Do we really know how much baryonic matter galaxies contain?

[TooMany]

Is there some kind of mechanism that would explain why H2 should be more prevalent in the outskirts of the Galaxy and local measurements should not be representative of overall hydrogen density?

Stellar population falls off exponentially beyond the cores of galaxies. A likely reason is that the gas density becomes insufficient for star formation. We don't really know what happens to the gas when the density falls below this threshold because it is dark. Any substantially sub-stellar objects that might form in the dark part cannot be detected. The radius of the major cutoff in star formation is much less than the radius of the overall disk. We know the disk extends far beyond the part populated by stars because of dust and H I emissions that we can detect. So there is the strong possibility that most of the hydrogen has been used up where stars are dense, but where they are not, the hydrogen still exists as it did before star formation occurred in the central part.

Imagine that the Galaxy originated from a large rotating cloud of hydrogen. The very densest part of the cloud would be near the center and this is where almost all of the stars form. The rest remains as it was, a much larger surface (in area) containing gas and perhaps condensed objects, but not dense enough to form stars. The rotation curves may be telling us that there is a lot of additional matter beyond the stellar disk that has not been converted into stars (e.g. 5 times the stellar mass).

[Nereid]

Stellar population falls off exponentially beyond the cores of galaxies.

They do?

Evidence please.

Especially evidence with regard to (giant) ellipticals.

A likely reason is that the gas density becomes insufficient for star formation.

It is?

Evidence please.

We don't really know what happens to the gas when the density falls below this threshold because it is dark.

What threshold is that (quantitatively)?

Any substantially sub-stellar objects that might form in the dark part cannot be detected.

They can't?

Evidence please, especially with respect to the various micro-lensing studies published (e.g. MACHO, OGLE).

The radius of the major cutoff in star formation is much less than the radius of the overall disk.

It is?

Evidence please. And how does this relate to galaxies which do not have a disk?

We know the disk extends far beyond the part populated by stars because of dust and H I emissions that we can detect.

We do?

Evidence please.

(I think that's enough for now)

[TooMany]

Originally Posted by TooMany
Stellar population falls off exponentially beyond the cores of galaxies.

They do?

Evidence please.

Especially evidence with regard to (giant) ellipticals.

I'm referring to spirals where we have rotation curves to measure. Are you seriously questioning that the literature shows exponential falloff in starlight or are you just trying to create doubt about whatever I say? You certainly know the answer to your own question, don't you?

Originally Posted by TooMany*
A likely reason is that the gas density becomes insufficient for star formation.

It is?

Evidence please.

That's my idea. What's your idea of why star formation falls off? Is it likely that suddenly there is no gas, even when we known that H I emissions occur far from the cutoff radius?

Originally Posted by TooMany*
We don't really know what happens to the gas when the density falls below this threshold because it is dark.

What threshold is that (quantitatively)?

I don't know precisely and I suppose it may depend on stellar luminosity which heats dust and gas, dust content, tidal forces etc. Do you feel that the gas content abruptly stops exactly at the radius where star formation stops? How do you explain the very extensive H I/dust regions? Why would they be devoid of molecular hydrogen? If molecular hydrogen can exist near the core, why not also beyond the stellar cutoff when there is a lot less radiation to dissociate molecules? There must have been enough molecular hydrogen in the stellar core to create all those stars. Why not more?

Originally Posted by TooMany
Any substantially sub-stellar objects that might form in the dark part cannot be detected.

They can't?

Evidence please, especially with respect to the various micro-lensing studies published (e.g. MACHO, OGLE).

No they cannot be directly detected. This is getting ridiculously repetitive, but we can't even detect the sub-planetary bodies beyond the outer solar system. We can only detect nearby brown dwarfs which are much larger than planets and thus warm enough to emit enough radiation to detect.

MACHO studies have several problems as yet:

1) They are only looking in the direction of the MCs and the core, because nowhere else are there enough background stars to study in reasonable time period. Therefore these studies tell us nothing about the outer parts of the disk.
2) They cannot detect anything smaller than a Jupiter.
3) They cannot detect any small objects moving rapidly relative to the background because they do not check each background star with enough frequency.
4) As pointed out by kzb the requirements for a detention are quite high.
5) The number of detections is very small so far and cannot be argued as statistically accurate. But even if they were accurate, see problem 1.


Originally Posted by TooMany*
The radius of the major cutoff in star formation is much less than the radius of the overall disk.

It is?

Evidence please. And how does this relate to galaxies which do not have a disk?

