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Thread: Galactic Rotation... no need for dark matter.

  1. #31
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    Quote Originally Posted by Nereid View Post
    What do you mean ("has never been rationally justified", "has not yet been justified rationally")?

    Specifically, what does the process of 'rational justification' entail?
    To put it bluntly: GR is non-local in nature, just like Newton's law.
    Nothing new, just another representation of the acceleration field (with a few artifacts, which are the inevitable consequence of the method itself, namely the full geometrization of gravity).

    The process of rational justification is a standard procedure.
    And in this case it consists of:
    localisation a sources of gravity (real, not only in the form of a name - quality), a mechanism for transferring energy - momentum, the discovery of various dependencies, circumstances, etc.
    Last edited by Hetman; 2012-Apr-27 at 09:19 PM.

  2. #32
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    Quote Originally Posted by Hetman View Post
    Quote Originally Posted by Nereid
    What do you mean ("has never been rationally justified", "has not yet been justified rationally")?

    Specifically, what does the process of 'rational justification' entail?
    To put it bluntly: GR is non-local in nature, just like Newton's law.
    Nothing new, just another representation of the acceleration field (with a few artifacts, which are the inevitable consequence of the method itself, namely the full geometrization of gravity).
    I have no idea what this means.

    Would you please explain, in some detail?

  3. #33
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    You seem to be arguing for a very old fashioned form of science - closer to natural philosophy than what we would regard as empirical science. Since the history of natural philosophy largely consists of people telling the universe how it should be and then getting upset when it doesn't obey them I'll stick with models based on evidence, thanks. GR works, it works astoundingly well. Is it the final word in gravity? Nope. But it is a good model.

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    Quote Originally Posted by Shaula View Post
    You seem to be arguing for a very old fashioned form of science - closer to natural philosophy than what we would regard as empirical science. Since the history of natural philosophy largely consists of people telling the universe how it should be and then getting upset when it doesn't obey them I'll stick with models based on evidence, thanks. GR works, it works astoundingly well. Is it the final word in gravity? Nope. But it is a good model.
    Whether GR works or not is irrelevant here.
    As long as there is no rational justification, so long will be questionable and unreliable.

    The consequences are obvious: soon we'll insert the green elephants among the stars.

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    Quote Originally Posted by kzb View Post
    The papers I posted are just one piece of the puzzle. They purport to show one important factor, and that is, in the outer galaxy, the efficiency of turning matter into stars appears to be the same as it is further in. There is apparently no gradient in star forming efficiency with galactic radius.

    Remember it is the ratio of light to mass which we are talking about, not the actual density of stars, which does decline with radius of course.

    This is despite the gradients that do exist, matter density, supernova density and metallicity.
    Can you explain a little about how "star forming efficiency" is defined and measured?

    What I concluded from the paper was different and was just that in the outer parts of the galaxy there do exist regions in which stars are formed much like in the inner parts. But in all likelihood the conditions are quite similar in those occasional regions. I don't see how that demonstrates a constant M/L ratio.

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    [QUOTE=Nereid;2012114]Deep Surface Brightness Profiles of Spiral Galaxies from SDSS Stripe82: Touching Stellar Halos (arXiv:1204.3082) is a recent paper that may be quite relevant to this.

    Interesting. The analysis is way over my head but there are some tantalizing things in there. There are varieties of halo distributions (visible halos I mean). There is the interesting detection of very red halos which may contradict Universal IMF at the small end of the scale.

    There is also a very interesting comparison of images formed with different surface brightness limits. The difference this makes it quite striking.

    I think it's fair to say, when we look closer, we often find more than what we expected. Put another way, the things that we cannot see don't seem to exist.

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    Quote Originally Posted by Hetman View Post
    Law of physics must be derived from the basic principles of symmetry or other axiomatic systems.
    Principle: gravitational energy cannot be destroyed or created (technically, the divergence of the gravitational field outside the source is zero)
    Result: Newton's law of gravity

    Good enough for a basic principle approach?

  8. #38
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    Quote Originally Posted by caveman1917 View Post
    Principle: gravitational energy cannot be destroyed or created (technically, the divergence of the gravitational field outside the source is zero)
    Result: Newton's law of gravity

    Good enough for a basic principle approach?
    Rather speculation.
    What constitutes the gravitational energy?
    Nothing is also preserved.

