the halo could be revealed from the resulting radition: The amount of gas in the intergalactic halo was found to be ten times the mass of all the the galaxies in the cluster.
Galaxies in clusters were found to be engulfed by large halos of rarified hot gas, at temperatures of 100 million kelvin. The galaxies fly through that hot gas at speeds of 600 miles/s. There were long discussions on the source of that heat, until someone came up with the fact that the speed of the galaxies is equivalent to the speed a hydrogen atom has when it is 100 million kervin hot. It is worth while reading articles and reports.
I could go on and on, but it is time to go to bed for me. It is almost 2 a.m. now.

Regards,

Günther

RussT
2007-Mar-08, 11:17 AM
http://kencroswell.com/FirstDarkGalaxy.html


A cloud of gas in the Virgo cluster may be the first dark galaxy ever found, say astronomers in Britain, Australia, Italy, and France. The mysterious object has one-tenth the Milky Way's mass but consists of hydrogen gas and dark matter--with no detectable stars.

Minchin's team estimates the cloud's mass is at least 90 billion Suns. The astronomers reached this conclusion because hydrogen atoms in the cloud move relative to one another at 220 kilometers per second (about half a million miles per hour). If the cloud had less than 90 billion solar masses, its gravity would be too weak to hold on to such speedy atoms.


http://www.narrabri.atnf.csiro.au/public/images/ngc2915/

The Non-baryonic DM is there, BUT they have so little constraint on what made it, where it came from, how tiny it really is, how fast it is really moving, that simulations are completly 'guesswork', IN addition to the fact that galaxy mergers from the bottom up is nowhere even near to being fact!

thecolorofash
2007-Mar-08, 01:07 PM
Dark matter is not going to stick to other dark matter. It is plausible that you could get swarms of dark matter particles orbiting around a common center of mass, with a concentration toward the center.

I do not understand...why would dark matter not form big objects of planet to star size and larger? Is there a know property of dark matter that prevent this?:confused:

torque of the town
2007-Mar-08, 02:51 PM
I do not understand...why would dark matter not form big objects of planet to star size and larger? Is there a know property of dark matter that prevent this?:confused:




Now thats what I call a question..........

01101001
2007-Mar-08, 03:28 PM
I do not understand...why would dark matter not form big objects of planet to star size and larger? Is there a know property of dark matter that prevent this?:confused:

A probable property: dark matter only interacts with matter or other dark matter gravitationally. It does not interact with the electromagnetic force. Dark matter particles do not collide. It cannot lose energy the way ordinary matter does and will not coalesce the same way.

See topic Dark Matter in the Sun? (http://www.bautforum.com/showthread.php?p=938730#post938730)

John Mendenhall
2007-Mar-08, 03:42 PM
QUOTE=GBendt;942512]

All we know is that our experimental designs and approaches have proven to be unable to detect a "dark matter" particle -yet. No reason to give up the quest.

But every now and then, you find a report on new findings. The diameter of a hydrogen gas halo around a galaxy was detected. High velocity hydrogen gas clouds were found plunging from intergalactic space into the andromeda galaxy, each with a mass of several million solar masses. The gas halo interaction of colliding galaxy clusters was observed and the mass of the halo could be revealed from the resulting radiation: The amount of gas in the intergalactic halo was found to be ten times the mass of all the galaxies in the cluster.

Regards,

Günther[/QUOTE]

You hypothesize, then, that the anomalous rotation curves and so forth are due to undetected baryonic matter? I’d love it if it proves to be true, but I just don’t see that enough baryonic matter exists – or has been detected - at least in the right places. And if the Pioneer anomaly turns out to be correct, it’s going to be really difficult to find that much mass in the solar system.

Keep in mind, I don’t like the exotic particle dark matter idea, and it’s very satisfying that nothing has been detected so far, and I agree with Rusty that they’ve got the cart before the horse with (most) simulations, but the example that started this thread is good support for DM.

And kudos to 01 for his good post above about clumping.

Addendum: Once again, despite my dim view of dark matter (maybe we could call it DiM), I find myself arguing the DM side.

Grand_Lunar
2007-Mar-08, 11:50 PM
I'm curious how one would know they've spotted a DM galaxy.

In addition, I wonder what the view would be if our own galaxy would be if it was a DM galaxy. Hmm, facinating thought...

RussT
2007-Mar-09, 02:15 AM
http://www.aas.org/publications/baas/v31n5/aas195/983.htm

There are many misconceptions that are being bandied around here.

First, this link shows that this is NOT true of most Dwarf Spheroidals.