Oh Nereid, have you not seen multi-spectral pictures of spiral galaxies? Are you not aware that the extent of flatness of rotation curves is born out by measuring doppler shift in H I emitting regions far outside of the stellar core? Don't you know that the extent of Andromeda has relatively recently found to be much larger than previous thought from H I studies (ironically through the detection of remote stars). Why do you fain ignorance? Just to discredit my statements?

What about galaxies that have no disk? Last I heard ellipticals haven't got much DM to find according to the viral motions of their stars. (I am aware that there is at least one paper denying the correctness of the mass estimate from viral motion. What a kicker! I guess we better rethink clusters too.)


Originally Posted by TooMany*
We know the disk extends far beyond the part populated by stars because of dust and H I emissions that we can detect.

We do?

Evidence please.

(I think that's enough for now)

Nereid. You already know these things. Why pretend that you don't? Yes that is quite enough of this diversion.

The real issue here is in the name of the thread "Do we really know how much baryonic matter galaxies contain?". If you don't want to address how we can completely exclude the possibility of sufficient baryonic matter to explain the rotation curves then don't post. If you have some evidence that proves this baryonic matter does not exist then please share it.

[R.A.F.]

If you have some evidence that proves this baryonic matter does not exist then please share it.

Really?...does someone really have to explain the burden of proof for the 10 zillionth time...really???

Yawn.......

[Nereid]

[snip]
Nereid. You already know these things. Why pretend that you don't? Yes that is quite enough of this diversion.

Thank you for the specific, concrete answers to my questions.

For the rest - particularly this part of your post, that I'm quoting - I'll merely say that this thread is in BAUT's ATM section, and that, as such, it has special rules.

Having said that, I look forward to your timely answers to my direct, pertinent questions. Note, of course, that "I don't know" and "I need more time to answer" (etc) are perfectly good answers.

The real issue here is in the name of the thread "Do we really know how much baryonic matter galaxies contain?". If you don't want to address how we can completely exclude the possibility of sufficient baryonic matter to explain the rotation curves then don't post. If you have some evidence that proves this baryonic matter does not exist then please share it.

It's "your" thread, in the ATM section of BAUT. As I understand it, the relevant BAUT rule encourages other BAUT members to ask penetrating, direct, pertinent questions, of the ATM ideas presented, as presented. I look forward to reading your direct, pertinent answers, to the questions that I - and other BAUTians - have posed, concerning your ATM ideas, as presented.

[TooMany]

Really?...does someone really have to explain the burden of proof for the 10 zillionth time...really???

Yawn.......

Your not paying attention. This is a question I asked the experts here. I am not trying to prove anything. I am asking is there proof of the mainstream assumption? If so what is that proof?

You see if you claim that enough baryonic matter to explain galactic rotation absolutely does not exist, then it is your burden to support such a conclusion, not mine!

By the way, my wife once met Mr. Asimov (in 1968). Both of us were admirers of his science books for the layman and his fiction. However, she was very disappointed in meeting him only to find discover that he was an arrogant Language. She is not usually inclined to pass judgment on people.

[TooMany]

I'll merely say that this thread is in BAUT's ATM section, and that, as such, it has special rules.

I'm well aware of the usual stoning procedure. I DID NOT POST THIS QUESTION TO THIS THREAD. It was moved without my concent. It was a question perfectly relevant to the thread called "Galactic Rotation Curves... no need for dark matter", not a claim. Do you understand the difference?

Having said that, I look forward to your timely answers to my direct, pertinent questions. Note, of course, that "I don't know" and "I need more time to answer" (etc) are perfectly good answers.

It's "your" thread, in the ATM section of BAUT. As I understand it, the relevant BAUT rule encourages other BAUT members to ask penetrating, direct, pertinent questions, of the ATM ideas presented, as presented. I look forward to reading your direct, pertinent answers, to the questions that I - and other BAUTians - have posed, concerning your ATM ideas, as presented.

I'm really not interested in your questions (which you already know the answers to anyway). What I'm interested in is getting my question answered which is why I posed it. Please read the title of my first post which was moved to this forum. It's full of questions, is it not?

Nereid. I just asked you not to post unless you have something specific to say about the question I posted that was placed here in this forum. But you babble on without the slightest contribution of relevant information. What's the point?

[Reality Check]

I'm referring to spirals where we have rotation curves to measure.

Rotation curves of elliptical galaxies are also measured, e.g.
Dynamics of early type galaxies. I - The rotation curve of the elliptical galaxy NGC 4697, Bertola, F. & Capaccioli, M. (1975)

[R.A.F.]

I DID NOT POST THIS QUESTION TO THIS THREAD. It was moved without my concent.

Too Bad.

If you don't like moderator decisions, then take it up with them, privately or in the proper forum section as per the rules of this board.

