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dgruss23
2003-Aug-29, 07:04 PM
A paper has just come out on the Los Alamos Preprint server which finds that three elliptical galaxies show a dearth (http://xxx.lanl.gov/abs/astro-ph/0308518) of dark matter.

This is not the first study to conclude that some elliptical galaxies have no dark matter. NGC 4697 (http://xxx.lanl.gov/abs/astro-ph/0109075), NGC 404 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001RMxAC..11..121D&db_key=AST&h igh=3e5c03c80a24015), NGC 5812 and NGC 7507 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1994A%26A...292..381B&db_key=AST &high=3e5c03c80a24042) have been shown to lack dark matter and this Mass to Light (http://xxx.lanl.gov/abs/astro-ph/0211323) ratio study found evidence that ellipticals frequently do not have dark matter.

This will throw standard Cold Dark Matter (CDM) models into flux because those models predict that all ellipticals should have dark matter halos. Of course this should spur on interest in baryonic dark matter scenarios as I have noted in the baryonic dark matter (http://www.badastronomy.com/phpBB/viewtopic.php?t=5133&start=0) thread. There is a potential connection between the evidence that spirals may be mostly baryonic dark matter. If the dark matter is mostly baryonic molecular hydrogen gas clouds in spirals then it makes sense that systems such as ellipticals which have limited amounts of gas would also have limited amounts of dark matter.

AgoraBasta
2003-Aug-29, 09:20 PM
A paper has just come out on the Los Alamos Preprint server which finds that three elliptical galaxies show a dearth (http://xxx.lanl.gov/abs/astro-ph/0308518) of dark matter. dgruss23,

Each time I see this kind of 'result' I keep asking myself if it's about time to introduce a kind of 'new theory of gravity'. And then my 'enemies' come up with yet another material field with properties to settle down the idiotically new evidence.

So - no, nothing can happen that would ever sway the orthodoxes from sticking to the classic GR. There's always a way to interpret any possible new experimental reality as a new 'matter field', AND THEN THEY GLADLY SWALLOW ANOTHER NOBEL PRIZE FOR YET ANOTHER MALICIOUS DECEPTION.

dgruss23
2003-Aug-30, 12:53 PM
agora wrote: Each time I see this kind of 'result' I keep asking myself if it's about time to introduce a kind of 'new theory of gravity'

Some have asked that question with regard to the very existence of dark matter. MOND offers one alternative, but I've seen observations that contradict it. Plasma Cosmologists propose magnetic fields do the job but that had been well shredded here.

I see no reason to doubt that GR is a very close approximation to the gravitational workings of the universe. This new result with elliptical galaxies is fully consistent with GR. What it is suggesting is that for many elliptical galaxies - no dark matter is needed to explain the rotational dynamics. That doesn't require scrapping GR. It does require that theorists go back and take a look at their CDM models for the universe.

I've pointed out on the baryonic dark matter thread, that some evidence is suggesting the possibility that most of the dark matter in spirals could be cold molecular hydrogen gas. (See that thread for references). If that is the case then GR does not have to be scrapped in favor of MOND or some other alternative to account for the flat rotation curves of many spiral galaxies.

The problem that this result with elliptical galaxies creates is that the CDM models expect all ellipticals to be surrounded by CDM halos. Now we see that is not the case. So it is the CDM models that need a close look.

And the implications could be larger. The current concordance big bang model says the universe is ~23% cold non-baryonic dark matter. If it turns out that most ellipticals lack this CDM and spiral rotational dynamics can be accounted for by baryonic dark matter, then the concordance model would be contradicted and it would be time for Big Bang theorists to assess how in their model it is possible to live in a universe that is in fact dominated by baryonic dark matter. But it probably will take at least another 10 years of observations to establish whether or not the dark matter is mostly baryonic.

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Cougar
2003-Aug-30, 04:18 PM
...the CDM models expect all ellipticals to be surrounded by CDM halos. Now we see that is not the case. So it is the CDM models that need a close look.

I took a quick look at the linked article. It sounded like these Italians' method for determining the dark matter content of the ellipticals was rather new and novel. And the percentage error was not particularly comforting?

