Do we really know how much baryonic matter galaxies contain?

[TooMany]

It is often stated in these threads that we know that there is not enough baryonic matter to explain the rotation curves.

Do we really know that? The visible part of the Galaxy has many clouds of molecular hydrogen that we can see. Somehow these clouds form in a relatively high radiation environment and become internally cold and dense enough to form stars. Star formation has been going for at least several billion years at the current rate (in the Galaxy).

"Studies of [the Galaxy's] visible disk indicate that most of the mass is in the stars, with only about 2% gas (mostly hydrogen) and about 0.01% dust."

Thus more than 98% of the hydrogen now exists in stars (at least in the visible disk). Where does the hydrogen come from to continue forming stars? The ISM is very sparse.

When galaxies interact, outer parts of arms are sometimes stretched away into long strings that then form stars at very high rates. Doesn't this imply some stable reservoir of hydrogen that need only be "disturbed" to begin forming stars?

We can detect atomic hydrogen out to two or three times the visible radius of galaxies. If there were large quantities of molecular hydrogen at temperatures in the 10 K range shielded by an atmosphere of atom hydrogen, could we detect it? Have we really ruled that out?

If it's so easy to detect H2 and eliminate this possibility, why are estimates based on CO detected? Shouldn't we be skeptical of the arguments that use CO as a tracer? Here there is an assumption that the H2/CO ratio is constant. But what if the outer part of the galaxy is more primordial and contains much less CO. Maybe it's just beyond the ability of our current instruments/techniques to correctly measure the molecular hydrogen content.

If the molecular hydrogen has condensed into planet sized or smaller objects in the outer galaxy (as it has in the solar system), would we have detected it?

Back in 1999, a group of astronomers reported finding large amounts of molecular hydrogen gas when viewing a couple of edge-on spirals. They were only able to detect it directly because it was "warm", IIRC more than 100 K (or higher I don't remember the exact figure). I never saw much follow up on this discovery.

The search for non-baryonic matter is massive, involving the most expensive scientific apparatus ever build and perhaps thousands of people. But I hear very little about the search for large amounts of baryonic matter in the galaxy. There seems to be little interest in that possibility. One cannot help but wonder whether there is little interest simply because it is not "expected" to exist. Scientist generally don't go looking directly for the unexpected. It is usually when they go looking for the expected and get a surprise that science advances.

[Tensor]

It is often stated in these threads that we know that there is not enough baryonic matter to explain the rotation curves.

Strawman. It is often stated that we know from observations that there isn't enough baryonic matter to explain rotation curves. That is a lot different from claiming we know there is not enough baryonic matter. How about you actually get the statements correct?

Do we really know that? The visible part of the Galaxy has many clouds of molecular hydrogen that we can see.

Well, if we can see them, they are part of the calculation for the amount of baryonic matter.

Somehow these clouds form in a relatively high radiation environment and become internally cold and dense enough to form stars. Star formation has been going for at least several billion years at the current rate (in the Galaxy).

And, so?

"Studies of [the Galaxy's] visible disk indicate that most of the mass is in the stars, with only about 2% gas (mostly hydrogen) and about 0.01% dust."

Thus more than 98% of the hydrogen now exists in stars (at least in the visible disk). Where does the hydrogen come from to continue forming stars? The ISM is very sparse.

Are you saying all of the ISM is very sparse? What support do you have for that statement?

When galaxies interact, outer parts of arms are sometimes stretched away into long strings that then form stars at very high rates. Doesn't this imply some stable reservoir of hydrogen that need only be "disturbed" to begin forming stars?

I don't know, does it? What type of disturbance would start the formation? How much hydrogen is needed? How does metallicity affect the amount needed? How much of that is in gas that would have been observed prior to the galactic interactions?

We can detect atomic hydrogen out to two or three times the visible radius of galaxies. If there were large quantities of molecular hydrogen at temperatures in the 10 K range shielded by an atmosphere of atom hydrogen, could we detect it? Have we really ruled that out?

What type of column density are you talking about for these so called large quantities? If there was so much hydrogen molecular gas, how well would we be able to see objects through this "shielded molecular hydrogen"? After all, you're proposing it, you should be able to tell us the column densities from the amount you think exists, right?

If it's so easy to detect H2 and eliminate this possibility, why are estimates based on CO detected?

You don't know this, AND YOU'RE CRITICIZING THE TECHNIQUE? Don't you think you might want to actually study and understand the why's of this?

Shouldn't we be skeptical of the arguments that use CO as a tracer?

By all means, show us where in the papers you think they get it so wrong that we show be skeptical. You're the one that says we should be skeptical, show us why. Here, I'll give you a hint why you won't: Dust extinction, γ-ray emission, and thermal dust emission.

