The dynamical evidence suggests that significant amounts of dark matter exist in spiral galaxies, but the nature of the dark matter is very uncertain. In the current models the dark matter halo is seen to contribute an increasing amount to the rotational velocity of a galaxy as distance from the nucleus increases:

http://nedwww.ipac.caltech.edu/level...er/frames.html

This can be seen in the lower panel of the first figure in the above link. The contribution from the halo increases while the contribution from the disk decreases.

An interesting possibility is that a significant portion of the dark matter could be normal baryonic dark matter. This possibility was put forward in 1994 by Pfenniger, Combes, & Martinet:

http://adsbit.harvard.edu/cgi-bin/np...PRINT&ext=.pdf

Pfenniger et al propose that the dark matter is primarily cold molecular gas such as H2 in a rotationally supported thick disk. The gas is thought to be in fractal subunits of about 30 AU and containing 10-3 solar masses. If dark matter is in the form of cold gas then several problems might be explained:

1. Cold gas could provide a reservoir to feed the relatively constant star formation rate in spiral galaxies.
2. The ratio of dark mass to stellar mass decreases along the Hubble sequence from Sd to Sa type galaxies, but the dark matter to HI mass remains constant. Since one scenario is that Sd galaxies involve into Sa galaxies, this would be expected if the dark matter is being converted into stars along with atomic hydrogen.
3. This might also explain the disk-halo conspiracy: http://nedwww.ipac.caltech.edu/level5/Bosma/frames.html

One difficulty is that molecular hydrogen is very difficult to detect. However, a number of recent studies indicate that there may be a significant amount of molecular gas and dust in the halo:

1. Velentijn find that NGC 891 has enough molecular hydrogen to explain the dark matter at least in the optical disk: http://adsabs.harvard.edu/cgi-bin/np...e5c03c80a07381
2. Kalberla et al find evidence for significant amounts of H2 in the galactic halo with the flattened distribution predicted by Pfenniger et al:
http://xxx.lanl.gov/abs/astro-ph/9909068
3. Walker&Wardle find evidence for significant amounts of Jupiter mass cool gas clouds in the galactic halo from Extreme Scattering Events:
http://www.journals.uchicago.edu/ApJ...985107.web.pdf
4. Trewhella et al find evidence for a significant component of cold dust in galaxies and conclude that: “But perhaps the most important implication of this result is the suggestion that the far-infared emission is associated with cold halo material that could, in turn make up some or all of the missing mass in galaxies. It is also significant to note that they find the cold dust is extended 10 x more radially than vertically which is consistent with the proposal of Pfenniger et al that the baryonic dark matter is in a flattened thick disk.
http://adsabs.harvard.edu/cgi-bin/np...e5c03c80a07903
5. Richter, Sembach, & Howk find evidence for H2 in a dense filament in the Milky Way Halo. They note that it is unknown whether structures such as this filament are related to the structures proposed by Pfenniger et al.
http://xxx.lanl.gov/PS_cache/astro-p...05/0305017.pdf

So in total there is an interesting possibility that a significant – if not all – of the dark matter in the Milky Way could be composed of normal baryonic matter.

Orion, where'd your message go. I didn't have a chance to respond. You had asked about Marmet, right?

This is to pointing out the stability of molecular hydrogen when formed and the fact than It cannot be argued that H2 does not exist (in large amount) in space because it could be ionized or dissociated by ultraviolet radiation. If there were enough ultraviolet radiation to ionize H2, that same radiation would also ionize atomic hydrogen. This is not the case, because non-ionized atomic hydrogen is observed, even though it requires less energy to ionize the atomic than the molecular form of hydrogen.

http://www.newtonphysics.on.ca/hydrogen/index.html

Nature of Molecular Hydrogen

Molecular hydrogen is rarely looked for in space. In most papers in astrophysics, the word hydrogen is mentioned without distinguishing whether it is atomic or molecular. Yet it is a well-known fact of basic chemistry that atomic hydrogen is extremely unstable, and that it reacts violently to produce molecular hydrogen, which is extremely stable. Given a bottle of pure atomic hydrogen, one would expect an immediate energetic explosion, producing molecular hydrogen at a very high temperature.