Before we all go back and take a closer look at the CDM models, don't you think we should first take a closer look at this new method and get at least one additional observation, preferably from an independent source and method, that agrees with the findings in this article?

dgruss23
2003-Aug-30, 06:42 PM
cougar wrote: I took a quick look at the linked article. It sounded like these Italians' method for determining the dark matter content of the ellipticals was rather new and novel. And the percentage error was not particularly comforting?

K.C. Freeman has a rather notable (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1970ApJ...160..811F&db_key=AST&h igh=3d4e81227e14258) record of important contributions.



cougar wrote: Before we all go back and take a closer look at the CDM models, don't you think we should first take a closer look at this new method and get at least one additional observation, preferably from an independent source and method, that agrees with the findings in this article?

For example the other articles I linked to that find similar results using different methods?

Cougar
2003-Aug-30, 09:14 PM
cougar wrote: Before we all go back and take a closer look at the CDM models, don't you think we should first take a closer look at this new method and get at least one additional observation, preferably from an independent source and method, that agrees with the findings in this article?

For example the other articles I linked to that find similar results using different methods?

Ah, the other articles. Got a little ahead of myself there.

But again, just glancing at the abstract of your last linked paper....


Within the European Southern Observatory (ESO) Key Programme 'A search for dark matter in elliptical galaxies', we have performed a detailed study of the large scale stellar kinematics of NGC 1400, 4946, 5812, 6721, 7507, and 7796. Our observations have produced kinematical data down to the 24.1 mag/sq arcsec in the B band, i.e. in some cases out to 2 Re and beyond. NGC 1400, 4946, and 6721 show significant rotation. The other three galaxies (NGC 5812, 7507, and 7796) can be described by spherical non-rotating equilibrium models. For two of these objects self-consistent stellar dynamical models with constant mass-to-light ratio are found to give good fits to the photometric and kinematical data, although the presence of dark matter is not excluded. In contrast, NGC 7796 shows evidence for the presence of a dark halo. This galaxy also exhibits counterrotation inside 4 sec and, correspondingly, a central peak in the velocity dispersion profile. On the basis of the small sample of objects studied so far, with radially extended spectroscopic observations, possible trends in the size of dark halos around elliptical galaxies are pointed out.

Without getting into the details, this does NOT seem to be a slam dunk for your position.

dgruss23
2003-Aug-30, 11:35 PM
cougar wrote: Without getting into the details, this does NOT seem to be a slam dunk for your position.

I think I've defined my position pretty clearly. Evidence is now accumulating that many ellipticals may not have dark matter halos. Since CDM models predict that they all should have dark matter halos this creates problems with those models and they will need closer scrutiny.

There are at least 10 ellipticals indicated among the papers I've cited which show no evidence for dark matter halos. Ever paper I cited with the exception of the NGC 404 paper was cited by the authors of the new study. As for the abstract you refer to - if you go into the paper you find it says this in section 4:


Therefore dark halos are not required to explain the decreasing velocity dispersion profiles of NGC 1400, 4946, 5812, 6721, and 7507, even if the best-fit models selected by the 2C method would include a sizable dark component (see Table 4).
NGC 7796 stands out as a different case. In the 2C analysis a satisfactory fit is found only by considering the presence of a sizable secondary component; ..."

and this in section 5 after eliminating three galaxies with significant amounts of rotation thus reducing the sample to the 3 galaxies which their modeling tool appears most valid:


In the previous section we have concluded that no evidence for dark matter is found for NGC 5812 and 7507, while NGC 7796 appears to possess a dark halo, much like two other galaxies (NGC 4472 and IC 4296) studied in the same program (SKP). We should try to find a way to interpret such apparently discordant results.

Thus using their method they find that 2 of the 5 galaxies that best fit their model do not require dark matter halos and 3 others excluded because of rotation velocity do not require dark matter halos either. Using the newer planetary nebula method 4 of 5 galaxies are indicated not to have dark matter halos. NGC 404 was shown to lack dark matter as well.

Now I'm not sure what you mean by "slam dunk" in this instance because the point is that all ellipticals are supposed to have dark matter halos and it appears that less than 50% of the ellipticals for which such measurements are possible actually indicate the presence of a dark halo. If a sizeable fraction of ellipticals lack dark matter halos you have a problem that requires the attention of CDM theorists. That is my position and its supported by these new results as well as the older studies.

In fact its not even a position so much as pointing out what the observational results are!

