Galactic Rotation... no need for dark matter.

[Don J]

Galactic Rotation Described with Bulge+Disk Gravitational Models
C. F. Gallo, James Q. Feng
(Submitted on 20 Apr 2008)

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

Observations reveal that mature spiral galaxies consist of stars, gases and plasma approximately distributed in a thin disk of circular shape, usually with a central bulge. The rotation velocities quickly increase from the galactic center and then achieve a constant velocity from the core to the periphery. The basic dynamic behavior of a mature spiral galaxy, such as the Milky Way, is well described by simple models balancing Newtonian gravitational forces against the centrifugal forces associated with a rotating thin axisymmetric disk. In this research, we investigate the effects of adding central bulges to thin disk gravitational models. Even with the addition of substantial central bulges, all the critical essential features of our thin disk gravitational models are preserved. (1) Balancing Newtonian gravitational and centrifugal forces at every point within the disk yields computed radial mass distributions that describe the measured rotation velocity profiles of mature spiral galaxies successfully. (2) There is no need for gravity deviations or ``massive peripheral spherical halos of mysterious Dark Matter''. (3) The calculated total galactic masses are in good agreement with star count data. (4) The addition of central bulges increases the calculated total galactic masses, possibly more consistent with the presence of galactic gases, dust, grains, lumps, planets and plasma in addition to stars. (5) Compared with the light distribution, our mass distributions within the disk are larger out toward the galactic periphery which is cooler with lower opactiy/emissivity (and thus darker). This is apparent from edge-on views of galaxies which display a dark disk-line against a much brighter galactic halo.

[grapes]

Centrifugal force, eh? Does the paper list their affiliation?

EDT: Superconix, in Minnesota.
http://superconix.com/

[Don J]

Centrifugal force, eh? Does the paper list their affiliation?

EDT: Superconix, in Minnesota.
http://superconix.com/

I find these affiliation:
http://arxiv.org/refs/0804.3203

[profloater]

we seem to have enough difficulty describing the grav attraction of a simple uniform cone let alone a bulging galaxy of gas dust and stars!

[Shaula]

From the other thread with this paper in.

The first paper is basically just a cold, baryonic dark matter model. They assume that star density is not as well correlated with overall mass distribution as current models, work back to a mass distribution that does fit observed rotational curves and say "if our assumptions are true then this must be the mass distribution of the galaxy". They then assign this extra mass to clouds of molecular hydrogen, assume that current estimates of its density are way too low and voila - a solution. So they replace DM (because it has such odd properties - it is invisible, we are not sure what it is, we only know it is there by its gravity) with molecular hydrogen (which has the preferable properties that it is invisible, we do not know how it behaves in their models as it doesn't quite seem to behave like we'd expect and we only know it is there by its gravity)... Umm...

Does quoting yourself make you go blind?

[John Mendenhall]

Curious. I have been discussing non-DM proposals with an administrator. There are MOND proposals, GR proposals, and here a Newtonian proposal. The last is probably the weakest. There is a good summary on DM at the link below.

http://scienceblogs.com/startswithab...ark_matter.php

Regards, John M.

[John Mendenhall]

Does quoting yourself make you go blind?

See the 'Back when I went to school' thread.

[kzb]

I think I've brought this paper up on here before (to little response). The "centrifugal" force grated on me too, but I think I've seen it used in astophysics papers from other sources as well.

I don't understand the maths, but the whole thing depends on the mass-to-light ratio systematically increasing with galactocentric radius. Trouble is, the Universal IMF (initial mass function) is almost a central dogma, with a lot of evidence to back it up.

The only place where it is more or less accepted that the M/L ratio might be higher is out beyond the traditional edge of the disk at 13-15kpc. This is because a large proportion of the stars beyond that were not formed there, they are ejectees from further in, hence an older population. But this type of effect is no way big enough to explain the rotation curve.

[TooMany]

What am I missing here? Isn't it rather intuitive that the density of matter declines with increasing radius and therefore the star formation rate declines with radius? There may be more or less a cutoff radius for star formation due to these conditions, especially if large stars do not form and the supernova rate is correspondingly low. That does not imply a cutoff radius for DM. Why would anyone expect M/L to be anything close to flat? Also, the disk surface area increases with the square of radius so if M/L gets larger with larger with radius, it's quite possible for there to be lots of DM out there. If that can explain the rotation curves then it sounds like a pretty good guess, in better agreement with known physics.

kzb,

Can you tell us a little about the Universal IMF dogma?