[R.A.F.]

By the way, my wife once met Mr. Asimov (in 1968)

Not to get nitpicky...ok I will...That would be DR. Asimov...

Both of us were admirers of his science books for the layman and his fiction. However, she was very disappointed in meeting him only to find discover that he was an arrogant Language.

Were you "aiming" at relevancy with this post?...you "missed" by a mile.

[pzkpfw]

TooMany, you have made assertions contrary to the mainstream science view. There is essentially one mainstream, and it's support by the current science. It is not up to anyone to explain the mainstream (to someone asserting it's wrong). Otherwise the same old ground would be covered again and again.

That's why your "question" was moved to the ATM forum and is why the burdon is on you to support your claim, not for the prevailing scientific view to be supported for your benefit.

[John Mendenhall]

In the case of big bang nucleosynthesis, the problem is that large amounts of ordinary matter means a denser early universe, more efficient conversion of matter to helium-4 and less unburned deuterium that can remain. If one assumes that all of the dark matter in the universe consists of baryons, then there is far too much deuterium in the universe. This could be resolved if there were some means of generating deuterium, but large efforts in the 1970s failed to come up with plausible mechanisms for this to occur. For instance, MACHOs, which include, for example, brown dwarfs (balls of hydrogen and helium with masses < 0.08M_\odot), never begin nuclear fusion of hydrogen, but they do burn deuterium. Other possibilities that were examined include "Jupiters", which are similar to brown dwarf but have masses \sim 0.001M_\odot and do not burn anything, and white dwarfs.

From Wiki, Baryonic Dark Matter

This is what your idea that not all the baryonic matter is accounted for runs into. Under the ATM rules, you would need to explain why the early universe nucleosynthesis is incorrect, and what needs to be done to correct it. As I said before, I don't like the DM idea, but it is the best 'place holder' currently available. The mainstream all fits together just as a glove fits your hand just right. Astronomers have got to be about the pickiest most skeptical group ever, with the possible exception of paleontologists. If in general they accept (and note that I do not say 'believe in') dark matter, then you can bet that they have peeked and poked and prodded at every aspect of it.

(sigh) Try the relevant Wiki articles. I recommend Missing Mass.

[Swift]

Too Bad.

If you don't like moderator decisions, then take it up with them, privately or in the proper forum section as per the rules of this board.

R.A.F.

Stop trying to play moderator. I know you know how to use the Report function. Frankly, posts like this, or several of your others, are not helpful to the discussion.

[TooMany]

From Wiki, Baryonic Dark Matter

This is what your idea that not all the baryonic matter is accounted for runs into. Under the ATM rules, you would need to explain why the early universe nucleosynthesis is incorrect, and what needs to be done to correct it. As I said before, I don't like the DM idea, but it is the best 'place holder' currently available. The mainstream all fits together just as a glove fits your hand just right. Astronomers have got to be about the pickiest most skeptical group ever, with the possible exception of paleontologists. If in general they accept (and note that I do not say 'believe in') dark matter, then you can bet that they have peeked and poked and prodded at every aspect of it.

(sigh) Try the relevant Wiki articles. I recommend Missing Mass.

Fine. I understand that in the BB theory the nucleosynthesis calculations must be consistent with observed primordial isotopic ratios and also consistent with the interpretation of the CMB. The result of these calculations demand that baryonic matter cannot constitute more than about 1/5 of the total mass. The additional 4/5 (DM) must exist to explain gravitational observations (rotation curves, velocity dispersions, lensing). Also, there would be problems with galaxy formation without this additional mass to aid in the collapse of gas. However, (and now I'm little less certain) it is proposed that this DM cannot interact in any significant way except gravitationally. In part, this requirement may be necessary so that the DM has little or no effect on the nucleosythesis calculations (not sure). In addition, because it is not seen, it must not interact significantly with baryonic matter. Hence it is deemed to be neutral in charge and collisionless. Further, in order to meet some requirements for galaxy formation and large structure formation, it must be cold. Otherwise it would not be able to concentrate sufficiently to be effective in formation.

So in BB theory, there are compelling reason to believe that there is no additional baryonic matter and the rest is CDM with the stated properties.

Whether everything truly fits like a glove is debatable and is being debated right now. Several issues exist with respect to real galaxies that seem inconsistent with CDM (core/cusp, disk/halo conspiracy, dwarf galaxy shortage, insufficient mergers for bottom up formation, large scale structure, lithium problem etc). Since DM has never been directly detected, we do not actually know what its properties are; rather the properties of DM have been carefully chosen to fit the theory and observations, so in that sense it's not surprising that some things fit very well.

I'm saying all this so that you understand I am aware of the reasons for hypothesizing CDM. However, all of this does not answer the question asked in the first post. "Do we really know [from observation] how much baryonic matter galaxies contain"? I have reasons to doubt that we do. For example some of the theoretically required baryonic matter is still "missing" (undetected). Also (for the millionth time) cold molecular hydrogen and sub-stellar objects are difficult, if not impossible, to detect in the outer parts of galaxies.

My question is whether and how we have established that it is actually not there. I cannot prove that much additional baryonic matter is there and I'm not trying to prove that. If it were there however, BB theory would have serious problems; on the other hand if it is there, several problems concerning galaxies would be solved. So it is very important (to BB theory) to truly know and not just assume by discounting the difficulty of detection.

[Reality Check]

However, (and now I'm little less certain) it is proposed that this DM cannot interact in any significant way except gravitationally.

That is an observation, not a proposal or requirement., e.g. in the Bullet Cluster.

For example some of the theoretically required baryonic matter is still "missing" (undetected).

TooMany: Citation for this claim please (remener that this is the ATM forum)

Also (for the millionth time) cold molecular hydrogen and sub-stellar objects are difficult, if not impossible, to detect in the outer parts of galaxies.

Measurement dfficulty (for the first time!) does not mean that cold molecular hydrogen and sub-stellar objects have not been detected.

MACHOs ("sub-stellar objects") have been ruled out as a significant part of DM.
If you mean rocks or planets or planetoids then there are problems:
JREF Post by ben m on 13th November 2009

I went ahead and did a calculation: how collisionless would "rocky" dark matter be?
...
That calculation is done for the Earth's "local" dark matter: isotropic 220 km/s orbits through a 0.3 GeV/cm^3 mean density. I gave it 5g/cm^3 density, somewhere between stone and iron.

Look at those numbers. If you built the Milky Way using Volkswagen-sized rocks as the dark matter, they'd last four thousand years between collisions; they'd be dust and plasma. Use 500 m asteroids, they'd last a million years before colliding and pulverizing. (Remember, these are 220 km/s collisions; they make Shoemaker-Levy look wimpy.) A 10^6 m planetoid could last for a gigayear---at least that survives a full Galactic orbit!---but at that point we're into the stuff that the EROS surveys have ruled out. Sub-meter-scale dust, of course, is not collisionless at all which is why it's never been even in the ballpark of viable dark matter candidates.

ben m is a particle physicist so he knows a lot about collisions. I suspect that an astronomer might have even more reasons why rocks are not a candidate for DM (maybe they would be so numerous that Earth would be destroyed!).

[John Mendenhall]

See the ben m quote above. So much for lots of rocks. Again, these guys are thorough. I'm willing to bet that they (astronomers et al.) have covered all the bases in being satisfied with the current estimate of baryonic matter. And I only bet on sure things.

[Nereid]

I'm well aware of the usual stoning procedure. I DID NOT POST THIS QUESTION TO THIS THREAD. It was moved without my concent. It was a question perfectly relevant to the thread called "Galactic Rotation Curves... no need for dark matter", not a claim. Do you understand the difference?



I'm really not interested in your questions (which you already know the answers to anyway).

Are my questions not clear? Are they not directly relevant and pertinent to the ATM claims you have made in this thread?

I would be only too happy to clarify any such.

However, if they are directly pertinent, and clear, then I look forward to your timely responses.

What I'm interested in is getting my question answered which is why I posed it. Please read the title of my first post which was moved to this forum. It's full of questions, is it not?

Nereid. I just asked you not to post unless you have something specific to say about the question I posted that was placed here in this forum. But you babble on without the slightest contribution of relevant information. What's the point?

As I said, this thread is in the ATM section of BAUT. And as such is subject to somewhat different rules than apply in other parts of BAUT. Specifically, the asking of questions - direct questions, questions pertinent to the ATM claims presented, as presented - is strongly encouraged by those rules.

I look forward to your timely responses to my direct, pertinent questions.

[Nereid]

Whether everything truly fits like a glove is debatable and is being debated right now. Several issues exist with respect to real galaxies that seem inconsistent with CDM ([...], insufficient mergers for bottom up formation, large scale structure, lithium problem etc).

Please explain how these three are "inconsistent with CDM".

Also (for the millionth time) cold molecular hydrogen and sub-stellar objects are difficult, if not impossible, to detect in the outer parts of galaxies.

What would be the observational signature of "cold molecular hydrogen and sub-stellar objects ... in the outer parts of galaxies", with a combined mass and distribution consistent with other observations, in the ionisation cone and/or jet from an AGN?

Specifically, how would such distributions of baryonic matter affect the appearance of DRAGNs (double-lobed radio AGNs)?

My question is whether and how we have established that it is actually not there. I cannot prove that much additional baryonic matter is there and I'm not trying to prove that.

I'm confused. What is the ATM claim you are making (and defending) in this thread?

From the OP, it seems to me that it is exactly that: that there is strong observational evidence in favour of the idea that the bulk of the DM inferred to exist in galaxies (and clusters of galaxies) is in the form of hithertofore undetected baryonic matter.

[Nereid]

Also (for the millionth time) cold molecular hydrogen and sub-stellar objects are difficult, if not impossible, to detect in the outer parts of galaxies.

Measurement dfficulty (for the first time!) does not mean that cold molecular hydrogen and sub-stellar objects have not been detected.

MACHOs ("sub-stellar objects") have been ruled out as a significant part of DM.
If you mean rocks or planets or planetoids then there are problems:
JREF Post by ben m on 13th November 2009

I went ahead and did a calculation: how collisionless would "rocky" dark matter be?
...
That calculation is done for the Earth's "local" dark matter: isotropic 220 km/s orbits through a 0.3 GeV/cm^3 mean density. I gave it 5g/cm^3 density, somewhere between stone and iron.

Look at those numbers. If you built the Milky Way using Volkswagen-sized rocks as the dark matter, they'd last four thousand years between collisions; they'd be dust and plasma. Use 500 m asteroids, they'd last a million years before colliding and pulverizing. (Remember, these are 220 km/s collisions; they make Shoemaker-Levy look wimpy.) A 10^6 m planetoid could last for a gigayear---at least that survives a full Galactic orbit!---but at that point we're into the stuff that the EROS surveys have ruled out. Sub-meter-scale dust, of course, is not collisionless at all which is why it's never been even in the ballpark of viable dark matter candidates.

ben m is a particle physicist so he knows a lot about collisions. I suspect that an astronomer might have even more reasons why rocks are not a candidate for DM (maybe they would be so numerous that Earth would be destroyed!).

TooMany, what calculations have you done, to show that a population of "sub-stellar objects", with a mass sufficient to account for a significant fraction of the estimated DM, in the outer disks/halos of spiral galaxies like the MW and M31, would have a velocity-space structure that would produce collision debris consistent with observations (e.g. very few collisions, or collisions which produce 'invisible' debris)?

Also, please show that there is at least one, realistic, distribution of cold molecular hydrogen, with a mass sufficient to account for a significant fraction of the estimated DM, in the outer disks/halos of spiral galaxies like the MW and M31, consistent with observables such as Lyman-alpha emission and absorption in sight lines to distant UV sources.

[TooMany]

From the OP, it seems to me that it is exactly that: that there is strong observational evidence in favour of the idea that the bulk of the DM inferred to exist in galaxies (and clusters of galaxies) is in the form of hithertofore undetected baryonic matter.

Because you are so adverse to any discussion that potentially does not support BBT, you attempt to turn my question into a claim instead of just providing some answers. Reality Check has at least contributed some links to support the case that there is no additional baryonic matter. I will read them soon, but I have already addressed the MACHO survey issue.

I think the Davies paper shows at least one way in which the necessary baryonic could be there but have escaped detection. If you disagree, show how the baryonic matter has be eliminated by observation. Site some papers. That's what I'm fishing for: convincing evidence that it is not there.

[TooMany]

If you mean rocks or planets or planetoids then there are problems:
JREF Post by ben m on 13th November 2009

ben m is a particle physicist so he knows a lot about collisions. I suspect that an astronomer might have even more reasons why rocks are not a candidate for DM (maybe they would be so numerous that Earth would be destroyed!).

Can you site an actual paper on this subject? Ben may be a great guy but he does not explain his assumptions.

Objects orbiting the galactic center are not in random motion with an average velocity of 200 km/s. What would be the correct velocity dispersion in deriving a probability of collision? If the objects are in fact collisional, then over time the collisions will decrease the velocity dispersion and the probably of collision will grow smaller and smaller. I'm assuming here that if such objects can form then they can eventually reform from the collision debris. An analogy is the asteroid belt in the solar system. Such a process of collision and reformation has assumed to have happened early in the history of the solar system. The asteroid belt has survived in spite of collisions for 4.5 billion years. The largest objects are way, way bigger than Volkswagens.

Even though dust particles are the most likely to collide (because of there numeric density), they still exist in the ISM because they continually reform.

Referring to the objects containing additional baryons as a bunch of "rocks" is pejorative intended to dismiss the idea. The objects would be condensations of dust and hydrogen rather like the outer planets of the solar system or perhaps much smaller. They would not have a high metallic content.

[Swift]

Because you are so adverse to any discussion that potentially does not support BBT, you attempt to turn my question into a claim instead of just providing some answers. Reality Check has at least contributed some links to support the case that there is no additional baryonic matter. I will read them soon, but I have already addressed the MACHO survey issue.

I think the Davies paper shows at least one way in which the necessary baryonic could be there but have escaped detection. If you disagree, show how the baryonic matter has be eliminated by observation. Site some papers. That's what I'm fishing for: convincing evidence that it is not there.

It has been explained to you why your thread was moved to ATM. You might not like that, but that's the way it is. Under ATM rules, it is your requirement to answer questions and to show evidence. There is no obligation on anyone else to prove the mainstream or answer your questions; you may ask, but you can not demand it.

You have two choices: follow these rules, or ask this thread to be closed. If you ask it to be closed, you may not bring up this topic again.

[Reality Check]

Can you site an actual paper on this subject? Ben may be a great guy but he does not explain his assumptions.

Read the post - ben m did explain his assumptions: "That calculation is done for the Earth's "local" dark matter: isotropic 220 km/s orbits through a 0.3 GeV/cm^3 mean density. I gave it 5g/cm^3 density, somewhere between stone and iron."

[Reality Check]

P.S. TooMany - you missed this

Originally Posted by TooMany
For example some of the theoretically required baryonic matter is still "missing" (undetected).

TooMany: Citation for this claim please (remener that this is the ATM forum)

[Reality Check]

I would point out that mainstream astronomers do not investigate the idea that galaxy velocity curves can be explained using "rocks" (your "sub-stellar objects") or MACHOs or gas. THey must have reasons for this (the non-observance of enough MACHOs is one!)
The gas idea has been brought up by you. You need to support it with evidence.

TooMany, Question 2: Can you cite the published papers that have found enough gas, e.g. H2, to explain galaxy rotation curves?

The "rocks" invalidation is possibly so simple that astromoners do not bother writing papers about it, e.g.

• The collision rate from back of the envelope calculations as ben m did.
• No observation of enough of these rocks existing in or passing through the Solar System.
• No mechanism to create these rocks. We need 5-10 trimes the mass of all of the stars in the galaxy in rocks. But the Solar System and other solar systems (so far!) only have a small % of their mass as dust, rocks, planetoids or planets. The Sun has 99.8% of the total mass of the Solar System.

Dust could be also ruled out because it is easy to detect AFAIK.

ETA:
The answer to the question in the OP title ("Do we really know how much baryonic matter galaxies contain? ") is yes we really do.
Astonomers can measure the usual suspects of stars, dust and gas quite easily.
Astronomers can measure molecular H2, e.g. Warm Molecular Gas in M51: Mapping the Excitation Temperature and Mass of H2 with the Spitzer Infrared Spectrograph (2008). This is a direct measurement rather then the indirect measurement of CO and then extrapolating to H2.
Astronomers can put upper limits on MACHOs (about 2% of the required mass to explain galaxy rotation curves).
There are no mainstream mechanisms to create enough "rocks" or "sub-stellar objects" to change the baryonic matter by much (~1% of the total stellar mass and !0.1% of the required mass to explain galaxy rotation curves).

[TooMany]

Read the post - ben m did explain his assumptions: "That calculation is done for the Earth's "local" dark matter: isotropic 220 km/s orbits through a 0.3 GeV/cm^3 mean density. I gave it 5g/cm^3 density, somewhere between stone and iron."

What does ben m mean by "isotropic 220 km/s orbits" through a 0.3 GeV/cm^3 mean density? Can you interpret that for me? Isotropic means "identical in all directions". Surely he doesn't suppose that the objects would be moving uniformly in all directions at this speed? This speed is the orbital speed of the sun around the galactic center. How does that relate this to relative speeds of potentially colliding objects? Another question is why does he define a density in terms of energy? Am I supposed to divide by c^2 to get the mass density he is assuming? Why did he pick that density and what does he mean by orbits through that density?

To summarize, does he mean to assume that objects have space-averaged density of 0.3 GeV/cm^3/c^2 and are moving in random directions at 220 km/s? Such motion makes no sense. Why would one assume that?

Asteroids orbit the sun at about 23 km/s, yet they rarely collide because they move in nearly circular orbits each orbit having uniform velocity. The same idea applies to galactic orbits. What you would really need to know is what is the velocity dispersion of such objects, not their orbital velocities. Even assuming that initially such objects had large relative velocities uniformly in all directions, what would happen? Well they would collide with one another (in a highly non-elastic manner) thus averaging the velocity of the debris. Through gravitation the debris would reform into objects which would also collide if some relative velocity still exists. Over billions of years, the relative velocities would approach zero and collisions would very rare, but the tendency to reform would remain unchanged.

