HST observations confirm SOME galaxies lack dark matter

[notFritzArgelander]

https://phys.org/news/2021-06-hubble-ga ... -dark.html

It's nail in coffin time for MOND. If a galaxy is found that has no DM then alternative gravity theories intended to explain DM away are all falsified. They need DM in every galaxy without exception. Will MONDophiles and MONDomaniacs recant? I'm not holding my breath.

[turboscrew]

"While DF2 and DF4 are both comparable in size to the Milky Way galaxy, their total masses are only about one percent of the Milky Way's mass."
How does it keep together? One would expect that the gravitational pull couldn't keep it together, and slower rotation would have made it smaller.

[notFritzArgelander]

"While DF2 and DF4 are both comparable in size to the Milky Way galaxy, their total masses are only about one percent of the Milky Way's mass."
How does it keep together? One would expect that the gravitational pull couldn't keep it together, and slower rotation would have made it smaller.

It keeps together by self gravitation. It's not the mass that matters, it's whether the internal velocities are small enough. For the stability under gravity the Virial Theorem states that twice the average internal kinetic energy is balanced in magnitude by the average gravitational potential energy. Headache alert:

https://en.wikipedia.org/wiki/Virial_theorem

http://hosting.astro.cornell.edu/academ ... 201/vt.htm

The virial theorem is in fact how Fritz Zwicky discovered dark matter. Looking at the Coma Cluster of galaxies he saw that the velocities were too high for the observed masses. These diffuse galaxies have low enough internal velocities that the gravity of the observed objects is enough to hold it together. Not so for the Milky Way and more common galaxies.

[turboscrew]

"While DF2 and DF4 are both comparable in size to the Milky Way galaxy, their total masses are only about one percent of the Milky Way's mass."
How does it keep together? One would expect that the gravitational pull couldn't keep it together, and slower rotation would have made it smaller.

It keeps together by self gravitation. It's not the mass that matters, it's whether the internal velocities are small enough. For the stability under gravity the Virial Theorem states that twice the average internal kinetic energy is balanced in magnitude by the average gravitational potential energy. Headache alert:

https://en.wikipedia.org/wiki/Virial_theorem

http://hosting.astro.cornell.edu/academ ... 201/vt.htm

The virial theorem is in fact how Fritz Zwicky discovered dark matter. Looking at the Coma Cluster of galaxies he saw that the velocities were too high for the observed masses. These diffuse galaxies have low enough internal velocities that the gravity of the observed objects is enough to hold it together. Not so for the Milky Way and more common galaxies.

Thanks! I think it requires some more concentration than I'm capable of now (it's almost 1 AM).
I think I got the idea how that is derived, but I need to go through it later with more rested brain.

[helicon]

I need to read up on the Viriam.

[notFritzArgelander]

I need to read up on the Viriam.

The virial theorem is one of the most important formulae in astrophysics. At the large scale you can get the mass of a cluster of galaxies and more in the neighborhood you can get the average inner temperature of the Sun. It's everywhere.

[GCoyote]

Stray thought: Several papers have posited a role for DM in galaxy formation and evolution have they not? And I'm sure I've seen simulations that demonstrate how both normal and DM redistribute themselves during galactic mergers. So do we suppose these galaxies to have formed in the absence of DM or to have somehow lost their DM in some manner? I can readily see the implications for MOND but does this tell us something we didn't know about DM?

[turboscrew]

Hmm for a single particle, the force due to the potential is equal to the change of radial component of the momentum to keep the particle on the orbit?

[turboscrew]

It looks like the harder part is figuring out the potential caused by a "fog" of other particles in such a way that it applies to "any particle" (or average particle).

[notFritzArgelander]

Stray thought: Several papers have posited a role for DM in galaxy formation and evolution have they not? And I'm sure I've seen simulations that demonstrate how both normal and DM redistribute themselves during galactic mergers. So do we suppose these galaxies to have formed in the absence of DM or to have somehow lost their DM in some manner? I can readily see the implications for MOND but does this tell us something we didn't know about DM?

Indeed there are many papers on the essential role of DM in galaxy formation. Without DM one can only make globular cluster size objects. IIRC there’s an older thread where folks propose that these diffuse galaxies have lost their DM through tidal interactions with the main galaxy in their neighborhood. I’d post a link but my fingers are fat and I’m still lounging in bed with only a phone. Later?

