HST observations confirm SOME galaxies lack dark matter
- notFritzArgelander
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HST observations confirm SOME galaxies lack dark matter
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.
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.
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Re: HST observations confirm SOME galaxies lack dark matter
"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.
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.
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Re: HST observations confirm SOME galaxies lack dark matter
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:turboscrew wrote: ↑Thu Jun 17, 2021 9:23 pm "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.
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
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Re: HST observations confirm SOME galaxies lack dark matter
Thanks! I think it requires some more concentration than I'm capable of now (it's almost 1 AM).notFritzArgelander wrote: ↑Thu Jun 17, 2021 9:37 pmIt 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:turboscrew wrote: ↑Thu Jun 17, 2021 9:23 pm "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.
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 theComa 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.
I think I got the idea how that is derived, but I need to go through it later with more rested brain.
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Re: HST observations confirm SOME galaxies lack dark matter
I need to read up on the Viriam.
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Re: HST observations confirm SOME galaxies lack dark matter
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.
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Re: HST observations confirm SOME galaxies lack dark matter
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?
Any metaphor will tear if stretched over too much reality.
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Re: HST observations confirm SOME galaxies lack dark matter
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?
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Re: HST observations confirm SOME galaxies lack dark matter
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).
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Re: HST observations confirm SOME galaxies lack dark matter
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?GCoyote wrote: ↑Fri Jun 18, 2021 12:44 pm 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?
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Re: HST observations confirm SOME galaxies lack dark matter
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.turboscrew wrote: ↑Fri Jun 18, 2021 5:06 pm 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).
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: HST observations confirm SOME galaxies lack dark matter
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.notFritzArgelander wrote: ↑Fri Jun 18, 2021 5:55 pmIndeed 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?GCoyote wrote: ↑Fri Jun 18, 2021 12:44 pm 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?
viewtopic.php?f=74&t=16376&p=137568&hil ... es#p137568
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Re: HST observations confirm SOME galaxies lack dark matter
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"?
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"?
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Re: HST observations confirm SOME galaxies lack dark matter
I don't seem to get what the dot product of F and r represent.
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I don't suffer from insanity. I'm enjoying every minute of it.
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Telescope: OrionOptics XV12, Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Grab 'n go: Omegon AC 102/660 on AZ-3 mount
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 10 mm TV Delos, 6 mm Baader Classic Ortho, 5 mm TV DeLite, 4 mm and 3 mm TV Radians
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Re: HST observations confirm SOME galaxies lack dark matter
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.)turboscrew wrote: ↑Sat Jun 19, 2021 11:42 am I recall gravitational force is logarithmic outside the ball of mass, but linear inside the (homogeneous) ball. Thus the GM/R?
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.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"?
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: HST observations confirm SOME galaxies lack dark matter
It’s a measure of the potential energy per particle.turboscrew wrote: ↑Sat Jun 19, 2021 12:22 pm I don't seem to get what the dot product of F and r represent.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: HST observations confirm SOME galaxies lack dark matter
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?notFritzArgelander wrote: ↑Sat Jun 19, 2021 12:28 pmIt’s a measure of the potential energy per particle.turboscrew wrote: ↑Sat Jun 19, 2021 12:22 pm I don't seem to get what the dot product of F and r represent.
And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.
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Telescope: OrionOptics XV12, Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Grab 'n go: Omegon AC 102/660 on AZ-3 mount
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 10 mm TV Delos, 6 mm Baader Classic Ortho, 5 mm TV DeLite, 4 mm and 3 mm TV Radians
Cameras: ZWO ASI 294MM Pro, Omegon veLOX 178C
OAG: TS-Optics TSOAG09, ZWO EFW 7 x 36 mm, ZWO filter sets: LRGB and Ha/OIII/SII
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Re: HST observations confirm SOME galaxies lack dark matter
turboscrew wrote: ↑Sat Jun 19, 2021 12:36 pmI 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]notFritzArgelander wrote: ↑Sat Jun 19, 2021 12:28 pmIt’s a measure of the potential energy per particle.turboscrew wrote: ↑Sat Jun 19, 2021 12:22 pm I don't seem to get what the dot product of F and r represent.
That is an unnecessary complication. Since F(r) ~ GM/r^2 the dot product with r gives ~ GM/r the potential.
You're welcome.And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: HST observations confirm SOME galaxies lack dark matter
Aahh, now I'm starting to get it.notFritzArgelander wrote: ↑Sat Jun 19, 2021 12:51 pmThat is an unnecessary complication. Since F(r) ~ GM/r^2 the dot product with r gives ~ GM/r the potential.turboscrew wrote: ↑Sat Jun 19, 2021 12:36 pmI 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]notFritzArgelander wrote: ↑Sat Jun 19, 2021 12:28 pm
It’s a measure of the potential energy per particle.
You're welcome.And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.
Thanks for your patience with me.
- Juha
Senior Embedded SW Designer
Telescope: OrionOptics XV12, Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Grab 'n go: Omegon AC 102/660 on AZ-3 mount
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 10 mm TV Delos, 6 mm Baader Classic Ortho, 5 mm TV DeLite, 4 mm and 3 mm TV Radians
Cameras: ZWO ASI 294MM Pro, Omegon veLOX 178C
OAG: TS-Optics TSOAG09, ZWO EFW 7 x 36 mm, ZWO filter sets: LRGB and Ha/OIII/SII
Explore Scientific HR 2" coma corrector, Meade x3 1.25" Barlow, TV PowerMate 4x 2"
Some filters (#80A, ND-96, ND-09, Astronomik UHC)
Laptop: Acer Enduro Urban N3 semi-rugged, Windows 11
LAT 61° 28' 10.9" N, Bortle 5
I don't suffer from insanity. I'm enjoying every minute of it.
Senior Embedded SW Designer
Telescope: OrionOptics XV12, Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Grab 'n go: Omegon AC 102/660 on AZ-3 mount
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 10 mm TV Delos, 6 mm Baader Classic Ortho, 5 mm TV DeLite, 4 mm and 3 mm TV Radians
Cameras: ZWO ASI 294MM Pro, Omegon veLOX 178C
OAG: TS-Optics TSOAG09, ZWO EFW 7 x 36 mm, ZWO filter sets: LRGB and Ha/OIII/SII
Explore Scientific HR 2" coma corrector, Meade x3 1.25" Barlow, TV PowerMate 4x 2"
Some filters (#80A, ND-96, ND-09, Astronomik UHC)
Laptop: Acer Enduro Urban N3 semi-rugged, Windows 11
LAT 61° 28' 10.9" N, Bortle 5
I don't suffer from insanity. I'm enjoying every minute of it.
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Re: HST observations confirm SOME galaxies lack dark matter
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!turboscrew wrote: ↑Sat Jun 19, 2021 1:04 pmAahh, now I'm starting to get it.notFritzArgelander wrote: ↑Sat Jun 19, 2021 12:51 pmThat is an unnecessary complication. Since F(r) ~ GM/r^2 the dot product with r gives ~ GM/r the potential.turboscrew wrote: ↑Sat Jun 19, 2021 12:36 pm
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]
You're welcome.And thanks for reminding: I was mixing two things together. Understanding the cohesion AND the Virial theorem at the same time intermixed.
Thanks for your patience with me.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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