Thread: galactic rotation curve

The flat rotation curve of some galaxies is called unexpected by some. It is unexpected if most of the mass in the galaxy is in the center. But obviously most of the matter is not in the center.

Modeling rotation curves within masses based on shapes where the matter is distributed in various ways show a consistancy of rotation curve sloping more steeply upward when the relative proportions of mass in disk to mass in center is greatest. The steepest curve is in spherical shapes.

Given these obvious models of rotation and their observable fit to real rotation curve to mass distribution ratios in galaxies and and the solar system why do you think the flat rotation curve is unexpected by so many astronomers?

I'm not sure I understand your question. The observed rotations of galaxies is unexpected because they do not match the rotation curves calculated from the distribution of observable matter.

Originally Posted by Larry Caldwell

I'm not sure I understand your question.

I suspect you don't understand that obvious models of rotation curves can be calculated to fit the observed rotation and mass distributions.

The models being;

1; Nearly all mass in the center, objects rotate outside of the mass (Like the Solar System).

Rotation result; follows a "pure" Newtonian curve sloping sharply up (inside the mass) then (outside the mass) down from the center.

2; Some mass in the center and some mass in a disk. Objects rotate in the mass of the disk and in the bulge (Like a disk galaxy with a bulge).

Rotation result; Slopes sharply up from center (in the bulge) then flat from the center (in the disk).

3; Nearly all mass in a disk. Objects rotate in the disk (Like a low surface brightness galaxy).

Roatation result; Slopes up from center at a steep angle.

Originally Posted by astrotech

Given these obvious models of rotation and their observable fit to real rotation curve to mass distribution ratios in galaxies and and the solar system why do you think the flat rotation curve is unexpected by so many astronomers?

Isn't it the case that they've determined the mass density that would most likely result in the observed curve, but based upon the mass of the visible stars, there must be some dark matter that they can't see? I haven't really looked into it.

It's not the density it's the distribution, the shape (which includes density variables), that determines the rotation curve. It's a calculated fact that certain shapes result in certain rotation curves. It's not a likelyhood.

From any rotation curve we can determine the mass inside the orbit of a "test particle". The mass calculated, or derived from rotation (called dynamic mass) doesn't match the mass calculated from assumptions about the visibility of mass (called visible mass). This is not a rotation curve issue. It is an instumentation and visibility assumption issue.

The OP question is that since the gross observable shapes of mass distribution (the visible distribution) fit the rotation curve calculated from dynamic mass distribution why do astronomers call the rotation curve unexpected when it is in fact perfectly predictable.

Originally Posted by astrotech

It's not the density it's the distribution, the shape (which includes density variables), that determines the rotation curve.

Yes, I meant the density distribution, when I said density.

It's a calculated fact that certain shapes result in certain rotation curves. It's not a likelyhood.

The curves aren't totally unique, but with certain assumptions about the objects and their distribution, we can determine the most likely possibilities. "Likely" in that sense.

From any rotation curve we can determine the mass inside the orbit of a "test particle". The mass calculated, or derived from rotation (called dynamic mass) doesn't match the mass calculated from assumptions about the visibility of mass (called visible mass). This is not a rotation curve issue. It is an instumentation and visibility assumption issue.

The mass outside has an effect as well, so the solution is not necessarily unique.

The OP question is that since the gross observable shapes of mass distribution (the visible distribution) fit the rotation curve calculated from dynamic mass distribution why do astronomers call the rotation curve unexpected when it is in fact perfectly predictable.

I haven't read the papers. Maybe that's a good idea.

Originally Posted by grapes

I haven't read the papers. Maybe that's a good idea.

Not reading the papers is a good idea?

No, I haven't read the papers yet, but it's probably a good idea that I do.

Originally Posted by grapes

No, I haven't read the papers yet, but it's probably a good idea that I do.

Just watch out for the weasle words in the papers.

One example that comes to mind is "roughly spherical". Usually in a context of a "roughly spherical" dark matter halo. The rotation curve in a sphere slopes steeply upward from center. But papers written about "dark matter halos" purport that the flat rotation curve is evidence of a "roughly spherical" dark matter halo. Of course with the weasle words "roughly spherical" even a disk shape with a bulge and a flat rotation curve is "roughly spherical". Even a cube is "roughly spherical".

Originally Posted by astrotech

Just watch out for the weasle words in the papers.

One example that comes to mind is "roughly spherical". Usually in a context of a "roughly spherical" dark matter halo. The rotation curve in a sphere slopes steeply upward from center. But papers written about "dark matter halos" purport that the flat rotation curve is evidence of a "roughly spherical" dark matter halo. Of course with the weasle words "roughly spherical" even a disk shape with a bulge and a flat rotation curve is "roughly spherical". Even a cube is "roughly spherical".

Give me some examples of the distinctions you're making here. I'm not sure.