[notFritzArgelander]

It looks like the harder part is figuring out the potential caused by a "fog" of other particles in such a way that it applies to "any particle" (or average particle).

It’s done for galaxy clusters in this way. Detailed studies of galaxies yield a mass to luminosity ratio, M/L. Spirals and ellipticals have very different M/L. With a decent distance and angular size one has a mean radius. The magnitude of the gravitational potential energy is then given by GM/R.

[notFritzArgelander]

Stray thought: Several papers have posited a role for DM in galaxy formation and evolution have they not? And I'm sure I've seen simulations that demonstrate how both normal and DM redistribute themselves during galactic mergers. So do we suppose these galaxies to have formed in the absence of DM or to have somehow lost their DM in some manner? I can readily see the implications for MOND but does this tell us something we didn't know about DM?

Indeed there are many papers on the essential role of DM in galaxy formation. Without DM one can only make globular cluster size objects. IIRC there’s an older thread where folks propose that these diffuse galaxies have lost their DM through tidal interactions with the main galaxy in their neighborhood. I’d post a link but my fingers are fat and I’m still lounging in bed with only a phone. Later?

Well I've finally moved the lazy bones to the laptop and found the older thread on tidal stripping of DM from these DM free galaxies.

viewtopic.php?f=74&t=16376&p=137568&hil ... es#p137568

[turboscrew]

I recall gravitational force is logarithmic outside the ball of mass, but linear inside the (homogeneous) ball. Thus the GM/R?

First you calculate the kinetic energy of one "particle" on the average orbit.
Then you sum for all the "particles" on the average orbit.
Then you sum over the cross-section of the half disk.
If the results match, the system is stable. If not, the difference shows the speed of shrinking/expanding.

Am I even in the "hoods"?

[turboscrew]

I don't seem to get what the dot product of F and r represent.

[notFritzArgelander]

I recall gravitational force is logarithmic outside the ball of mass, but linear inside the (homogeneous) ball. Thus the GM/R?

Only this part has a problem. Outside a homogeneous sphere gravity is inverse square of R. So the potential is inverse R. (Logarithmic laws for potential happen for gravity in only 2 space dimensions with a result that force is 1/R. But you’re right that in a homogeneous sphere force is linear inside.)

First you calculate the kinetic energy of one "particle" on the average orbit.
Then you sum for all the "particles" on the average orbit.
Then you sum over the cross-section of the half disk.
If the results match, the system is stable. If not, the difference shows the speed of shrinking/expanding.

Am I even in the "hoods"?

Yes. But you’re making it more hard. All you need is the velocity dispersion of the particles, their number and masses. That gives the average kinetic energy. Compare that to the average potential energy given by the mass and size and you’re set.

[notFritzArgelander]

I don't seem to get what the dot product of F and r represent.

It’s a measure of the potential energy per particle.

[turboscrew]

I don't seem to get what the dot product of F and r represent.

It’s a measure of the potential energy per particle.

I could understand that if the F was constant when you bring the "particle" from the center of gravity to the distance r from the center of gravity. I understand that's not the case?

And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.

[notFritzArgelander]

I don't seem to get what the dot product of F and r represent.

It’s a measure of the potential energy per particle.

I could understand that if the F was constant when you bring the "particle" from the center of gravity to the distance r from the center of gravity. I understand that's not the case?[/url]

That is an unnecessary complication. Since F(r) ~ GM/r^2 the dot product with r gives ~ GM/r the potential.

And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.

You're welcome.

[turboscrew]

It’s a measure of the potential energy per particle.

I could understand that if the F was constant when you bring the "particle" from the center of gravity to the distance r from the center of gravity. I understand that's not the case?[/url]

That is an unnecessary complication. Since F(r) ~ GM/r^2 the dot product with r gives ~ GM/r the potential.

And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.

You're welcome.

Aahh, now I'm starting to get it.
Thanks for your patience with me.

[notFritzArgelander]

I could understand that if the F was constant when you bring the "particle" from the center of gravity to the distance r from the center of gravity. I understand that's not the case?[/url]

That is an unnecessary complication. Since F(r) ~ GM/r^2 the dot product with r gives ~ GM/r the potential.

And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.

You're welcome.

Aahh, now I'm starting to get it.
Thanks for your patience with me.

No worries. Patience doesn't enter into it. It is always a pleasure to talk physics with folks who are polite and don't take the name of Emmy Noether in vain!