Second, consider this...if galaxies formed by many mergers, making larger and larger galaxies, then this how would this 'tidal stripping of gas' ever (the stars that are already formed and the dark matter is what are being considered collisionless) allow any large galaxies to form?

trinitree88
2007-Mar-09, 02:31 AM
I'd be interested to see how much dark matter is in the Bootes void. :shifty: Pete

thecolorofash
2007-Mar-09, 08:09 AM
A probable property: dark matter only interacts with matter or other dark matter gravitationally. It does not interact with the electromagnetic force. Dark matter particles do not collide. It cannot lose energy the way ordinary matter does and will not coalesce the same way.

See topic Dark Matter in the Sun? (http://www.bautforum.com/showthread.php?p=938730#post938730)

Ok thanks - good explanation and link ;-)

RussT
2007-Mar-09, 10:31 AM
I'd be interested to see how much dark matter is in the Bootes void. :shifty: Pete

Well now...since scientists can ONLY see Non-baryon Dark Matter (I know I am going to get in BIG trouble here trinitree, because they are defining CDM based on the environment they 'see it in', and so 'assume' things about it to hypothesize a fit for it in that/those environments ie; galaxy rotation curves/clusters, and then MOST of their anylsis comes from N body simulations) as 'extra gravity', they can only see it in its relationships with baryonic matter, BUT the clumping ability or inabilty, its actually motions, and especially its velocity, are IMHO, being totally contrived.So the Bootes void and all the other voids is the 'flowing' of space and where all the Non-Baryonic Dark Matter is coming from.

Jerry
2007-Mar-11, 09:06 PM
A probable property: dark matter only interacts with matter or other dark matter gravitationally. It does not interact with the electromagnetic force. Dark matter particles do not collide. It cannot lose energy the way ordinary matter does and will not coalesce the same way.

See topic Dark Matter in the Sun? (http://www.bautforum.com/showthread.php?p=938730#post938730)

I think this is a key problem with exotic (non-baryonic) dark matter: We know light interacts with gravity, and since light is an electromagnetic phenomenon, we really have to conclude DM should likely have other identifiable properties. All of the effort to find additional attributes of exotic DM have turned up zilch. This leaves the question open: it is a force we do not understand, or a particle?

transreality
2007-Mar-11, 09:50 PM
Since the voids have been forming by expansion of space it is quite likely that there is no matter in the voids, dark or otherwise.

RussT
2007-Mar-12, 07:52 AM
I realize that it is Taboo to use logic in cosmology, BUT IF all the expansion that we see is in the Voids between the clusters, and those Voids are huge, AND "SPACE" is something 'physical' (Hmmmm, 'Extra Gravity'), THEN it would certainly seem prudent to at least entertain the idea that...

The Extra Gravity is entering into the Voids on a continual basis and makes up 'space'???

cbacba
2007-Mar-12, 06:58 PM
I think this is a key problem with exotic (non-baryonic) dark matter: We know light interacts with gravity, and since light is an electromagnetic phenomenon, we really have to conclude DM should likely have other identifiable properties. All of the effort to find additional attributes of exotic DM have turned up zilch. This leaves the question open: it is a force we do not understand, or a particle?

Perhaps the problem tends to be in the 'exotic' There seems to be assumptions that all baryonic matter becomes stars or remains clouds detectable by em means (emission/absorption). Obviously, astronomy is concerned with objects that emit em or at least with objects obscuring emiited em from behind. I've seen the term PFP primordial Fog Particle - referring to a planet sized mass which would seem to have what attributes seem to exist for DM. Perhaps these are far more common than the luminous variety, perhaps they are even future stars as they slowly collect material in rather low matter concentration areas, unlike the star forming regions that are virtually opaque with matter.

Somehow it seems to me to be too much sensationalism going on out there and perhaps not enough use of Ockham's razor. We seem to have exotic DM competing with Mond - when neither may actually be real or necessary.

John Mendenhall
2007-Mar-12, 08:31 PM
Somehow it seems to me to be too much sensationalism going on out there and perhaps not enough use of Ockham's razor. We seem to have exotic DM competing with Mond - when neither may actually be real or necessary.

You hit it. Maybe a large international conference to thrash things out? Remember what happened when the geologist/paleontoligists held one to put the Chicxulub idea to rest? They went away going "Uh-oh, looks like it's correct."

John Mendenhall
2007-Mar-12, 08:44 PM
http://www.aas.org/publications/baas/v31n5/aas195/983.htm

There are many misconceptions that are being bandied around here.

First, this link shows that this is NOT true of most Dwarf Spheroidals.

Second, consider this...if galaxies formed by many mergers, making larger and larger galaxies, then this how would this 'tidal stripping of gas' ever (the stars that are already formed and the dark matter is what are being considered collisionless) allow any large galaxies to form?

Big ellipticals are noted for being gas-free and composed of old stars. Fits the model.

Oh my, I'm arguing the DM side again! Sorry, Russ.

RussT
2007-Mar-13, 07:49 AM
Big ellipticals are noted for being gas-free and composed of old stars. Fits the model.

Oh my, I'm arguing the DM side again! Sorry, Russ.

Why are you appologizing to me?

All throughout this and many other threads I am agreeing that Non-baryonic DM exists, BUT that Mainstream in both their application of GR and QM have badly misidentified what it is and what it is doing.

And the reason that Large Ellipticals have so little DM is simple. They are the oldest galaxies and have made almost all of the stars they are ever going to make, so they have very little gas left to make stars with. The reason they have less DM is because they have way more stars that the DM is "IN" as the DM is traveling at "c" through the galaxy, so we don't 'see it' has having nearly as much DM as a 'blue' young galaxy, that has very little star formation compared to the elliptical.

The less stars any galaxy has, the more DM 'we see' it having.

Though the real truth of the matter is, there is just as much DM in every galaxy!!!

Can anyone tell me what I really mean by this?

01101001
2007-Mar-13, 02:10 PM
Can anyone tell me what I really mean by this?

Tell RussT in an ATM thread, please.

John Mendenhall
2007-Mar-13, 05:42 PM
Why are you appologizing to me?

Though the real truth of the matter is, there is just as much DM in every galaxy!!!

Can anyone tell me what I really mean by this?

I've taken strong positions against DM, and I think I may have to eat my words.

You propose all galaxies have plenty of DM. This makes me think about the central bulges of spirals which appear to be directly proportional to the sizes of the super-massive black holes at their centers. Is there a central bulge/DM relationship? If there is, then there would be an super-massive black hole/dark matter relationship. You propose non-baryonic exotic particles traveling at c for dark matter, so far undetected. I'm feeling better about the whole idea, I think this has less problems than MOND. I just wish a particle would show up in a detector.

Tensor
2007-Mar-13, 06:43 PM
I just wish a particle would show up in a detector.

I'm willing to bet that a lot of astrophysicists feel the same way.....

John Mendenhall
2007-Mar-13, 07:17 PM
I'm willing to bet that a lot of astrophysicists feel the same way.....

I'm thinking about putting together an 'Is / Is Not' analysis for DM.

For example, evidence for:

IS:
Rotation curves.
Pioneer anomaly?
Group velocities.
Light bending.
Non-baryonic matter (simulation based)?

IS NOT:
Any detected particle.
Baryonic matter.
Lots of black holes?
MOND?
Gravity under general relativity?
Machos.
Gas.
Dust.
Big Band dependent.

Each of the items above is probably worth a thread on its own . . .

And I'm sure there are others. Suggestions?

RussT
2007-Mar-14, 10:17 AM
I've taken strong positions against DM, and I think I may have to eat my words.

When we are talking about "if" non-baryonic DM exists as 'extra gravity' OR is there something else that can 'be there' as the extra gravity, then as I far as I can tell, maintream has done an extremely diligent and thorough job, of ruling out all the possibilites that have been thought of, Macho's, Dusts, Gases, ETC.

DM existing and being 'extra gravity' that is 'just there' in the galaxies to explain the rotation curves and in the clusters to explain galaxies not flying out, is NOT Big Bang theory dependent.

SO, it is there, but, what it is, where it is coming from, what makes it, and how fast it is going, COMPLICATES things tremendously!!!

Geeez, I'll just leave it at that for now.

John Mendenhall
2007-Mar-15, 08:50 PM
then as I far as I can tell, maintream has done an extremely diligent and thorough job, of ruling out all the possibilites that have been thought of, Macho's, Dusts, Gases, ETC.

DM existing and being 'extra gravity' that is 'just there' in the galaxies to explain the rotation curves and in the clusters to explain galaxies not flying out, is NOT Big Bang theory dependent.

So that's 4 more 'Is Not' items, right? Machos, dust, gas, and Big Bang dependent? The first three I'm familiar with, the last I'll take your word for it. Hmm . . . the 'Is Not' list is growing faster than the 'Is' list. But that's the nature of the problem. Ah, I'm going to edit the previous post, if I can, and add these. Thanks, Russ.

Blob
2007-Mar-15, 09:31 PM
Hum,

IS (probably)
Crude computer simulations of the universes large structures and voids
Non -baryonic matter.

IS Not (probably)
blue fairies

John Mendenhall
2007-Mar-19, 05:06 PM
Hum,

IS Not (probably)
blue fairies

And I was hoping it might be invisible dwarves in our backyards.

And I'll add N-B matter under 'Is'.