Here there is an assumption that the H2/CO ratio is constant. But what if the outer part of the galaxy is more primordial and contains much less CO.

And your observations and support for this is, what? Or are you just throwing out anything you can, without support, to save the idea of much larger amounts of baryonic matter. This is actually almost laughable as CO/H₂ ratios can vary widely in molecular clouds and it is not the ratio that is used. Again, hint: Velocity integration. Also, another hint: Different amounts for starburst galaxies.

Maybe it's just beyond the ability of our current instruments/techniques to correctly measure the molecular hydrogen content.

And maybe it isn't. Maybe it just that you don't understand how CO is used as a tracer to determine column density in H₂ gas. How about you provide why you think, specifically, the use of CO is in error? Instead of just throwing out as many non-supported ideas as you can. Apparently hoping something will stick.

If the molecular hydrogen has condensed into planet sized or smaller objects in the outer galaxy (as it has in the solar system), would we have detected it?

I don't know. Would we? What support do you have that shows we wouldn't?

Back in 1999, a group of astronomers reported finding large amounts of molecular hydrogen gas when viewing a couple of edge-on spirals. They were only able to detect it directly because it was "warm", IIRC more than 100 K (or higher I don't remember the exact figure).

You don't remember? Then how do you know it had to be "warm". Did the discovery have anything to do with the type of spiral? How about the location of the gas. How about the amount? You said large. How much, exactly, is large, according to that discovery?

I never saw much follow up on this discovery.

Well, where exactly have you searched? Who were the authors? What was the title of the paper? If you don't know those, it can be difficult to find out any follow-ups or cites on the paper

The search for non-baryonic matter is massive, involving the most expensive scientific apparatus ever build and perhaps thousands of people. But I hear very little about the search for large amounts of baryonic matter in the galaxy. There seems to be little interest in that possibility.

And you can show us that the reason for this is not because the observations indicate there is not any large amount of baryonic matter, but some other reason (besides a bunch of ifs, perhaps, maybes, etc), right?

Scientist generally don't go looking directly for the unexpected. It is usually when they go looking for the expected and get a surprise that science advances.

So, are you saying the current group of astronomers and astrophysicists are simply not doing their jobs? That they are ignoring possible avenues of exploration? That they simply ignored the possibilities you raised, because, after all, it wasn't obvious to them that those possibilities were there?

[tusenfem]

Posts moved to ATM with the associated rules.
TooMany you usually seem only to be here to criticize mainstream physics.
I moved this post and Tensor's answer/comments/questions to make you actually do some work apart from just talking away.

[Cougar]

Where does the hydrogen come from to continue forming stars? The ISM is very sparse.

Remarkably, galaxies are big into recycling. You'll want to read Cosmic Clouds: Birth, death, and recycling in the galaxy [1997] by James Kaler

[TooMany]

Posts moved to ATM with the associated rules.
TooMany you usually seem only to be here to criticize mainstream physics.
I moved this post and Tensor's answer/comments/questions to make you actually do some work apart from just talking away.

X

[TooMany]

Originally Posted by TooMany
It is often stated in these threads that we know that there is not enough baryonic matter to explain the rotation curves.
Strawman. It is often stated that we know from observations that there isn't enough baryonic matter to explain rotation curves. That is a lot different from claiming we know there is not enough baryonic matter. How about you actually get the statements correct?

Strawman. It is often stated that we know from observations that there isn't enough baryonic matter to explain rotation curves. That is a lot different from claiming we know there is not enough baryonic matter. How about you actually get the statements correct?

I'm only stating that the claim has been made. I am asking for the observations that eliminate the possibility with certainty.


Originally Posted by TooMany *
Somehow these clouds form in a relatively high radiation environment and become internally cold and dense enough to form stars. Star formation has been going for at least several billion years at the current rate (in the Galaxy).

Well, if we can see them, they are part of the calculation for the amount of baryonic matter.
And, so?

I mention the clouds simply to demonstrate that large concentrations of molecular hydrogen can and do occur in a relatively high radiation environment.
I suppose, although I'm not sure, that we detect these clouds because 1) they contain dust that obscures the background and radiates in the infrared and 2) they contain molecules like CO that are easily detected. I'm not sure how we evaluate how much H2 is present except perhaps if parts of these clouds are warm enough to directly detect H2 emissions? Perhaps we can detect H2 absorption to directly measure it? I am just asking.


Originally Posted by TooMany
"Studies of [the Galaxy's] visible disk indicate that most of the mass is in the stars, with only about 2% gas (mostly hydrogen) and about 0.01% dust."