Cougar
2003-Aug-31, 12:43 AM
I thought the baryonic molecular hydrogen gas clouds (which you propose as a replacement for the cold dark matter) were pretty well accounted for in our galaxy. Yet, as Rocky Kolb puts it, "We find that the galaxy has a much larger mass than the sum of all the stars, dust, and other things we "see." The shortfall is not just a few percentage points, but most of the mass of our galaxy seems to have been left unaccounted." I'm no expert in this field, but I expect the molecular hydrogen is one of those "other things" that is easily detectable. So what's flattening our galaxy's rotation curve?

dgruss23
2003-Aug-31, 02:41 AM
I thought the baryonic molecular hydrogen gas clouds (which you propose as a replacement for the cold dark matter) were pretty well accounted for in our galaxy. Yet, as Rocky Kolb puts it, "We find that the galaxy has a much larger mass than the sum of all the stars, dust, and other things we "see." The shortfall is not just a few percentage points, but most of the mass of our galaxy seems to have been left unaccounted."

This thread (http://www.badastronomy.com/phpBB/viewtopic.php?t=5133&start=0) is where I posted some links to papers that discuss the possibility that the Milky Way's dark matter could be entirely baryonic in the form of molecular hydrogen clouds. The key paper that proposed the idea are by Pfenniger et al. I pointed out some of their arguments and a few papers that find evidence for large amounts of baryonic dark matter. If Pfenniger is right, then Rocky Kolb is wrong.



I'm no expert in this field, but I expect the molecular hydrogen is one of those "other things" that is easily detectable. So what's flattening our galaxy's rotation curve?

Actually molecular hydrogen is not easily detectable and that is the problem. If it were easily detected the matter (Pun not intended :D ) would already be resolved. Atomic hydrogen is easily detected. Molecular hydrogen clouds could potentially exist in amounts that are capable of explaining the flattening of the galaxy's rotation curve.

I point to this as a possibility. The initial response to this is that it must be wrong because the Big Bang forbids it (nucleosynthesis predictions and so on). But the reality is that the baryonic dark matter content of the universe has not been observationally determined. As such it is an important test for the Big Bang. IF it is determined that the baryonic dark matter exists in the large amounts hinted at by some studies such as those I linked to on the other thread, then the Big Bang's current version will fail that test.

Cougar
2003-Aug-31, 02:47 AM
Regarding your "NGC 5812 and 7507" linked paper, "A search for dark matter in elliptical galaxies: radially extended spectroscopic observations for six objects" by Bertin, et al....

The English is not always stellar. What do you think is meant when the authors say, "We should stress that we cannot exclude that those objects for which the available kinematical data do not give convincing evidence for dark matter are actually embedded in massive diffuse halos, as optical and radio studies of spiral galaxies have shown to be the case for disk galaxies..." [page 390]

Also, I note the authors' final word on this matter states, "The few trends pointed out above have to be taken only as a simple practical way to direct our attention to possibly interesting clues within a very small and biased set of galaxies. We expect that future surveys will allow for a more objective selection of elliptical galaxies."

I must have missed that part. Why do they refer to their set as "biased"?

dgruss23
2003-Aug-31, 03:11 AM
The English is not always stellar.

Good one! :D


What do you think is meant when the authors say, "We should stress that we cannot exclude that those objects for which the available kinematical data do not give convincing evidence for dark matter are actually embedded in massive diffuse halos, as optical and radio studies of spiral galaxies have shown to be the case for disk galaxies..." [page 390]

They continue on and refer back to the introduction. In the introduction they talk about the hydrogen studies of spiral galaxies. Spirals have an optical disk - which is what we see in visible light and for most spirals a hydrogen disk extends well beyond the optical disk. This is where the 21cm hydrogen linewidth profiles come from that provide one method of determining rotational velocity. Optical rotation curves provide the other method.

What is observed is that the rotation curves remain flat beyond the optical disk in most spirals. So this is the evidence for a dark halo in spirals. If it had a keplerian fall off then there would be no evidence for a dark halo. (In the baryonic dark matter thread I was simply pointing to some evidence that this halo could actually be composed of molecular hydrogen gas - but the gravitational effect is still the same.)