Atomic hydrogen (H), composed of a single proton and electron, is the simplest existing stable atom. Because of the spin structure of the particle, it is easily detectable using a high frequency radio signal at 21-cm wavelength. Atomic hydrogen in galaxies and in intergalactic space can be detected very easily, because the atomic hydrogen can change its spin (which changes its energy).

Electromagnetic radiation is emitted at the wavelength of 21 cm, or an absorption line is observed (in the background radiation) at that wavelength. However, when two atoms of atomic hydrogen combine, forming molecular hydrogen (H2), their spins are coupled and completely cancel each other. The radio-frequency spectral line at 21 cm no longer exists, and the molecular hydrogen becomes totally invisible at that wavelength.



However, it is well known that atomic hydrogen in space was certainly naturally transformed into H2. Over billions of years, dust, three-body interactions, and even photon emission have produced H2. Once molecular hydrogen is formed, it is so stable that it has little probability of dissociation. It cannot be argued that H2 does not exist in space because it could be ionized or dissociated by ultraviolet radiation. If there were enough ultraviolet radiation to ionize H2, that same radiation would also ionize atomic hydrogen. This is not the case, because non-ionized atomic hydrogen is observed, even though it requires less energy to ionize the atomic than the molecular form of hydrogen.

These considerations show, that as a result of the large amount of atomic hydrogen already observed in space, and the extreme stability of molecular hydrogen, the chemical equilibrium giving the relative abundance between atomic hydrogen and molecular hydrogen in space, strongly favors the formation of the diatomic form (H2) over the monoatomic form. We must thus conclude that the recent discovery of H2, is no surprise, and should have been expected from the known facts concerning the natural equilibrium between H2 and H. It is expected that much more colder H2 will also be discovered.

You raise an important issue Orion. If molecular hydrogen clouds exist in the galactic halo, then it must be explained how they remain stable without undergoing gravitational collapse. This question is considered by Wardle & Walker:

http://xxx.lanl.gov/abs/astro-ph/9907023

They propose that the clouds are stabilized by precipitation and sublimation of particles of solid hydrogen. Apparently if the particles are destroyed which then makes cooling less efficient with increasing temperature.

I am constantly amazed how proposed solutions to current astrophysical problems so often involve something "that we cannot observe". It's either too far away, too this, not enough that, or just plain too mysterious. This makes for a great story around the camp fire, but not good science.

First, molecular hydrogen does make itself known via the Lyman-Werner bands in the UV (near 1000-1200 Angstroms), usually in absorption with some bright continuum source behind. FUSE regularly measures H2 in sight lines through the Galaxy. Second, if there is H2, then there ought to be CO and CO2 and H20 and all kinds of abundant molecules that have dipole moments and emit in the far-IR or radio wavelengths. This is how we observe molecular clouds in the disk of the Milky Way. Even if these clouds were somehow primordial, how could they stay that way with the MW Galaxy constantly blowing out metal enriched gas from its disk (via overpressures of hotstars and supernovae near star forming regions)?

Certainly, discoveries of molecular H at great galactocentric distances is interesting, but it's kinda jumpin' the gun to make it some mysterious solution to the "dark matter" problem. That isn't to say that we shouldn't pursue a complete census of baryonic material in the Milky Way - we should, and for reasons beyond the dark matter problem.

I'm not clear how this statement is relevant to anything I've said on this thread. I never used the phrase "we cannot observe". I said "very difficult to detect" - which is true. However, if that is your criticism then you must feel that non-baryonic dark matter is a great campfire story too?.

I would expect so.

The proposal of Pfenniger et al is that these molecular clouds are in the halo, not the disk. I pointed to other references that indicate the same possibility.

If these clouds are in the halo, then they are not near any star forming regions and they may very well be near the temp. of the CMB. Again the references I cited discuss this.

Nobody's "jumpin' the gun". My posts on this topic are appropriately peppered with "If" and "possibility".

Sure, but its important to determine how much of the missing mass problem can be accounted for by baryonic dark matter. At this time what would be a more important implication of this possibility.

I was mainly responding to some of the comments of Orion38, both here and also as a common theme that appears in many of Orion38's posts. While I don't work in the field, non-baryonic dark matter leaves me feeling queezy, at least until we have some real idea as to what it is (IF it is). Until then, it's certainly best to measure all important sources of baryonic matter present in the MW.

I understood that. I was merely pointing out that for the most part we infer the molecular H2 content in the disk of the MW by measuring CO emission (and other relatively common molecules). So even if H2 is hard to observe itself, many other species should normally be more easily detectable.

Again, this comment was pointed more in the direction of Orion38.

Agree completely with the first point. On the question of importance, I also agree. I was simply pointing out that many fields of astronomy would be impacted by the discovery of any significant H2 in the halo, nevermind whether or not it contributes significantly to the mass budget or addresses the dark matter problem.

Spaceman Spiff wrote:
I was mainly responding to some of the comments of Orion38, both here and also as a common theme that appears in many of Orion38's posts

Note than I have especially delete and replace a previous post to try to escape this kind of remark so I dont see in my post an emphasis other than to pointing out an objection done by JS on another thread about the same subject.
-It cannot be argued that H2 does not exist (in large amount) in space because it could be ionized or dissociated by ultraviolet radiation-

Maybe I must have better writing
It cannot be argued that H2 does not exist -PROBABLY-...(in large amount) in space because it could be ionized or dissociated by ultraviolet radiation- Read my post for more details.

So the tone would sound less provocative _PROBABLY_ :wink:

Sorry Spaceman! I didn't think Orion had said enough on this thread for your points to be directed at him.

Hey - I'm glad we're in pretty good agreement!

That's my feeling on the non-baryonic dark matter. I do find it interesting that at least three different lines of evidence all are hinting that baryonic dark matter may be a significant component of the dark matter(direct measurements, extreme scattering events, and cold dust studies).

Ok dgruss23 and Orion38. Maybe we're all on the same page afterall.

As for Orion38's last comment: no we cannot assume hydrogen is mainly atomic, but we also have a pretty good idea about the conditions necessary for H2 to form and then exist for significant time periods (without being dissociated, for example).

There are several arguments [i]for[\i] non-baryonic dark matter, though,
including Big-Bang Nucleosynthesis constraints on the total amount of
baryonic matter (whether or not we can see it) and the power spectrum of
the Cosmic Microwave Background, which disagrees with a completely
baryonic universe at a significant level. Other arguments can be found
in these review articles: Baryonic Dark Matter by Bernard Carr and Particle
Dark Matter by David Spergel.

__________________
http://www.as.utexas.edu/~kurtis

Excellent point kurtisw! I was wondering if anybody would raise that objection. Here are the three reasons Spergel gives for non-baryonic dark matter:

The studies I've noted speak directly to Spergel's first point. It appears that the baryonic dark matter is now being detected. If further studies do not confirm the baryonic dark matter exists in these large amounts then non-baryonic DM would be the only other option and the status-quo works.

The last two points are interesting because empirical evidence trumps theoretical models. IF the baryonic dark matter is observed in large enough amounts to account for the rotational dynamics of galaxies, then we cannot say the verified observations are wrong because they do not fit the Big Bang's predictions. Rather it would have to be that the Big Bang itself would need modification - or an alternative model that could account for the baryonic matter would need to be developed.

Until the baryonic matter content hinted at in the above studies is verified, there is no reason to panic about that, but in the end the theory must be consistent with the observations. In fact, since the total baryonic content of the universe has been theoretically determined, but not yet observationally determined, we do not know if the baryonic content will confirm the Big Bang's predictions or refute them. So the baryonic content is an important test of the Big Bang model.

Cold, non-baryonic (mainly non-dissipative; it doesn't radiate) is apparently needed in many types of theoretical models of structure formation, from large scale to galaxy scale. Big Bang Nucleosynthesis (coupled with the helium, lithium and deutrerium abundance measurements) predicts and with data from the cosmic microwave background we infer similar amounts of baryonic matter. There is a lot of converging evidence for the presence of non-baryonic dark matter, but it would sure be nice if we could detect it more directly (and so independently of these other constructs and their constraints) and have a theoretical description of what the heck it is.

You're right. There are a lot of theoretical reasons for thinking non-baryonic dark matter exists. I'm not aware of a single observation that independent of theory that requires the dark matter be specifically non-baryonic. Can anybody think of one?

I can't think of a single observation, but I can think of several observations
that, when taken together, imply that the dark matter is likely not baryons.

The velocity dispersions of galaxies in clusters of galaxies
imply large amounts of unseen matter (doesn't matter in this case what this
is). But x-ray observations find large reservoirs of hot gas in these clusters.
If the gas is in hydrostatic equilibrium, then large amounts of unseen matter
must also exist. The mass of the hot gas, which can be determined directly
from the observations, is not nearly enough to account for the missing mass.
Cool gas would not be stable in this environment, so it cannot be neutral
atomic or molecular gas. If it is warm ionized gas, which can be stable,
we would see emission lines which are not observed. So, in my mind, this
pretty much leaves MACHOS as the only baryonic dark matter candidate
in clusters of galaxies.

The MACHO and OGLE searches for MACHOs in our galactic halo have
turned up that less than ~20% of our galactic halo is composed of
MACHOs. Unless one can propose a mechanism for creating a much larger
fraction of MACHOs in galaxy clusters, I think that we can rule out MACHOs
as the primary source of the dark matter.

I believe that observations of the Lyman alpha forest in quasar spectra
also provide evidence that while vast amounts of baryonic dark matter
exist in the inter-galactic medium, there is not nearly the quantity
required to make up the total matter density in the universe. But this is
far from my areas of research, so I personally can't talk about this point
much.

Also, if much of the dark matter were in the form of molecular clouds
(that somehow don't have CO and other tracers, even though these
elements exist in the galactic halo and the IGM), wouldn't these clouds
absorb UV light? In this case, all-sky UV surveys (like the
recently-launched GALEX) should be able to detect the absorption.
This is again far from my areas of research, so I can't press the point
too much.

But let's look at the candidates for non-baryonic dark matter. Primordial
black holes are one. From what I understand, these would be black holes
with event horizons on the scale of the Plank radius, and they require a
theory of quantum gravity to explain. I can't say much more about them,
but it sounds kinda fishy to me.

What about WIMPS? Theories of supersymmetry (and other models
beyond the Standard Model) require these particles to exist. And
because neutrinos oscillate (and therefore have some mass), we know
that there must be some physics beyond the Standard Model. So I
don't find it that much of a stretch to think that there may be particles out
there with seemingly-odd characteristics.

I also am reluctant to say that evidence such as Big-Bang Nucleosynthesis
and the CMBR power spectrum are theoretical arguments. Certainly they
have pinning in theory, but they have made predictions of observables
that have been shown to be correct. (Lots of observers, including myself,
were very suspicious of Big Bang + Inflationary models that required a flat
Universe even just a few years ago, but supernova Ia results and
WMAP results now agree with that flat universe prediction. It was a
powerful prediction that seems to be holding true.)

Lastly, let me say that I fully agree that we need to account for all the
baryons in the Universe and come up with a full census. I also think
we need to keep working on searches for non-baryonic dark matter,
because, as has been mentioned above, we must verify that it exists.
There's at one Nobel Prize to the group that finds it, and perhaps
a second for the theoretical underpinnings if the two groups don't have
to share.

I think the evidence is very strong that non-baryonic dark matter exists,
but we certainly need to find it to prove it!

__________________
http://www.as.utexas.edu/~kurtis

Do you mean not likely to be entirely baryons?