Thus more than 98% of the hydrogen now exists in stars (at least in the visible disk). Where does the hydrogen come from to continue forming stars? The ISM is very sparse.

Are you saying all of the ISM is very sparse? What support do you have for that statement?

In the original thread, kzb stated that the hydrogen column from here to the nearest star amounted to the thickness of 1 micron at atmospheric pressure. (How do we know that when molecular hydrogen is so hard to detect?) No one contradicted kzb so I supposed it was true. Also we have the above claim that less that 2% of all the hydrogen is not already in stars. That implies some difficultly in gathering enough to form a new star, which is perhaps consistent with the low starformation rate of 1/yr in the visible disk. Yet how do these dense clouds form? These clouds disperse when stars form in them so there must be some mechanism that keeps forming them from a reservoir of hydrogen.


Originally Posted by TooMany *
When galaxies interact, outer parts of arms are sometimes stretched away into long strings that then form stars at very high rates. Doesn't this imply some stable reservoir of hydrogen that need only be "disturbed" to begin forming stars?

I don't know, does it? What type of disturbance would start the formation? How much hydrogen is needed? How does metallicity affect the amount needed? How much of that is in gas that would have been observed prior to the galactic interactions?

"Does it?" Of course it does. The hydrogen must have been there for a very long time without forming stars. These cases of rapid star formation in interacting galaxies produce many globular clusters which are quite dense in stars.
"What kind of disturbance?" The changes in tidal gradient alone may be a sufficient disturbance because there doesn't always appear to be a direct collision between the gaseous arms of the galaxies involved where the stars are forming.
"How much hydrogen?" In all probability, many times more than the total mass of the stars that form. The star formation process is self limiting since clouds cannot condense in the high radiation environments created.
"Metallicity effect?" AFAIK, metallicity is helpful in star formation because it allows the gas to cool through emission much faster than just molecular hydrogen can cool. Exactly quantitatively how much metallicity is needed versus hydrogen? I don't think enough is known about star formation to answer that question.
"How much prior to interaction?" I don't really know although I have read that it comes somewhat as a surprise that star formation can be so vigorous in these parts of galaxies. Do you know how much? Has the H2 been measured?


Originally Posted by TooMany *
We can detect atomic hydrogen out to two or three times the visible radius of galaxies. If there were large quantities of molecular hydrogen at temperatures in the 10 K range shielded by an atmosphere of atom hydrogen, could we detect it? Have we really ruled that out?

What type of column density are you talking about for these so called large quantities? If there was so much hydrogen molecular gas, how well would we be able to see objects through this "shielded molecular hydrogen"? After all, you're proposing it, you should be able to tell us the column densities from the amount you think exists, right?

Enough density to account for the outer mass of the galaxy. Molecular hydrogen is rather special in that the symmetry of the molecule cancels out any dipole moment and spin. Because of this, it is extremely transparent and hard to detect. The difficulty of detection is inversely proportional to temperature. So there are two points here. H2 could exist in some dynamic equilibrium if the outer layers of atomic hydrogen can shield it from radiation sufficiently to limit dissociation. How much can be protected in this way I cannot calculate. How much is required to shield it? I think Davies addressed this in his paper.

No, I cannot tell you the exact H2 density; my point is that we cannot detect the column densities of H2 directly. They are inferred by HI and CO emissions which are not necessarily an accurate representation.


Originally Posted by TooMany *
If it's so easy to detect H2 and eliminate this possibility, why are estimates based on CO detected?

You don't know this, AND YOU'RE CRITICIZING THE TECHNIQUE? Don't you think you might want to actually study and understand the why's of this?

Oh but I do know that CO is used as a proxy for H2. That information is in many, many papers. I have also read several papers that question the use of CO as a proxy outside the visible part of the galaxy. I've seen it suggested that estimates could be off by a factor of 10.


Originally Posted by TooMany
Shouldn't we be skeptical of the arguments that use CO as a tracer?

By all means, show us where in the papers you think they get it so wrong that we show be skeptical. You're the one that says we should be skeptical, show us why. Here, I'll give you a hint why you won't: Dust extinction, γ-ray emission, and thermal dust emission.

I'll have to do some digging to recover the papers that criticize the use of CO as a proxy, but they do exist.
Dust extinction and thermal dust emission cannot correctly estimate the H2 any more than CO. Why would we expect H2/dust to be constant if metallicity declines with radius?
I'm not sure what you are suggesting about gamma ray emission. Can you explain?


Originally Posted by TooMany
Here there is an assumption that the H2/CO ratio is constant. But what if the outer part of the galaxy is more primordial and contains much less CO?