BTW, I read this paper about a year ago. I searched the literature for information about "what if's" for disk mass distributions accounting for rotation curves and this is one of the few papers I was able to find. Most papers only address "spherical" halos of mystery matter.

[StupendousMan]

TooMany,

What you are missing is several years of experience reading about the observations and modeling which have been done to investigate the rotation curves of galaxies. May I suggest that, as a tiny little step, you pick some resource which describes some of these observations and read it? Perhaps you might start with one of the lectures from a class aimed at advanced undergraduates, such as

http://spiff.rit.edu/classes/phys443.../gal_dark.html

This lecture glosses over almost all the details. There are some figures and references to the primary literature. You could read those, and then look at the papers which refer to THOSE.

[TooMany]

TooMany,

http://spiff.rit.edu/classes/phys443.../gal_dark.html

This lecture glosses over almost all the details. There are some figures and references to the primary literature. You could read those, and then look at the papers which refer to THOSE.

From that lecture:

Could the answer be as simple as a change in the stellar population with radius? Well, if so, then we'd expect to see a gradient in color with radius: galaxies would have to be much redder in their outer regions. Azzollini, Trujillo and Beckman (ApJ 679, L69, 2008) look at a set of galaxies of different types, running from spiral-ish ones on the left to elliptical-ish ones on the right.

Figure taken from Azzollini, Trujillo and Beckman (ApJ 679, L69, 2008)

We don't see such gradients, so it's not possible to explain the rotation curves with a change in the stellar population.

This isn't quite what I was suggesting. Of course I would have to look into this more, but this sounds to me like very weak evidence for constant M/L. I don't doubt the observations of little change in stellar types, I just doubt the conclusion that this implies constant M/L. We know that star forming regions that we can study are mostly massive, dense, cold molecular clouds. If such clouds do not form in the outskirts, then there are going to be fewer stars relative to dark normal matter.

This assumption of constant M/L is on it's face amazingly unlikely. The exponential star falloff may simple indicate how rapidly stellar density falls off when the conditions for star formation are no longer met. You would need lots more evidence to prove that M/L is constant. My guess is that presently, normal dark matter that may be there is not observable.

If this is the way alternative ideas (to BB cosmology-linked conclusions) are dismissed in school, it's no wonder we have at serious problems explaining galactic rotation.

Consider the solar system as a sort of analogy. It's like concluding that there are only the few chunks of matter beyond Pluto that we can see. What you see is all you get? But we can infer from observations of planetary disks (and other things) that matter probably extends in some frozen dark form far beyond the radius of the planetary system we can observe. If we cannot directly detect the matter we believe lies well beyond Pluto in our solar system, how can we possibly claim that there is no such matter surrounding the luminous portions of galaxies?

[Don J]

From the other thread with this paper in.

The first paper is basically just a cold, baryonic dark matter model. They assume that star density is not as well correlated with overall mass distribution as current models, work back to a mass distribution that does fit observed rotational curves and say "if our assumptions are true then this must be the mass distribution of the galaxy". They then assign this extra mass to clouds of molecular hydrogen, assume that current estimates of its density are way too low and voila - a solution. So they replace DM (because it has such odd properties - it is invisible, we are not sure what it is, we only know it is there by its gravity) with molecular hydrogen (which has the preferable properties that it is invisible, we do not know how it behaves in their models as it doesn't quite seem to behave like we'd expect and we only know it is there by its gravity)... Umm...

Does quoting yourself make you go blind?

Based on what is described in 1.2 i was thinking that it was the the orbital velocity law applied to rotating galactic disk they were challenging for the reason they are giving .So it seem that they are doing more than just replacing dark matter by molecular hydrogen.

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

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.

[Shaula]

Based on what is described in 1.2 i was thinking that it was the the orbital velocity law applied to rotating galactic disk they were challenging for the reason they are giving .So it seem that they are doing more than just replacing dark matter by undetected molecular hydrogen.