This is perhaps the strongest traditional observational evidence for non-baryonic dark matter. But it relies upon the interpretation that velocity dispersions of clusters are in fact entirely caused by gravitationally induced velocities.

What is interesting here is the local observations of baryonic matter are indicating just the opposite (potentially if you consider the papers I've cited). Again if the velocity dispersions are not accurate measures of gravitational potential, then baryonic dark matter could be sufficient.

Isn't this also one of the objections to cold gas clouds in the Milky Way halo - how to keep them stable? Yet researchers are now starting to find that it may be possible for such clouds to exist.

I agree here. I don't think MACHOs are the answer. If the baryonic dark matter is eventually shown not to exist, then the non-baryonic dark matter is the best option.

You're right. This is an uncertain factor. But the very existence of the Ly alpha forest suggests that large amounts of molecular gas could also exist.

Actually, it was suggested in at least one of the papers I've read that the best place to look for the signature of these clouds would be at temperatures slightly warmer than the microwave background.


The model is consistent as currently formulated with the available data. However, Rowan-Robinson pointed to some reasons to be more cautious about the validity of the Supernova Ia results. There are also some concerns about the amplitudes of several of the WMAP peaks. (I'll have to go back and pull out the reference I had on that one and the Rowan-Robinson reference). And while the nucleosynthesis looks good, if the baryonic dark matter turns out to be enough to account for the dark matter, then that would be an observational contradiction with the current version of the Big Bang.

Thanks for considering the issue!

Yes. Sorry for being a tad sloppy.

[quote]
This is perhaps the strongest traditional observational evidence for
non-baryonic dark matter. But it relies upon the interpretation that velocity
dispersions of clusters are in fact entirely caused by gravitationally induced
velocities.
[\quote]

Given that the temperature/density of the hot x-ray gas results in
mass measurements generally consistent with the velocity dispersion
measurements, and that the two are independent methods of measuring
the depth of the potential well, I consider the mass measurements fairly
sound.

Just a note to others reading, MACHOs are a form of baryonic dark
matter, and some do exist (but not enough to explain the mass of
the Galactic halo.

Agreed. I seriously doubt that this will turn out to be the case, however.
But that opinion makes it no less important to search for baryons.

__________________
http://www.as.utexas.edu/~kurtis

Grey wrote:
Orion38 wrote:
"Can you invalidate this alternative view given by Marmet?"

>I can at least point out some of the flaws in his argument, if you'd like,

Orion 38
Some members in the past have pointing out than this density is not observed but
i make a second try based on new data from the Baryonic Dark Matter thread. Is it this point you are talking about?

Cosmic Matter and the Nonexpanding Universe.
http://www.newtonphysics.on.ca/UNIVERSE/Universe.html
Quote:
An average density of matter is space of about 0.01 atom/cm3 is derived. It is known that the density of matter is compatible with many reliable observations. These results lead to a nonexpanding cosmological universe.

A New Non-Doppler Redshift
http://www.newtonphysics.on.ca/HUBBLE/Hubble.html

Orion, I'll respond to this, since you pulled it over to this conversation, though as I pointed out, I'm not certain that I can materially add to this discussion. Since it looks like this conversation had mostly ended last month, hopefully no one will object. Kurtisw has already pointed out some of the reasons that expecting large quantities of undetected molecular hydrogen is unlikely and that in fact the unobserved matter needs to be non-baryonic. This is certainly my largest criticism of the link you provided.

I confess that I also question a scientific essay which begins with a discussion of which prestigious scientists said things which support ones theories. I understand that he's trying to provide some support for a non-mainstream view, but it shouldn't be necessary if the ideas stand on their own. And, of course, prestigious scientists can be wrong, just like anyone else.

Hmm, I also notice that most of this has already been discussed here, and I think JSPrinceton probably did a better job speaking than I would, so perhaps it's unnecessary to recover this ground.

Because the discussion was limited to H2 but if you include also ordinairy Hydrogen the average density calculated by Marmet look correct.