And your observations and support for this is, what? Or are you just throwing out anything you can, without support, to save the idea of much larger amounts of baryonic matter. This is actually almost laughable as CO/H₂ ratios can vary widely in molecular clouds and it is not the ratio that is used. Again, hint: Velocity integration. Also, another hint: Different amounts for starburst galaxies.

So you are in strong agreement that CO is not a good tracer for H2? Then why is it used? What ratio is used instead? The outer parts of galaxies are generally thought to be more primordial than the inner and thus they should be lower in metalicity including CO. In fact, I think the rotation of outer parts of the galaxies is measured using H I emissions and not CO. That seems to imply there is not enough CO or it is not warm enough to make these observations.

Can you explain how "velocity integration" or "different amounts in starburst galaxies" are relevant?


Originally Posted by TooMany
Maybe it's just beyond the ability of our current instruments/techniques to correctly measure the molecular hydrogen content.

And maybe it isn't. Maybe it just that you don't understand how CO is used as a tracer to determine column density in H₂ gas. How about you provide why you think, specifically, the use of CO is in error? Instead of just throwing out as many non-supported ideas as you can. Apparently hoping something will stick.

I thought I explained that. I assume there have been some measurements in warm gas where H2 can be directly detected and compared with the quantity of CO. Then an assumption must be made about how the H2/CO ratio varies to determine how much H2 is about by detecting CO. What I'm saying is that especially in the outer parts, this ratio may be much larger because the CO content should be much smaller due to the primordial nature of the gas.


Originally Posted by TooMany
If the molecular hydrogen has condensed into planet sized or smaller objects in the outer galaxy (as it has in the solar system), would we have detected it?

I don't know. Would we? What support do you have that shows we wouldn't?

We have the Kuiper belt very close by in our solar system but we can only detect only the most massive objects. We are pretty sure that an Oort cloud exists within two light years as the reservoir of long period comets. We believe it contains a trillion objects but we cannot detect any of them unless they enter the inner solar system.


Originally Posted by TooMany
Back in 1999, a group of astronomers reported finding large amounts of molecular hydrogen gas when viewing a couple of edge-on spirals. They were only able to detect it directly because it was "warm", IIRC more than 100 K (or higher I don't remember the exact figure). I never saw much follow up on this discovery.

You don't remember? Then how do you know it had to be "warm". Did the discovery have anything to do with the type of spiral? How about the location of the gas. How about the amount? You said large. How much, exactly, is large, according to that discovery?

Here is a link to the original paper.

The observed line ratios indicate relatively warm (T = 150–230 K) molecular clouds scattered throughout the disk in addition to a massive cooler (T 5 80–90 K) component which dominates the signal in the outer regions. For H2 ortho/para ratios of 2–3, the cool gas has typical edge-on column densities of (1-3) x 10^23 cm^-2 (or »3000 Msolar pc^-2), in which case it outweighs the H i by a factor of 5–15. This factor matches well the mass required to resolve the problem of the missing matter of spiral galaxies within at least the optical disk.

Well, where exactly have you searched? Who were the authors? What was the title of the paper? If you don't know those, it can be difficult to find out any follow-ups or cites on the paper.

Apparently I have not searched enough. There seems to be at least one recent (2012) paper on the subject which confirms detection of warm molecular gas but denies it's significance saying it only amounts to only to a maximum of 60 M_solar pc^-2.

Therefore, warm molecular hydrogen cannot account for dark matter in these disk galaxies, contrary to what was implied by a previous ISO study of the nearby edge-on galaxy NGC 891.

I do need to read some of these more recent papers. However, 60 versus 3000 is different by a factor of fifty! What's with that I wonder. The second paper was studying some more distant edge-ons. There is also this point. I believe it is true that we are only able to detect H2 when it is warm, e.g. about 100 K. H2 at 10 K is 10 times as difficult to detect. So even if the warm H2 is insufficient, there may exist much colder H2 (e.g. 10 K or less) that we simply cannot detect.

[Nereid]

TooMany, that last post of yours seems to have some formatting problems; would you mind having a go at editing it, to make it more accurately reflect both what you (and Tensor) wrote (i.e. what you're quoting, and what's new), and what it is that you're trying to say?

[pzkpfw]

(Quotes can be nested:
[quote=Bob said]A=B[quote=Alice said]No it doesn't.[/quote][/quote]

A=B

No it doesn't.

If you see a tag in your post, e.g. [/quote], you've got things unbalanced.)

[TooMany]

TooMany, that last post of yours seems to have some formatting problems; would you mind having a go at editing it, to make it more accurately reflect both what you (and Tensor) wrote (i.e. what you're quoting, and what's new), and what it is that you're trying to say?

Sorry, I clicked the wrong button, long before I was done. I have updated the reply. I have to put the rest of the reply into another message.

[TooMany]

Here is the rest of my reply:

Originally Posted by TooMany
The search for non-baryonic matter is massive, involving the most expensive scientific apparatus ever build and perhaps thousands of people. But I hear very little about the search for large amounts of baryonic matter in the galaxy. There seems to be little interest in that possibility.

And you can show us that the reason for this is not because the observations indicate there is not any large amount of baryonic matter, but some other reason (besides a bunch of ifs, perhaps, maybes, etc), right?

I cannot say exactly what the reasons are; I can only guess. I don't think we would even be looking for non-baryonic dark matter to explain rotation curves if it were not for the fact that the BBT and the study of the CMB does not allow there to be enough baryonic matter. I would suggest that the belief that the BBT and the interpretation of the CMB are correct has caused astronomers to believe that the missing matter must be non-baryonic and that therefore most of the effort has gone into looking for some new kind of non-baryonic matter rather than hidden baryonic matter.

After all, I have been often told that most astronomers regard the BBT as correct.

So, are you saying the current group of astronomers and astrophysicists are simply not doing their jobs? That they are ignoring possible avenues of exploration? That they simply ignored the possibilities you raised, because, after all, it wasn't obvious to them that those possibilities were there?

Not exactly "not doing their jobs". They are doing what they perceive to be their job; They are trying to find the non-baryonic matter that their theory predicts. I'm just saying that they are investing most of their effort in finding what they expect to find rather looking for that which they do not expect to find, but might be there. Would LHC be looking so hard for the Higgs particle if they did not expect such a particle to exist? Would LHC have even been built?

Neither do I think they have "ignored those possibilities; I think they simply don't put much stock in the baryonic possibility. They seem to have long ago (at least by 1986) found reasons to exclude the possibility without substantial direct evidence one way or the other. I refer to 1986 because of a paper from that year that suggested that the missing mass cannot be baryonic and gave various reasons for this.

Beside, as I am trying to argue, it appears to be very difficult to eliminate the possibility of much more baryonic matter because it is so difficult to detect H2 gas and non-stellar objects. I'm not sure whether existing instruments can do it, but maybe they can.

[Cougar]

These clouds disperse when stars form in them so there must be some mechanism that keeps forming them from a reservoir of hydrogen.

As I mentioned, galaxies recycle a lot of their hydrogen ("a proper time-dependent treatment of the gas return from stars shows that recycling extends the gas lifetimes of disks by factors of 1.5-4 for typical disk parameters" - source).

Perhaps more importantly, 1 or 2% of a trillion solar masses is still a lot of solar masses.

[TooMany]

As I mentioned, galaxies recycle a lot of their hydrogen ("a proper time-dependent treatment of the gas return from stars shows that recycling extends the gas lifetimes of disks by factors of 1.5-4 for typical disk parameters" - source).

I think what you are getting at is that supernovas and other stellar events can convert stars or parts of stars back into gas and dust. No doubt. But I also usually hear that supernovas are thought to be an important cause of star formation, again implying some significant reservoir of hydrogen. If it is correct that supernovas contribute most of the mass that forms new stars, how can it also be that supernovas could still be causing formation of multiple stars. You cannot get back more than you give. An interesting question is what is the star formation rate and the average mass of stars formed versus the rate of supernovas and the mass that they contribute back to the ISM? (I'll read the paper you referenced.)

Perhaps more importantly, 1 or 2% of a trillion solar masses is still a lot of solar masses.

Indeed there is much hydrogen but how does it ever get concentrated enough to form a star? The overall density is at most 1/50 of what it once was, however the Galaxy has churned out at least 1 star per year for the last billion years so in that time 1% additional stars were created from about 3% remaining gas, leaving us 2%. That rate seems remarkably efficient for so little gas. So it seems we live in a special time at which our galaxy is just about to run out of gas completely.

[Cougar]

I think what you are getting at is that supernovas and other stellar events can convert stars or parts of stars back into gas and dust. No doubt. But I also usually hear that supernovas are thought to be an important cause of star formation, again implying some significant reservoir of hydrogen.

Well, reservoir is your word, which may not be merited. And it's not just supernovas. Stars are blowing off mass all the time. (That's another great Kaler book: Extreme Stars, At the Edge of Creation [2001].)

So it seems we live in a special time at which our galaxy is just about to run out of gas completely.

Did you want to try to quantify that? Yes, the Universe is evolving. The peak star formation rate has already passed. Fortunately, "running out of gas" does not mean running out of stars, some of which last a very long time.

[Hornblower]

Too Many,

The burden is on you to walk us through the research methods of numerous experts who have studied these topics in great detail, and to show us in appropriate technical detail where you think they have messed up, and to explain why you think they messed up. It is my understanding that they are confident that if there was enough gas and other non-luminous baryonic stuff out there to have a total mass of several times that of the stars, they would be able to detect it in ways that were not yet doable when Zwicky and others suggested the possibility of large amounts of dark matter several decades ago.

[pzkpfw]

Too Many,

The burden is on you ...

Yes.

[Shaula]

But I also usually hear that supernovas are thought to be an important cause of star formation, again implying some significant reservoir of hydrogen. If it is correct that supernovas contribute most of the mass that forms new stars, how can it also be that supernovas could still be causing formation of multiple stars.

Suggest you go read up on supernovae. It is not thought, at all, that supernovae contribute most of the mass that forms new stars. Not by anyone as far as I know. Their contribution to stellar formation is totally different.

[captain swoop]

A Supernova sends out a shockwave that can compress gas enough to cause a cloud to collapse and start star formation.

[Nereid]

Here is the rest of my reply:

Thanks.

Originally Posted by TooMany
The search for non-baryonic matter is massive, involving the most expensive scientific apparatus ever build and perhaps thousands of people. But I hear very little about the search for large amounts of baryonic matter in the galaxy. There seems to be little interest in that possibility.

And you can show us that the reason for this is not because the observations indicate there is not any large amount of baryonic matter, but some other reason (besides a bunch of ifs, perhaps, maybes, etc), right?

I cannot say exactly what the reasons are; I can only guess. I don't think we would even be looking for non-baryonic dark matter to explain rotation curves if it were not for the fact that the BBT and the study of the CMB does not allow there to be enough baryonic matter. I would suggest that the belief that the BBT and the interpretation of the CMB are correct has caused astronomers to believe that the missing matter must be non-baryonic and that therefore most of the effort has gone into looking for some new kind of non-baryonic matter rather than hidden baryonic matter.

After all, I have been often told that most astronomers regard the BBT as correct.

(bold added)

How relevant is this?

I ask primarily because, it seems to me, you have mis-read the relevant history.

However, if your speculation concerning motivations - both historical and contemporary - is essentially irrelevant to whatever main point you are trying to make, my question can be ignored.

So, are you saying the current group of astronomers and astrophysicists are simply not doing their jobs? That they are ignoring possible avenues of exploration? That they simply ignored the possibilities you raised, because, after all, it wasn't obvious to them that those possibilities were there?

Not exactly "not doing their jobs". They are doing what they perceive to be their job; They are trying to find the non-baryonic matter that their theory predicts. I'm just saying that they are investing most of their effort in finding what they expect to find rather looking for that which they do not expect to find, but might be there. Would LHC be looking so hard for the Higgs particle if they did not expect such a particle to exist? Would LHC have even been built?

Neither do I think they have "ignored those possibilities; I think they simply don't put much stock in the baryonic possibility. They seem to have long ago (at least by 1986) found reasons to exclude the possibility without substantial direct evidence one way or the other.

Again, is this relevant? If so, why?

I refer to 1986 because of a paper from that year that suggested that the missing mass cannot be baryonic and gave various reasons for this.

Would you please provide a reference to that paper? You may have done so before, but I think it would help if you provided the citation directly, here.

Beside, as I am trying to argue, it appears to be very difficult to eliminate the possibility of much more baryonic matter because it is so difficult to detect H2 gas and non-stellar objects. I'm not sure whether existing instruments can do it, but maybe they can.

(bold added)

There are at least three distinct scales at which DM shows up, cosmological (e.g. in the CMB angular power spectrum, acoustic baryon oscillations), cluster (e.g. galaxy motions, lensing, SZE), and galactic (e.g. spiral galaxy rotation curves, elliptical galaxy velocity dispersions, lensing).

Have you made quantitative estimates - even BotE (back of the envelope) ones - of how much extra baryonic matter there would need to be, to be consistent with mass estimates at all three scales?

[TooMany]

Well, reservoir is your word, which may not be merited. And it's not just supernovas. Stars are blowing off mass all the time. (That's another great Kaler book: Extreme Stars, At the Edge of Creation [2001].)

Did you want to try to quantify that? Yes, the Universe is evolving. The peak star formation rate has already passed. Fortunately, "running out of gas" does not mean running out of stars, some of which last a very long time.

Right I read the paper 1994 paper that you mentioned and now I understand that most of the recycled gas is thought to be from normal stars as they evolve and loose much of their mass. That can apparently explain how star formation can continue at the current rate which would deplete all of the gas in a few Gy. If that is the answer to the hydrogen supply OK. Let's leave the subject. I have heard mention of in-fall of extra-galactic gas as a source of hydrogen but perhaps that idea has been completely discarded in favor of recycling.

I don't think this subject is particularly relevant to the question at hand "Do we really know how much baryonic matter galaxies contain?" I'm sorry that I brought it up because its a distraction.

[TooMany]

Too Many,

The burden is on you to walk us through the research methods of numerous experts who have studied these topics in great detail, and to show us in appropriate technical detail where you think they have messed up, and to explain why you think they messed up. It is my understanding that they are confident that if there was enough gas and other non-luminous baryonic stuff out there to have a total mass of several times that of the stars, they would be able to detect it in ways that were not yet doable when Zwicky and others suggested the possibility of large amounts of dark matter several decades ago.

You are turning this upside down. I am not asserting that I can show that there is sufficient baryonic matter to explain galactic rotation curves. In another thread called "Galactic Rotation... no need for dark matter" I posted the first entry in this thread. A moderator decided that asking this question constituted a reason to move my post into the ATM forum. I am not asserting, I'm asking "Do we really know how much baryonic matter galaxies contain?". I am asking whether the possibility of the existence of sufficient baryonic matter has been completely excluded and, if so, how it has been excluded.

I have pointed out that molecular hydrogen and condensations of molecular hydrogen would be extremely difficult to detect. This makes me wonder if we have truly excluded the possibility.

[TooMany]

Suggest you go read up on supernovae. It is not thought, at all, that supernovae contribute most of the mass that forms new stars. Not by anyone as far as I know. Their contribution to stellar formation is totally different.

Thanks. I get that now. The biggest contribution comes from stars leaving the main sequence when they blow off much of their mass, correct?

[TooMany]

A Supernova sends out a shockwave that can compress gas enough to cause a cloud to collapse and start star formation.

Yes and there has to be enough hydrogen out there to compress for this to be an important cause of star formation.

[korjik]

You are turning this upside down. I am not asserting that I can show that there is sufficient baryonic matter to explain galactic rotation curves. In another thread called "Galactic Rotation... no need for dark matter" I posted the first entry in this thread. A moderator decided that asking this question constituted a reason to move my post into the ATM forum. I am not asserting, I'm asking "Do we really know how much baryonic matter galaxies contain?". I am asking whether the possibility of the existence of sufficient baryonic matter has been completely excluded and, if so, how it has been excluded.

I have pointed out that molecular hydrogen and condensations of molecular hydrogen would be extremely difficult to detect. This makes me wonder if we have truly excluded the possibility.

21cm radiation makes neutral hydrogen fairly easy to detect actually.

[John Mendenhall]

(edit)
There are at least three distinct scales at which DM shows up, cosmological (e.g. in the CMB angular power spectrum, acoustic baryon oscillations), cluster (e.g. galaxy motions, lensing, SZE), and galactic (e.g. spiral galaxy rotation curves, elliptical galaxy velocity dispersions, lensing).

Have you made quantitative estimates - even BotE (back of the envelope) ones - of how much extra baryonic matter there would need to be, to be consistent with mass estimates at all three scales?

Nereid has some very good points here. But first, let me say that I am no fan of the dark matter solution, and have said so on this forum. The result was having enough dark matter thrown at me to make bruises (a first detection?). Anyhow, DM is the best of many ideas, I must admit. The amount of baryonic matter needed is way too much to have not been detected, like 3 to 4 times what we see.

Good thread, regards, John M.

[TooMany]

I said:
I would suggest that the belief that the BBT and the interpretation of the CMB are correct has caused astronomers to believe that the missing matter must be non-baryonic and that therefore most of the effort has gone into looking for some new kind of non-baryonic matter rather than hidden baryonic matter.

How relevant is this?

I ask primarily because, it seems to me, you have mis-read the relevant history.

However, if your speculation concerning motivations - both historical and contemporary - is essentially irrelevant to whatever main point you are trying to make, my question can be ignored.

Again, is this relevant? If so, why?

Isn't that obvious? It appears that detecting molecular hydrogen is very difficult and generally indirect methods are used when the gas is very cold. Moreover, if the molecular hydrogen actually condenses into planet size or smaller objects, it is nearly impossible to detect. It seems that eliminating this possibility would be very difficult, perhaps almost as difficult as finding the non-baryonic dark matter particle. I am suggesting that since the overwhelming majority of astronomers firmly believe that the missing mass must be non-baryonic that relatively little effort is directed at finding baryonic matter in comparison with the effort being made to find the "DM particle".