If it were not for this process, there would be no planets, no asteroid belt, no Kuiper belt in the solar system after 4.5 billion years. There would just be dust and gas or perhaps nothing except the sun would remain if the angular momentum of the dust and gas could be transfered.

"I gave it [the objects] 5g/cm^3 density"? That doesn't make any sense for condensed hydrogen and dust, does it? I don't think anybody believes that significant amounts of baryonic matter exist with such densities excepting perhaps dust particles and solar system type rocky/metallic objects. Of course assuming such high object densities reduces the probability of collisions for a given space density.

[TooMany]

I would point out that mainstream astronomers do not investigate the idea that galaxy velocity curves can be explained using "rocks" (your "sub-stellar objects") or MACHOs or gas. THey must have reasons for this (the non-observance of enough MACHOs is one!)

There are a lot of issues with MACHO studies. Please see post #33 where I already discussed this issue. I am not expecting to find the bulk of the MACHOs in the halo and that's all that's been ruled out as a sufficient source of DM (albeit at Jupiter mass and above). Interestingly, microlensing toward the galactic center has turned up an "unexpected" population of Jupiter sized objects not associated with stars. The population detected as yet is far too small to be a significant contributor to DM. However, the detections are right at the bottom of the detection threshold so it's not clear how many such objects exist or whether there is an even larger population of smaller objects toward the center of the galaxy. Note also that we don't need a great deal of DM to explain the rotation curve out to the orbit of the sun.

Halo estimates vary widely. While one group (in 2000) thought the halo might contain 20% of DM in the form of MACHOs, a more recent paper suggests 8% of DM.

The gas idea has been brought up by you. You need to support it with evidence.

Davies paper on baryonic dark matter in the disk suggests that the uniformity of max perceived column density of H I seen in external galaxies is a sign of saturation, i.e. opaqueness. Thus the true amount of H I may be much more than what it appears. Moreover, atomic hydrogen (H I emitting) can shield H2 from dissociating radiation by absorbing it and re-emitting. Thus it provides a mechanism through with H2 can survive (particularly in the outer parts of galaxies) such that some concentration can develop. H2 is probably the hardest plentiful gas to detect in the universe. Rarely is it possible to directly measure H2. Direct measurement of warm H2 (~ 100 K) is possible and such warm H2 has been detected. A 1999 paper on a couple of nearby edge-on nearby galaxies concluded that there may be enough warm H2 to account for all of the dark matter. A more recent paper (studying different galaxies with different instruments) concluded that there was some warm H2, but not nearly enough to account for DM. I wonder if anyone has tried to reproduce the 1999 detection with better instruments?

But the big problem is this: how do we know much cold (e.g. 10 K or lower) H2 is present, particularly in the outer parts of our galaxy and other galaxies?

So all I'm saying is that I don't see how the possibility has been eliminated.

Question 2: Can you cite the published papers that have found enough gas, e.g. H2, to explain galaxy rotation curves?

The only one that I can recall that claims to have directly detected sufficient H2 is the 1999 paper. Here is a link to the original paper.

I already posted this information in post #6. This is not police headquarters where you get to ask the same questions over and over again. If you want to reply, please read the previous posts before asking me redundant questions.

BTW, how's direct detection of CDM going?

The "rocks" invalidation is possibly so simple that astromoners do not bother writing papers about it, e.g.

Well as we know, "rocks" is a silly idea. If they can't be bothered to prove their point, maybe they haven't got one.

• The collision rate from back of the envelope calculations as ben m did.
• No observation of enough of these rocks existing in or passing through the Solar System.
• No mechanism to create these rocks. We need 5-10 trimes the mass of all of the stars in the galaxy in rocks. But the Solar System and other solar systems (so far!) only have a small % of their mass as dust, rocks, planetoids or planets. The Sun has 99.8% of the total mass of the Solar System.

Dust could be also ruled out because it is easy to detect AFAIK.

ben m's calculation is based on an absurd assumption, as already explained.
Why would they have to pass through the solar system? After all we are talking mainly about the outer disk here. If they did pass by and they were small, would we even detect them? Maybe some do pass through as comets.
Right, there is no mechanism to create "rocks" because there are not enough metals. However, it is possible for H2 to condense on dust particles and it is possible for such particles to merge and gradually form larger objects. If that were not so, how do you explain the ubiquitous gas planets like Jupiter that are roughly 90% hydrogen and 10% helium?