"An average density of matter is space of about 0.01 atom/cm3 is derived. It is known that the density of matter is compatible with many reliable observations. These results lead to a nonexpanding cosmological universe."

A New Non-Doppler Redshift

Quote
One must then conclude that a slight redshift is produced, due to hydrogen in the space crossed by electromagnetic radiation, according to equation 12. This redshift appears undistinguishable from the ordinary Doppler redshift. The energy loss of the initial radiation appears separately as very low frequency radio waves.
http://www.newtonphysics.on.ca/HUBBLE/Hubble.html

I don't even get past the opening paragraph before I disagree here. In the cases he cites, the absorption spectrum shows a smaller redshift than the emission spectrum, which is exactly what one would expect if the absorbing material is closer than the source.

Moreover, the density of matter he needs in intergalactic space is a few orders of magnitude larger than what's observed,. These observations include tests of density by observing how light from quasars is absorbed. He's explaining data which already has a valid explanation, and does it in a manner that conflicts with observation. That suggests that he's mistaken.

I agree that this is the expectation largely from theoretical considerations within the Big Bang framework. I would only caution that as the papers cited above note, there is a chance that the baryonic dark matter may be out there in much larger quantities than previously supposed. So the amount of baryonic dark matter is an important test for the current Big Bang Concordance model.

I'll agree that's a possibility, and I heartily concur that it's a very good idea to continue a survey of baryonic matter.

But does the actual observations include the large amount of H2 detected?

I don`t think that is the case at least from past discussion on the subject.

http://www.newtonphysics.on.ca/hydrogen/index.html

Grey wrote
>These observations include tests of density by observing how light from quasars is absorbed.

Are you talking about The Layman Alpha Forest?

There certainly has been a fair (even surprising) amount of molecular hydrogen observed in the disks of some galaxies. However, though some of the researchers involved claim that this obviates the need for dark matter in the halo, that's simply not the case. Nor does this address intergalactic matter. While it is conceivable that this could be accounted for by molecular hydrogen as well, it has not been observed to my knowledge, and people have been looking. We should, of course, continue to look, both for molecular hydrogen, and anything else which this matter might be.

Orion, To summarize, we can say that some studies suggest that there might be large amounts of baryonic dark matter, but those studies are inconclusive and require that further studies be done to resolve the matter. That will take time so at this point I'm satisfied with the agreement we seem to have here that large amounts of baryonic dark matter is a possibility that needs further study but which is not expected from theoretical considerations.

There would certainly be more extensive searches for baryonic dark matter if it was predicted by theory - as we see with the ongoing studies for non-baryonic dark matter.

Marmet adress this issue,
http://www.newtonphysics.on.ca/UNIVERSE/Universe.html
Quote:
"All values are theoretical and model dependent. The value given by the current cosmological model that assumes the big bang gives a characteristic density of about 10-8 atom/cm3. Other models based on particle physics predict different values. For example, it has been reported recently [18] that the standard hot-universe theory predicts a value that is 15 orders of magnitude higher. Models based on one version of supergravity contradict [18] the cosmological model by 10 orders of magnitude. The typical energy density stored in the Polonyi fields brings a contradiction [18] of 15 orders of magnitude, while most of the higher-dimensional Kalusa-Klein theories considered in the early 1980's predict [18]a density larger by 125 orders of magnitude! Finally, no consistent cosmological model [18] based on superstrings has been suggested so far. As seen above, the problem related to the amount of matter in intergalactic space is extremely complex, and the predictions are completely incompatible."

I agree with that but the point I want to adress is about the RED SHIFT model proposed by Marmet.

http://www.newtonphysics.on.ca/UNIVERSE/Universe.html
An average density of matter is space of about 0.01 atom/cm3 is derived. It is known that the density of matter is compatible with many reliable observations. These results lead to a nonexpanding cosmological universe.

I don't mean values predicted by models, I mean observations. If there are in fact reliable measurements placing the density of the intergalactic medium close to the value Marmet's model needs, I'm unaware of them, and would be interested in citations.