    Just note that solar cycles are strongly correlated with orbital periods of planets.

  9. #39
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    Rather speculation.
    Can you give an example of a theory that meets your criteria for being rationally derived/justified?

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    Quote Originally Posted by TooMany View Post
    Quote Originally Posted by Nereid View Post
    Deep Surface Brightness Profiles of Spiral Galaxies from SDSS Stripe82: Touching Stellar Halos (arXiv:1204.3082) is a recent paper that may be quite relevant to this.
    Interesting. The analysis is way over my head but there are some tantalizing things in there. There are varieties of halo distributions (visible halos I mean). There is the interesting detection of very red halos which may contradict Universal IMF at the small end of the scale.

    There is also a very interesting comparison of images formed with different surface brightness limits. The difference this makes it quite striking.
    I'm glad you found the paper interesting (I fixed the missing [ c o d e ]).

    Are you at all familiar with LSBs (low surface brightness galaxies)? Images of well-known, big, local spiral galaxies in unfamiliar wavebands (e.g. gamma rays, x-ray, FUV, NUV, NIR, MIR, FIR, microwaves, radio lines)? How well do you think you understand the descriptions of the (optical/visual) radial surface brightness profiles of galaxies, and their components?

    I think it's fair to say, when we look closer, we often find more than what we expected.
    I'm not sure about "often", but yes, one of the central aims of big, and deep, surveys (and those in new parts of the electromagnetic spectrum) is to make discoveries of the unexpected.

    Put another way, the things that we cannot see don't seem to exist.
    Huh?

    Neither you nor I can see gamma rays (well, we'd be dead if they were intense enough to 'see'), yet they certainly exist!

    Perhaps you could be a bit less cryptic?

  11. #41
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    Quote Originally Posted by Hetman View Post
    Just note that solar cycles are strongly correlated with orbital periods of planets.
    Hetman, this whole discussion of what constitutes a scientific theory is probably off topic. The quoted sentence is completely off topic. Talk about Galactic Rotation or start a new thread.
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  12. #42
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    Quote Originally Posted by Shaula View Post
    Can you give an example of a theory that meets your criteria for being rationally derived/justified?
    There are hundreds of such laws, models and theories.

    http://en.wikipedia.org/wiki/Theory
    Down there are a few examples.

  13. #43
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    Quote Originally Posted by Hetman View Post
    There are hundreds of such laws, models and theories.

    http://en.wikipedia.org/wiki/Theory
    Down there are a few examples.
    OK, apparently my suggestion didn't work, so I'll make it even more official. The entire discussion in this thread about what constitutes a scientific theory stops now. And frankly, responding to "can you give me an example of a theory" with a link to the wikipedia article on "theory" is insulting. Hetman, you have earned yourself another infraction.

    If you want to discuss the nature of scientific theories, start your own thread. This thread is ONLY about galactic rotation and dark matter.
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  14. #44
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    The response was adequate to the questions.

    And finally one more small note, namely: discussion on the rotation of galaxies, in which no one can speak about gravitation, is simply a farce.
    Thanks for your attention.

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    Quote Originally Posted by Hetman View Post
    The response was adequate to the questions.

    And finally one more small note, namely: discussion on the rotation of galaxies, in which no one can speak about gravitation, is simply a farce.
    Thanks for your attention.
    And one more small note, namely: that sort of comment back will earn you a suspension.
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  16. #46
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    Quote Originally Posted by Nereid View Post
    Are you at all familiar with LSBs (low surface brightness galaxies)?
    Just aware that some are almost entirely dark matter which makes them pretty interesting.

    Quote Originally Posted by Nereid View Post
    Images of well-known, big, local spiral galaxies in unfamiliar wavebands (e.g. gamma rays, x-ray, FUV, NUV, NIR, MIR, FIR, microwaves, radio lines)? How well do you think you understand the descriptions of the (optical/visual) radial surface brightness profiles of galaxies, and their components?
    No, no, no. I have much to learn.

    Quote Originally Posted by Nereid View Post
    Neither you nor I can see gamma rays (well, we'd be dead if they were intense enough to 'see'), yet they certainly exist!

    Perhaps you could be a bit less cryptic?
    Perhaps you could take things less literally? Seriously though, I'll try to use "detect" instead.

  17. #47
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    Low surface brightness galaxies: I've a paper somewhere that says properly correcting these for dust extinction puts them firmly back on the Tulley-Fisher plot. They're not underluminous for their mass, they're just exceptionally dusty. (Maybe this conclusion has been superseded I don't know.)

    Deep Surface Brightness Profiles of Spiral Galaxies from SDSS Stripe82: Touching Stellar Halos (arXiv:1204.3082)

    Yeh this is interesting (why don't I find things like this?). There's been quite a few papers on so-called galaxy disk truncations in the past. The later papers evolved a classification system, Type I = smoothly exponential disk, Type II = light profile turning down at large radius and Type III light profile deviating upwards at large radius.

    This paper says that these effects are a function of the amount of blending with the halo population in the outer disk.

    The important thing for this thread is their conclusion about the stellar mass profile. Even though there are step changes (they call them breaks) in the surface brightness plots in outer disks, there is no such step change in the surface stellar mass. The step changes in surface brightness are due to a change to a less luminous stellar population.

    Also, the outer disk seems to gradually fizzle out and merge with the halo. There is no edge to the disk.

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    Quote Originally Posted by kzb View Post
    Low surface brightness galaxies: I've a paper somewhere that says properly correcting these for dust extinction puts them firmly back on the Tulley-Fisher plot. They're not underluminous for their mass, they're just exceptionally dusty. (Maybe this conclusion has been superseded I don't know.)
    That's interesting. Does this apply to local dwarfs or just large galaxies? Wonder why there is so much dust in some galaxies. I'll have to do some looking.

    Quote Originally Posted by kzb View Post
    Deep Surface Brightness Profiles of Spiral Galaxies from SDSS Stripe82: Touching Stellar Halos (arXiv:1204.3082)

    The important thing for this thread is their conclusion about the stellar mass profile. Even though there are step changes (they call them breaks) in the surface brightness plots in outer disks, there is no such step change in the surface stellar mass. The step changes in surface brightness are due to a change to a less luminous stellar population.

    Also, the outer disk seems to gradually fizzle out and merge with the halo. There is no edge to the disk.
    To quote directly from the abstract:

    We have also explored the integrated (g'-r') color of the stellar halo of our galaxies. We find (g'-r') colors ranging from ~ 0.4 to ~ 1.2. By confronting these colors with model predictions, we encounter problems to fit our very red colors onto stellar population grids with conventional IMFs. Very red halo colors can be attributed to stellar populations dominated by very low mass stars of low to intermediate metallicity produced by bottom-heavy IMFs.
    This implies existence of a population we were not aware of. Could damage the concept of a Universal IMF.

    Something I need clarification on is the use of the word "halo". Apparently they are not talking about the disk, but the large dimly luminous sphere that is visible in some galaxies? It is something separate because they refer to Bulge/Disk/Stellar Halo.

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    The word "halo" in this context refers to the population of stars which are scattered thinly over a very large volume, surrounding the entire visible extent of a spiral (or elliptical) galaxy. If the visible disk galaxy is a goldfish, the halo is the water filling the entire bowl.

    The standard model of halos states that they are full of very old stars, stars which formed long ago, before the galaxy took on its current size and shape. At that time, the gas in the proto-galaxy had fewer heavy elements and was mostly hydrogen and helium -- hence the low metallicity of stars in the halo. The low metallicity could very well be responsible for some of the difference between the IMF at that time, and the IMF at the current time.

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    Quote Originally Posted by StupendousMan View Post
    If the visible disk galaxy is a goldfish, the halo is the water filling the entire bowl.
    Nice analogy.

    And I assume you do mean the entire bowl? I have had discussions with a lot of people who think the use of the word "halo" implies a ring or shell of matter (dark or otherwise) outside the galaxy.

    This appears to show a poor understanding of both cosmology and art history.

  21. #51
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    Quote Originally Posted by StupendousMan View Post
    The word "halo"...
    Nice answer. Thanks.

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    Good point. Since 'cloud' seems to be the buzz word of the year, how about 'cloud of dark matter'? Except that then the EU people will want lightning, and thunder in a vacuum, and DM currents . . . .

    Regards, John M.
    I'm not a hardnosed mainstreamer; I just like the observations, theories, predictions, and results to match.

    "Mainstream isnít a faith system. It is a verified body of work that must be taken into account if you wish to add to that body of work, or if you want to change the conclusions of that body of work." - korjik

  23. #53
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    Quote Originally Posted by kzb View Post
    Low surface brightness galaxies: I've a paper somewhere that says properly correcting these for dust extinction puts them firmly back on the Tulley-Fisher plot. They're not underluminous for their mass, they're just exceptionally dusty. (Maybe this conclusion has been superseded I don't know.)
    If you could dig that reference up, I (for one) would appreciate it.

    No doubt some LSBs are dusty, but some of the large, face-on ones surely aren't, because background galaxies shine right through (IIRC). I'll see if I can dig up some Hubble images; for now, here's an early study of three LSBs, by Hubble: Hubble Space Telescope WFPC2 Imaging of Three Low Surface Brightness Dwarf Elliptical Galaxies in the Virgo Cluster (despite the title, one of them may actually be a large LSB, far in the background!)

    Also, the outer disk seems to gradually fizzle out and merge with the halo. There is no edge to the disk.
    IIRC, it depends, greatly, on which part of the electromagnetic spectrum you observe in. For example, seen in HI 21-cm 'light', while the outer disk of many spirals flares, or seems warped, the halo remains invisible.

  24. #54
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    Neried wrote:
    If you could dig that reference up, I (for one) would appreciate it.

    I've tried to re-find it on Googel Scholar to no avail. I've got it printed off, somewhere in a large pile of other papers. I'll try and find it though. Hope I remembered it's conclusion correctly.


    IIRC, it depends, greatly, on which part of the electromagnetic spectrum you observe in.

    Sorry, to clarify this point, they are discussing the stellar populations only. The gas disk does not get a mention. So I should have said something like, "the outer disk stellar population fizzles out and merges with the halo population".

    But it's an interesting point. You'd think the gas would be a tracer for the ultimate possible extent of the disk population.

  25. #55
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    Quote Originally Posted by kzb View Post
    [...]

    IIRC, it depends, greatly, on which part of the electromagnetic spectrum you observe in.

    Sorry, to clarify this point, they are discussing the stellar populations only. The gas disk does not get a mention. So I should have said something like, "the outer disk stellar population fizzles out and merges with the halo population".

    But it's an interesting point. You'd think the gas would be a tracer for the ultimate possible extent of the disk population.
    It's even more complicated - and interesting! - than that.

    I've referred, in several of my posts, to the Galaxy Zoo citizen science project (dear reader, if you're not an active member, clicking for science, I urge you to sign up ... NOW!).

    Well before that project got under way a class of 'gas-less' spirals was known (the 'lenticulars', or 'S0' galaxies); what was a bit of a surprise, shortly after citizen scientists (zooites) got going with their clicks, was the discovery of 'red spirals' and 'blue ellipticals'. Not just one or two either, but dozens and dozens of them. And these were not dwarf galaxies, but regular, honest-to-goodness Messier-like galaxies.

    Also, well before zooites were invented, some mysterious HI clouds, at high galactic latitudes, with rather large apparent line-of-sight velocities. But how far away are they? Are they, for example, in our galaxy's halo? or are they more remote, like some of our galaxy's satellite galaxies (perhaps even mini-halos)?

    Locally, the Magellanic Stream has been known, via its radio emissions, for decades. Several times, in several ways, astronomers have searched for stars in this stream, to no avail (as far as I know).

    So, we have (disk) stars without gas, and (certainly in some LSBs, as well as the Magellanic Stream, etc) gas without stars. Isn't astronomy fun!

  26. #56
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    Quote Originally Posted by Shaula View Post
    That is not hugely new! See this 1963 paper. There have been a few disk gravity models (their approach is actually a hybrid of two well known ones) -
    I am surprised than you have not commented about their drastic approach of the subject including calculations in the paper.

    The major point is :

    Because the gravitational field of a thin disk is not spherically symmetric, the orbital velocity law is not applicable.
    see quote in context below

    http://search.arxiv.org:8081/details...df/0804.3203v1

    quote in context
    The observed galactic rotation curves (Eq(1) and Fig.1) can not (Ref.[7])be explained by simply applying the so-called orbital velocity law, derived for a spherically symmetric gravitational field applicable to the Keplerian rotation of our solar-planet system (where most mass is located at the center), but not to galaxies with substantial mass distributed in a disk-like shape. In fact, the galactic mass distribution calculated by the orbital velocity law applied to these constant (flat) galactic rotation curves yields an increasing mass density with radius, contrary to the measured galactic luminosity curves which decrease exponentially with radius.

    1.2 Thin-Disk Gravitational Models with Bulge Added

    For a thin rotating galactic disk, we impose a balance between the Newtonian gravitational forces and centrifugal forces at each and every point.
    Because the gravitational field of a thin disk is not spherically symmetric, the orbital velocity law is not applicable. As illustrated by Feng & Gallo [8] [9], an axisymmetric thin disk gravitational model successfully describes the basic rotational dynamics of mature spiral galaxies with a mass density decreasing from the center to periphery. And the calculated total galactic masses are in good agreement with star count data.

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    To my mind, the Bullet Cluster is actually strong evidence against non-baryonic dark matter, and here's why:

    The tightness of the Tully-Fisher Relation. Originally the TFR was observed to be a tight relation between the optical luminosity of spiral galaxies and the HI line width, which was a measure of the outer circular velocity. The scatter in the plot is about 18%, which could fairly be attributed entirely to measurement errors. So, at least within a certain mass range, the stellar population somehow knows exactly how much dark matter there is in the galaxy (that is, if there is any there at all).

    The TFR breaks down with low luminosity galaxies. However take a look at the paper I link to below. DO not be put off by the fact that it is supporting MOND, simply look what it has to say about the Baryonic TFR. This is simply replacing the luminosity with total detected baryonic mass in the plot. The small galaxies in this study are gas-dominated. Any error in the assumed stellar mass function is only a small part of a small part, and therefore is too small to affect the conclusion either way.

    Quotes:

    The BTFR appears to be the
    fundamental physical relation underpinning the empiri-
    cal Tully-Fisher relation.


    ...essentially all of the scatter can be accounted for
    by observational uncertainty and the expected variation
    in stellar mass-to-light ratios


    The Baryonic Tully-Fisher Relation of Gas Rich Galaxies as a Test of LCDM and MOND

    http://arxiv.org/abs/1107.2934

    SO it seems to be established that the rotation speed is tightly correlated with the baryonic mass, over many orders of magnitude of galaxy mass.

    So back to the Bullet Cluster. This is held up as an example of baryonic and non-baryonic matter becoming separated. But if this is possible, how come the BTFR has so little (if any) intrinsic scatter? This point alone makes me think non-baryonic matter is not reasonable.

  28. #58
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    Quote Originally Posted by kzb View Post
    To my mind, the Bullet Cluster is actually strong evidence against non-baryonic dark matter, and here's why:

    The tightness of the Tully-Fisher Relation. Originally the TFR was observed to be a tight relation between the optical luminosity of spiral galaxies and the HI line width, which was a measure of the outer circular velocity. The scatter in the plot is about 18%, which could fairly be attributed entirely to measurement errors. So, at least within a certain mass range, the stellar population somehow knows exactly how much dark matter there is in the galaxy (that is, if there is any there at all).

    The TFR breaks down with low luminosity galaxies. However take a look at the paper I link to below. DO not be put off by the fact that it is supporting MOND, simply look what it has to say about the Baryonic TFR. This is simply replacing the luminosity with total detected baryonic mass in the plot. The small galaxies in this study are gas-dominated. Any error in the assumed stellar mass function is only a small part of a small part, and therefore is too small to affect the conclusion either way.

    Quotes:

    The BTFR appears to be the
    fundamental physical relation underpinning the empiri-
    cal Tully-Fisher relation.


    ...essentially all of the scatter can be accounted for
    by observational uncertainty and the expected variation
    in stellar mass-to-light ratios


    The Baryonic Tully-Fisher Relation of Gas Rich Galaxies as a Test of LCDM and MOND

    http://arxiv.org/abs/1107.2934

    SO it seems to be established that the rotation speed is tightly correlated with the baryonic mass, over many orders of magnitude of galaxy mass.

    So back to the Bullet Cluster. This is held up as an example of baryonic and non-baryonic matter becoming separated. But if this is possible, how come the BTFR has so little (if any) intrinsic scatter? This point alone makes me think non-baryonic matter is not reasonable.
    Apples and oranges.

    Clusters of galaxies - well, at least rich clusters (and yes, definitions are important) - are prominent extra-galactic x-ray sources. Their diffuse x-ray emission (i.e. excluding AGNs, etc) is, for many clusters, well-modelled as a 'halo' of hot, inter-galactic gas (actually a plasma; the ICM - intra-cluster medium). This plasma also shows up as the Sunyaaev-Zel'dovich effect (SZE), in microwave images of the same clusters (several independent, blind searches for clusters, using the microwave SZE signal, have been very successful, including ones using data from Planck). The estimated mass of this ICM plasma exceeds that of the total estimated mass of constituent galaxies (including whatever CDM they may, or may not, have) by a factor of many. Yet the estimated total mass of the cluster - derived from analyses of line-of-sight galaxy velocities - exceeds the estimated mass of the ICM plasma, again by a factor of many.

    Or, at the 'TooMany level of generalisation over-simplification', in rich clusters, galaxies are mere bystanders; whatever TFR or BTFR the galaxies seem to follow, it's irrelevant wrt clusters.

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    Quote Originally Posted by Nereid View Post
    Their diffuse x-ray emission (i.e. excluding AGNs, etc) is, for many clusters, well-modelled as a 'halo' of hot, inter-galactic gas (actually a plasma; the ICM - intra-cluster medium). This plasma also shows up as the Sunyaaev-Zel'dovich effect (SZE), in microwave images of the same clusters (several independent, blind searches for clusters, using the microwave SZE signal, have been very successful, including ones using data from Planck). The estimated mass of this ICM plasma exceeds that of the total estimated mass of constituent galaxies (including whatever CDM they may, or may not, have) by a factor of many. Yet the estimated total mass of the cluster - derived from analyses of line-of-sight galaxy velocities - exceeds the estimated mass of the ICM plasma, again by a factor of many.
    I believe kzb's point is that the amount of baryonic matter that can be detected is tightly correlated with rotation curves. (Is this related to the disk-halo conspiracy?) This is not to say that there is no hidden additional baryonic matter. The point is that it's hard to explain this tight correlation if baryonic and non-baryonic matter are easily separated (as claimed in the Bullet Cluster). A simple explanation of this correlation is that we consistently detect approximately the same fraction of all the baryonic matter actually present and there is no non-baryonic matter (in galaxies).

    You then point to another issue. When we look at clusters we find even more mass (by gravitation) than exists in galaxies and in the X-ray emitting medium combined. However, if we are failing to detect large amounts of baryonic matter in galaxies, then it is quite possible that we are also failing to detect much baryonic matter that is not in the form of visible galaxies. Possibly many LSB galaxies exist almost void of stars. The assumption that the hot plasma in clusters represents all the matter that is not in visible stars or detectable galactic gas seems unjustified.

    Also, I wonder how we can simultaneously hold that 20% of all matter is baryonic and that non-baryonic matter comprises 90% of galaxy clusters? How is that explained?

    BTW, I'd be interested in reading about the "blind searches for clusters, using the microwave SZE signal" with Planck data.

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    Quote Originally Posted by TooMany View Post
    I believe kzb's point is that the amount of baryonic matter that can be detected is tightly correlated with rotation curves. (Is this related to the disk-halo conspiracy?) This is not to say that there is no hidden additional baryonic matter. The point is that it's hard to explain this tight correlation if baryonic and non-baryonic matter are easily separated (as claimed in the Bullet Cluster).
    The two are not easily separated. The ram pressure required is not insignificant and requires a collisional fluid. The collision speeds are approaching 6 million MPH (~1% c). The mean free path for the particles is ~1000 parsecs, compared to the size of the entire system which is on the order of ~1,000,000. With those dimensions, the baryonic particles are almost guaranteed to collide, meaning you can treat the ICM as a collisional fluid, for baryons. If the system was not the size it is, or the collision speeds were not what they were, the baryonic and non-baryonic matter would not be separated, as you couldn't treat the ICM as a collisional fluid, for baryons.

    Quote Originally Posted by TooMany View Post
    A simple explanation of this correlation is that we consistently detect approximately the same fraction of all the baryonic matter actually present and there is no non-baryonic matter (in galaxies).
    Well then, the models showing how the baryonic matter would not collide, collect as observed, and produce the observed emission should be easy to find and link to, right?

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