As explained in my last post - given the collisionless property of the hypothesized CDM particles, the distribution of baryonic matter (which produces the galaxies light) and the CDM should not be coupled. References are provided in this post. I'm going to try and select just a handful from the numerous papers in my files.
Over 20 years ago now
Bahcall and Casertano (1985) noted that one fact of the observed rotation curves is that there is no feature to mark a transition between the inner region of the spiral galaxy where the visible matter dominates the gravitational dynamics and the outer region where the DM halo dominates the dynamics.
This image may help clarify the problem. The observed rotation curve is represented by the plotted data points with error bars. The contribution to the rotation curve derived from the observed disk is represented by the long dashed lines. The short dashed line represents the portion of the curve that comes from the hydrogen (mostly) gas in the galaxy. These two lines comprise the baryonic mass contribution. The halo contribution is added in to make up the difference between the baryonic mass contribution and the observed rotation. Note that at small radii the baryonic matter dominates the rotation curve, but at larger radii the DM halo becomes more important.
However, what is missing in this rotation curve is some feature to indicate a transition between the zones in which the normal matter dominates to the zone in which the DM dominates. The DM and normal matter must be precisely distributed in such a way that they "conspire" to keep the rotation curve flat. But this is an odd conspiracy (known as the "disk-halo conspiracy") if the DM is collisionless CDM. As Bahcall and Casertano noted in 1985:
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Originally Posted by Bahcall&Casertano
Featureless rotation curves require fine tuning if the inner galactic material (stars and gas) is physically different from the halo (dark mass).
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Palunas&Williams (2000) demonstrated with the maximum disk model that the distribution of the DM must be tightly coupled to the distribution of normal matter:
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Originally Posted by Palunas&Williams
These results show that within the optical regions of most spiral galaxies, the radial mass distribution is tightly coupled to the luminosity distribution. This is a much stronger constraint than that due to global correlations such as the TF relation. It implies that either the mass of dark matter must be small within the optical radius or that the distribution of dark matter must be precisely coupled to the distribution of luminous matter. A dark halo which is independent of, and unresponsive to the luminous disk cannot dominate the mass within the optical radius.
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They go on to note that this result is in conflict with simulations which predict that DM should dominate the inner dynamics of even the most massive (fastest rotating) spiral galaxies.
Renzo Sancisi (2004) takes this coupling even further by noting that even on the scale of wiggles and bumps in the light profile of a spiral galaxy, the rotation and the wiggles/bumps are correlated - again at odds with what would be expected from a collisionless particle such as CDM:
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Originally Posted by Sancisi
There is a striking correspondence between the shape of the rotation curves and the shape of the radial distribution of luminosity and spiral galaxies ... In other words, the correspondence between rotation curve and distribution of light is close and detailed. There is a simple rule - to an excess of light corresponds an excess of rotation and vice versa - which seems to apply to all spiral galaxies including the late type dwarfs and the low surface brightness objects. I do not know of any counterexample
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Sancisi provides an example of this phenomenon in Figure 2 of the paper.
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Originally Posted by Sancisi
The unavoidable conclusion from the observed correspondence between the shapes of the rotation curves and those of the luminosity profiles is that the gravitational potential is strongly correlated with the distribution of luminous matter: either the luminous mass dominates or there is a close coupling between luminous and dark matter.
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Donato, Gentile and Salucci (2004) demonstrated that the core radius for the DM halo is correlated with the disk scale length which is derived from the luminous mass:
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Originally Posted by Donato, Gentile, and Salucci
The second level is the realization that the core radius, the quantity
that defines the unexplained feature of the distribution of the dark
matter, correlates with the main quantity controlling the distribution
of the luminous matter as well as the total mass within this radius.
This indicates that the DM density in galaxies has been shaped
by dark-to-luminous matter coupling, a very challenging task if
the former is collisionless.
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It is worth noting that in version 1 of this paper on arXiv the words "very challenging task" were simply a single word: "impossible".
And that is the issue with all of these cases I've noted and it constitutes a falsification of the expectations for CDM particles - and any collisionless DM particles. If DM is collisionless it should not be coupled to the distribution of luminous matter. On the scale of individual galaxies CDM is a failure. And in this regard there are plenty more references where these came from. I could go into the Tully-Fisher relation and how CDM fails to explain it. I could go into the fact that the mass discrepancy from which DM is inferred only appears at a specific acceleration scale. I could go into the references that discuss the failure of CDM simulations to predict the correct shape for the DM cores (cuspy core problem). And there are still more issues.
I've explained one problem on galaxy scales in this post - the distribution of DM is coupled to the distribution of luminous matter - a contradiction with the properties of collisionless particles such as CDM.