ETA:
The answer to the question in the OP title ("Do we really know how much baryonic matter galaxies contain? ") is yes we really do.
Prove that you can accurately detect Neptune size or smaller gas planets in the outer galaxy and show me the studies. Prove that you can accurately detect nearly pure cold H2 in the outer parts of galaxies and show me the studies. Then I will believe you.

Astonomers can measure the usual suspects of stars, dust and gas quite easily.
So? But they cannot detect planetoids nor any sub-stellar objects in the outer disk. They cannot detect cold H2 in the outer disk.

Astronomers can measure molecular H2, e.g. Warm Molecular Gas in M51: Mapping the Excitation Temperature and Mass of H2 with the Spitzer Infrared Spectrograph (2008). This is a direct measurement rather then the indirect measurement of CO and then extrapolating to H2.
That's right as AFAIK. The keyword here is warm as already mentioned.

Astronomers can put upper limits on MACHOs (about 2% of the required mass to explain galaxy rotation curves).
One last time and I'm done with this question. MACHO studies use the MCs and galactic center as backgrounds so that can only detect halo and central objects (down to about Jupiter size). Also 2% sounds like a lowball. Here's a 2006 paper that says 8% in the halo.

There are no mainstream mechanisms to create enough "rocks" or "sub-stellar objects" to change the baryonic matter by much (~1% of the total stellar mass and !0.1% of the required mass to explain galaxy rotation curves).

If the mainstream can find no such mechanism, I suggest they need to give it some more thought. When it comes to CDM they are pretty creative.

[TooMany]

P.S. TooMany - you missed this

Originally Posted by TooMany
For example some of the theoretically required baryonic matter is still "missing" (undetected).

TooMany: Citation for this claim please (remener that this is the ATM forum)

OK, here are some papers that focus on the subject:

http://arxiv.org/abs/1204.3377
http://arxiv.org/abs/1112.2706
http://arxiv.org/abs/1108.0037
http://arxiv.org/abs/1102.0201
http://arxiv.org/abs/1011.2530
http://arxiv.org/abs/1007.1980
http://arxiv.org/abs/1005.0923
http://arxiv.org/abs/1003.3273
http://arxiv.org/abs/1002.0844
http://arxiv.org/abs/0912.3943
http://arxiv.org/abs/0911.2700
http://arxiv.org/abs/0907.3831
http://arxiv.org/abs/0906.4993
http://arxiv.org/abs/0902.4695
http://arxiv.org/abs/0811.2643
http://arxiv.org/abs/0809.2991
http://arxiv.org/abs/0801.3606
http://arxiv.org/abs/0708.1761
http://arxiv.org/abs/0707.3991
http://arxiv.org/abs/0707.3795
http://arxiv.org/abs/0705.1356 (recommend wrt to the possibility of much additional baryonic dark matter)

They are very interesting; give them a read. Pick your own favorites.

BTW, another interesting item in today's news is this one: Multiple Mergers Generate Ultraluminous Infrared Galaxy

These apparently merging galaxies, designated Arp 220 and thought to be "powered mostly by a large number of massive stars, are comparable to the high luminosity of quasars." This raises a question: If these relatively nearby galaxies contain only 2% hydrogen by mass (as the MW is supposed to), is that enough hydrogen to explain such a furious star burst?

[Amber Robot]

Moreover, atomic hydrogen (H I emitting) can shield H2 from dissociating radiation by absorbing it and re-emitting. Thus it provides a mechanism through with H2 can survive (particularly in the outer parts of galaxies) such that some concentration can develop.

This doesn't happen. Dust can shield H2 and H2 can shield itself, but HI doesn't shield H2.

[TooMany]

This doesn't happen. Dust can shield H2 and H2 can shield itself, but HI doesn't shield H2.

H I is a spectral line (21 cm wavelength) used to detect atomic hydrogen so it actually cannot shield anything. What I meant is that H2 can be dissociated into atomic H by radiation, thus absorbing the radiation. If the conditions are right at some point this atomic H will become dense enough to recombine (most easily with the help of dust particles) and reform H2. So in this sense atomic H can shield H2 from complete dissociation. If you continually removed the atomic H, the H2 would eventually all dissociate in the presence of high energy photons (> 4.5eV). Perhaps this is what you meant by H2 can shield H2, but it does so by becoming H which forms a shield by reforming H2.

Atomic H can also absorbs at various UV wavelengths (in the Lyman series) which have higher energies than 4.5eV. I don't really know how significant that would be without doing a lot more research.

Edit: My apologies, you said HI which means atomic hydrogen which I incorrectly interpreted as H I, the 21 cm line. The rest stands.