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) - they still need dark matter as this paper does. The most you can say is that in some versions you do not need as big a halo as the one you need assuming the spherical galaxy. The authors just chose not to call it dark matter and ignore that their different M/L values are all about matter that is dark.

[Don J]

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) - they still need dark matter as this paper does. The most you can say is that in some versions you do not need as big a halo as the one you need assuming the spherical galaxy. The authors just chose not to call it dark matter and ignore that their different M/L values are all about matter that is dark.

I see the point you are trying to make :Matter that is dark = Baryonic dark matter

http://en.wikipedia.org/wiki/Baryonic_dark_matter

My point was rather no need for Nonbaryonic dark matter. DM

http://en.wikipedia.org/wiki/Nonbaryonic_dark_matter

Good try but no cigar!

[Shaula]

Then you should have made that point up front, and they should have worded their conclusions more carefully. Dark matter is not just non-baryonic dark matter. It could be anything, subject to observational constraints. In current dark matter models there is a component of baryonic material. So what this paper is actually saying is:

"Using a disk model and assuming a certain mass distribution of just baryonic dark matter we can remove the need for non-baryonic dark matter at the expense of introducing a mass distribution that has not been shown to be a feasible product of accepted models of galactic and structure evolution. We have not addressed the effect of this model on other pieces of evidence for the nature of dark matter."

It is an interesting result but there is a lot more work to be done on it. The key things about the mixed dark matter models we have is that they explain more than just rotation curves. Explaining away rotation curves at the expense of destroying theories on structure formation, spread of velocities of clusters around galaxies, gravitation lensing studies and oscillations in the CMBR seems a bit weak just to be rid of non-baryonic DM.

[Hetman]

I suspect that this problem is not properly recognized.

There is, in general, the distribution of additional mass (invisible - this presumed), which correctly reconstruct the dynamics of the galaxy - not only the rotation?

This is simply a lack of gravity - centripetal acceleration, and not some mass.

Traditional descriptive model of gravity is simply too weak and inadequate because it does not even touch the merits of the issue: principles of operation of the phenomenon, popularly known as gravity.

Here the mechanism is completely unknown, there are only empirical formulas - developed 300 years ago!

After all, it is abundantly clear that the forces pulling from a distance do not exist at all.
It is extremely abnormal situation that such a model could survive so much time, in virtually intact form - where are the physicists?

[tusenfem]

I suspect that this problem is not properly recognized.

There is, in general, the distribution of additional mass (invisible - this presumed), which correctly reconstruct the dynamics of the galaxy - not only the rotation?

This is simply a lack of gravity - centripetal acceleration, and not some mass.

Traditional descriptive model of gravity is simply too weak and inadequate because it does not even touch the merits of the issue: principles of operation of the phenomenon, popularly known as gravity.

Here the mechanism is completely unknown, there are only empirical formulas - developed 300 years ago!

After all, it is abundantly clear that the forces pulling from a distance do not exist at all.
It is extremely abnormal situation that such a model could survive so much time, in virtually intact form - where are the physicists?

Hetman, please keep your ATM ideas out of the Astronomy section.
If you don't agree with gravity theory (which has well been studied/developed since Newton) then take it to ATM.

[kzb]

TooMany and others interested in star formation in the outer galaxy:

Molecular gas and a new young stellar cluster in the far outer Galaxy

Quote: We estimate a value of  6% for the star-formation eciency of this molecular region
[similar to the inner disk]

http://arxiv.org/abs/0904.3502

And here we have a cloud at 19kpc galactocentric radius, and the star forming efficiency is the same as it is around here:

STAR FORMATION IN THE EXTREME OUTER GALAXY
DIGEL CLOUD 2 CLUSTERS
http://arxiv.org/abs/0711.0257

One more:

A star cluster at the edge of the Galaxy [20.2kpc galactocentric radius]

http://arxiv.org/abs/astro-ph/0702541

[TooMany]

Yes they could have been more precise, but I think it was clear that they were talking about the need for non-baryonic dark matter and trying to demonstrate that a flat disk where M/L increases at some radius is compatible with observation and does not require as much missing mass as the non-baryonic distribution. You can point to other evidence for missing mass all you want, but we do not have to know what goes on in clusters to understand the dynamics of a single galaxy.

Galaxies can be studied independently and perhaps completely understood if we can find out how much ordinary dark matter is there and how it is distributed.

[Hetman]

Hetman, please keep your ATM ideas out of the Astronomy section.
If you don't agree with gravity theory (which has well been studied/developed since Newton) then take it to ATM.

Excuse me, but I never heard that in the standard medelu there is any theory of gravity.

There is still the old empirical formula: GM/r^2, which was discovered several centuries ago, and without any further examine the merits of such a thing - even in the elementary sense of rationalism.

The problem is open, means there is no theory of gravity opposite to the main stream, because there is no theory of gravity in the textbooks.

By the way, I would ask for clarification of the status quo.
There are physicists in the U.S.?

[Swift]

Excuse me, but I never heard that in the standard medelu there is any theory of gravity.

There is still the old empirical formula: GM/r^2, which was discovered several centuries ago, and without any further examine the merits of such a thing - even in the elementary sense of rationalism.

The problem is open, means there is no theory of gravity opposite to the main stream, because there is no theory of gravity in the textbooks.

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[TooMany]

TooMany and others interested in star formation in the outer galaxy:

Molecular gas and a new young stellar cluster in the far outer Galaxy

Quote: We estimate a value of 6% for the star-formation eciency of this molecular region
[similar to the inner disk]

http://arxiv.org/abs/0904.3502

And here we have a cloud at 19kpc galactocentric radius, and the star forming efficiency is the same as it is around here:

STAR FORMATION IN THE EXTREME OUTER GALAXY
DIGEL CLOUD 2 CLUSTERS
http://arxiv.org/abs/0711.0257

One more:

A star cluster at the edge of the Galaxy [20.2kpc galactocentric radius]

http://arxiv.org/abs/astro-ph/0702541

From the first paper:

These findings reinforce previous results that the distant outer Galaxy continues to be active in the production of new and rich stellar clusters, with the physical conditions required for the formation of rich clusters continuing to be met in the very distant environment of the outer Galactic disc.

Continuing to be met occasionally.

The matter density at a given radius is uniform only when you average it. In reality there are clumps of gas along the spiral arms in which stars form. It is not surprising that occasional clumps perhaps little different from local ones exist to a wide radius. The issue is whether there exists a significant amount of very cold dark matter where conditions have fallen below those needed for star formation.

Imagine for a moment that early galaxies formed from a condensation of cold gas. A very large diffuse cloud of slowly rotating gas would gradually become denser and most dense in the center. A some point near the center, the density would eventually be high enough and the gas would become cold enough to create stars. We would certainly not conclude that beyond the radii where those stars formed there was no significant amount of matter and that the matter distribution precisely followed the light from stars. Nor need we conclude that for our own galaxy.

I can think of several factors that should lead to a decline in star formation relative to mass in the outer extremes. 1) The existence of fewer clouds dense and large enough, 2) the scarcity of supernova to drive additional star formation and 3) the low metallicity of the outer parts imposes limits on cooling of dense clouds.

[Cougar]

After all, it is abundantly clear that the forces pulling from a distance do not exist at all.

I'm not sure what you're talking about, but as distance from the galactic center increases, the gravitational force from the galaxy's visible mass appears to be greater than one would expect, hence the need for additional non-visible mass. Contrary to any modified gravity theory, the dark matter hypothesis has gained very strong support in the last few years through weak lensing studies.

[Hetman]

I'm not sure what you're talking about, but as distance from the galactic center increases, the gravitational force from the galaxy's visible mass appears to be greater than one would expect, hence the need for additional non-visible mass. Contrary to any modified gravity theory, the dark matter hypothesis has gained very strong support in the last few years through weak lensing studies.

I said what the problem is: the standard Newton's empirical formula has never been rationally justified.

Thus, it is still only a working hypothesis of Newton, nothing more.

Law of physics must be derived from the basic principles of symmetry or other axiomatic systems.

For example, Hooke's law, gas laws, Planck's law.
They were reasonably explained, derived, and there are no problems with them, because we know that those rights are limited in scope.

In the case of: g = GM/r^2 - what is the scope of applicability here: universal, anywhere, anytime, unlimited?
Such a thing is impossible, in principle!

As regards the ideas of dark matter:
invisible things do not exist just because it was non-existent, and not vice versa.

[Nereid]

I said what the problem is: the standard Newton's empirical formula has never been rationally justified.

What is this ("the standard Newton's empirical formula")?

[kzb]

TooMany wrote:
Continuing to be met occasionally.

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.

When I get chance I'll find what I have on galaxy "truncation". That's an interesting topic as well. At some large galactic radius there is indeed almost a step change in the M/L ratio. But I think we can assume the amount of mass involved in this "truncation" is not enough to account for the rotation curve. (It isn't literally "truncated", I think it got called that because that was what it was thought to be originally, but there is a step change in the stellar population giving a higher M/L ratio).

[Nereid]

TooMany wrote:
Continuing to be met occasionally.

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.

When I get chance I'll find what I have on galaxy "truncation". That's an interesting topic as well. At some large galactic radius there is indeed almost a step change in the M/L ratio. But I think we can assume the amount of mass involved in this "truncation" is not enough to account for the rotation curve. (It isn't literally "truncated", I think it got called that because that was what it was thought to be originally, but there is a step change in the stellar population giving a higher M/L ratio).

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.

In particular, this, from the Introduction:

Based on their behavior galaxy surface brightness profiles are classified into three major classes termed as Type I, Type II, and Type III. Type I galaxies follow pure exponential profiles. The Type II morphology represents all galaxies with a “downbending break”: a double exponential profile with a steeper outer exponential (Pohlen et al. 2002; PT06; Erwin et al. 2008). This class is the revisited and extended version of the Freeman Type II class (Freeman 1970) now including the so-called truncations discovered by van der Kruit (1979). Type III galaxies show an “upbending break”, a double exponential profile with a less inclined outer exponential discovered by Erwin et al. (2005). PT06 showed that only a minority of the surface brightness profiles of late-type disks follow a single exponential decline. The majority of late-type disks, ∼ 90%, exhibits so called breaks in the faint outer regions. Apart from the fact that these features can be linked to different morphological features (such as bars, rings, spiral arms, etc) inside the disk, the properties of breaks follow tight correlations with the host galaxy properties, like Hubble-type, total luminosity or stellar mass, and disk scale-length. Type II phenomenon is not limited to the local universe, but has also been found in galaxies at higher redshift (Pérez 2004; Trujillo & Pohlen 2005). Moreover, Azzollini et al. (2008b) showed that the break radius could be used as a tracer of size evolution of disk galaxies. In the last ∼ 8 Gyrs, Type II disks went through significant (size) evolution.

[Hetman]

What is this ("the standard Newton's empirical formula")?

http://en.wikipedia.org/wiki/Newton%...al_gravitation

It should be noted that in the last paragraph only offered a different interpretation of the old Newton's law.

GR is based on this law. It's clearly seen in the calculations: constant of integration a = 2m, could not be determined without the Newton's law, so the solution of equations would have no practical value.

But on the other hand, by using Newton's law, explicitly or implicity, in the calculations, the whole model is just hypothetical, not less than the used law. Loop is closed.

You can not generalize a law which has not yet been justified rationally;
in this way, you can create only castles in the air.

[Nereid]

I said what the problem is: the standard Newton's empirical formula has never been rationally justified.

What is this ("the standard Newton's empirical formula")?

http://en.wikipedia.org/wiki/Newton%...al_gravitation

[...]

You can not generalize a law which has not yet been justified rationally; ...

What do you mean ("has never been rationally justified", "has not yet been justified rationally")?

Specifically, what does the process of 'rational justification' entail?

[Cougar]

I said what the problem is: the standard Newton's empirical formula has never been rationally justified.

Excellent correspondence with observations in typical human settings seems to be a pretty good rational justification to me.

Thus, it is still only a working hypothesis of Newton, nothing more.

That can be said about every scientific hypothesis or theory ever devised.

Law of physics must be derived from the basic principles of symmetry or other axiomatic systems.

Boy, I don't know about that one....

In the case of: g = GM/r^2 - what is the scope of applicability here: universal, anywhere, anytime, unlimited? Such a thing is impossible, in principle!

We know Newton's laws have limited applicability. Where they're applicable, they're certainly simple, elegant, and accurate. That doesn't sound like a problem....

As regards the ideas of dark matter:
invisible things do not exist just because it was non-existent, and not vice versa.

Does not compute.