Would you please provide a reference to that paper? You may have done so before, but I think it would help if you provided the citation directly, here.

I think this is the paper I remember reading. It seems to be frequently sited and may be representative of the reasons that astronomers discount the possibility of a much larger mass of baryons. Frankly I did not find their arguments particularly convincing. They were primarily simplistic theoretical arguments to explain how various forms of baryonic matter could not exist.

There are at least three distinct scales at which DM shows up, cosmological (e.g. in the CMB angular power spectrum, acoustic baryon oscillations), cluster (e.g. galaxy motions, lensing, SZE), and galactic (e.g. spiral galaxy rotation curves, elliptical galaxy velocity dispersions, lensing).

None of this excludes the possibility of sufficient baryons to explain galactic rotation, certainly not by direct observation.

Have you made quantitative estimates - even BotE (back of the envelope) ones - of how much extra baryonic matter there would need to be, to be consistent with mass estimates at all three scales?

No I haven't but some astronomers have. In elliptical galaxies the answer seems to be close to zero. In typical spiral galaxies, about a factor of 5-7 more than stars. In DM dominated dwarf or LSB galaxies a factor of 10 -100 times the stellar mass. If much more baryonic matter does exist in typical spiral galaxies, my guess is that it would be mainly in the disk beyond the part densly populated by stars, where we find H I emissions. These large differences in the DM content of various kinds of galaxy can be interpreted as evidence that DM is in fact baryonic. Thus it seems important to exclude this possibility through highly reliable observations.

[TooMany]

21cm radiation makes neutral hydrogen fairly easy to detect actually.

Yes, this is the H I line, but it is emitted by atomic hydrogen, not molecular. This line is used to measure the rotation curves of galaxies at large radii where there is insufficient starlight. It has also been used to estimate the additional H I mass which has been assumed to comprise most of the gas in the outer regions, whereas molecular hydrogen has been largely discounted. If only the inferred H I mass exists, it is not nearly enough to account for the rotation curves.

[TooMany]

The amount of baryonic matter needed is way too much to have not been detected, like 3 to 4 times what we see.

Good thread, regards, John M.

But the question remains, would we really see it if it were there? How has observation completely ruled this out? I don't understand because molecular hydrogen is measured indirectly and condensed hydrogen is practically impossible to detect.

[Amber Robot]

But the question remains, would we really see it if it were there? How has observation completely ruled this out? I don't understand because molecular hydrogen is measured indirectly and condensed hydrogen is practically impossible to detect.

Molecular hydrogen can be detected directly in the far-ultraviolet. See, for example, Gillmon et al. 2005 for a survey of high-latitude AGNs against which H2 is observed in absorption. These allow for a determination of H2 column density looking out of the MW disk.

[TooMany]

Molecular hydrogen can be detected directly in the far-ultraviolet. See, for example, Gillmon et al. 2005 for a survey of high-latitude AGNs against which H2 is observed in absorption. These allow for a determination of H2 column density looking out of the MW disk.

Do you mean this article?

We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE) survey of interstellar molecular hydrogen (H2) along 45 sight lines to AGNs at high Galactic latitudes ( > 20).

Haven't read all of it yet but it sounds like they are not measuring H2 in the disk, but rather at high latitudes through the halo. The object of the paper seems to be to find lines of sight relatively uncontaminated by Galactic H2 at high latitudes. These lines of sight can then be used for deep space studies. They infer that infrared measurements may be used to approximate the location of H2.

Moreover disk measurements seem to rely on OB stars as UV sources. I don't think we find many OB stars in the outskirts of the galaxy so I assume that the disk measurements referenced are the local H2 density in the disk.

[Amber Robot]

Haven't read all of it yet but it sounds like they are not measuring H2 in the disk, but rather at high latitudes through the halo. The object of the paper seems to be to find lines of sight relatively uncontaminated by Galactic H2 at high latitudes. These lines of sight can then be used for deep space studies. They infer that infrared measurements may be used to approximate the location of H2.

Yes, but even the "uncontaminated" sightlines have H2 in them and it is directly measured. Yes, this will give more accurate numbers for halo rather than disk, though there are several studies of H2 that had disk targets as well.

Moreover disk measurements seem to rely on OB stars as UV sources. I don't think we find many OB stars in the outskirts of the galaxy so I assume that the disk measurements referenced are the local H2 density in the disk.

The problem with probing too far away in the far-ultraviolet is extinction. So yes, a good fraction of the sightlines are limited to more 'local' parts of the MW. There have been some distant sightlines probed (a couple of kpc) but these are somewhat biased towards less dense material necessarily.

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?