What they're referring to on page 390 is that their data is within the optical portion of the elliptical galaxies. So they cannot rule out the presence of a dark halo beyond the optical extent of the galaxy simply because their data does not extend out that far. However, the kinematic data that they have been able to recover only indicates the presence of a dark halo for NGC 7796. But there should be some signature of the dark halo on the optical kinematics of the galaxies too - which is why they conclude that they have no evidence for those galaxies that the dark halo is actually there.




Also, I note the authors' final word on this matter states, "The few trends pointed out above have to be taken only as a simple practical way to direct our attention to possibly interesting clues within a very small and biased set of galaxies. We expect that future surveys will allow for a more objective selection of elliptical galaxies."

I must have missed that part. Why do they refer to their set as "biased"?

Their sample was biased because they had to select very specific types of ellipticals to conduct their study.

They wanted high surface brightness ellipticals to improve signal to noise ratios. They wanted galaxies with angular diameters small enough so that they could sample the sky background because that light must be subtracted. They wanted objects with large central velocity dispersions and they wanted round ellipticals classified as E0 or E1.

So in selecting these galaxies they are not sampling the full range of elliptical galaxy morphology, surface brightness, central velocity dispersion and size. That is their selection bias and they have to acknowledge it because it cannot be certain that their results extend to all the different types of ellipticals.

cyrek1
2003-Aug-31, 02:21 PM
Cyreks comment:

This is just an example that will eventually prove that dark matter does not exist as I wrote in my post about this subject.

dgruss23
2003-Aug-31, 02:46 PM
Cyreks comment:

This is just an example that will eventually prove that dark matter does not exist as I wrote in my post about this subject.

It depends upon what type of dark matter you're talking about. If additional samples verify that ellipticals frequently lack dark matter then that would be a serious strike against the cold dark matter scenarios. But if ellipticals lack cold dark matter then spirals probably do as well because ellipticals are thought to form from merging of spirals with dark halos in the CDM scenarios. So the evidence for baryonic dark matter I've noted would become much more relevant in explaining the flat rotation curves of spiral galaxies because w/o CDM, baryonic is what you have left.

Cougar
2003-Sep-05, 08:05 PM
After their galactic collision, "Strong tidal forces from NGC 2207 have distorted the shape of IC 2163, flinging out stars and gas into long streamers stretching out a hundred thousand light-years..."

Perhaps the dark matter also gets considerably thinned out - as ellipticals form from collisions of spirals.... (?)

dgruss23
2003-Sep-06, 02:57 AM
After their galactic collision, "Strong tidal forces from NGC 2207 have distorted the shape of IC 2163, flinging out stars and gas into long streamers stretching out a hundred thousand light-years..."

Perhaps the dark matter also gets considerably thinned out - as ellipticals form from collisions of spirals.... (?)

But that's not what the merger theories predict. They say that large dark matter halos are built up by the collisions.

John Kierein
2003-Sep-06, 02:01 PM
Molecular hydrogen is indeed hard to detect spectroscopically. This is what led Marmet to beleive that there is sufficient intergalactic molecular hydrogen to cause the red shift. Photons from background galaxies hit the intergalactic gas, lose momentum and energy to it to cause the red shift according to Marmet. Marmet is a spectroscopist.

This is similar to my ideas. However, I think the red shift is due to light hitting electrons (+ionized hygrogen protons) causing a Compton effect red shift.

Cougar
2003-Sep-06, 02:48 PM
caltech.edu is a pretty reputable source....


"Roughly speaking, (http://nedwww.ipac.caltech.edu/level5/Struck/St5.html) a pair of spherical, interpenetrating dark halos interact as if they were single-component systems: the orbital angular momentum of the two halos is transferred to internal degrees of freedom, imparting spin and creating broad tails. ...embedded disks and/or bulges, are not much braked by the tidal forces retarding the dark matter; instead, these components lose orbital angular momentum mostly by interacting with their own surrounding halos, once the latter have been decelerated."

Not only is the dark matter disrupted in a collision, it sounds like some collisions might even send the involved central galaxies outrunning their dark matter envelopes, or at least temporarily losing their central location, confounding earth observers noticing those galaxies' relative lack of dark matter.

dgruss23
2003-Sep-06, 03:45 PM
Thanks for the reference Cougar! The Caltech website is excellent. I'll take a look at the article as soon as I get a chance. It may be a few days or even a week. I'm extremely busy these days so I've had to cut way back on my BABB time. :cry: