Why doesn't the centrifugal force cause rotating dark halos to flatten?












2












$begingroup$



  • Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below below


  • Assuming that the halo is purely made up of dark matter which has only gravitational interaction and nothing else. Therefore, it cannot dissipate energy with time.


  • Now imagine the system is given a uniform rotation at $t=0$ about the axis shown in the figure.



Wouldn't such a motion cause the dark matter distribution to flatten out and take up the shape of a disc? After all, these dark matter particles will experience a centrifugal force. But this is not observed for the galactic halo. The halo retains its spherical shape even though the galaxy is rotating. What is wrong with my reasoning? Isn't the halo also expected to rotate like the visible matter in the galaxy?






cosmology astrophysics dark-matter







share|cite|improve this question











$endgroup$

















    2












    $begingroup$



    • Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below below


    • Assuming that the halo is purely made up of dark matter which has only gravitational interaction and nothing else. Therefore, it cannot dissipate energy with time.


    • Now imagine the system is given a uniform rotation at $t=0$ about the axis shown in the figure.



    Wouldn't such a motion cause the dark matter distribution to flatten out and take up the shape of a disc? After all, these dark matter particles will experience a centrifugal force. But this is not observed for the galactic halo. The halo retains its spherical shape even though the galaxy is rotating. What is wrong with my reasoning? Isn't the halo also expected to rotate like the visible matter in the galaxy?






    cosmology astrophysics dark-matter







    share|cite|improve this question











    $endgroup$















      2












      2








      2





      $begingroup$



      • Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below below


      • Assuming that the halo is purely made up of dark matter which has only gravitational interaction and nothing else. Therefore, it cannot dissipate energy with time.


      • Now imagine the system is given a uniform rotation at $t=0$ about the axis shown in the figure.



      Wouldn't such a motion cause the dark matter distribution to flatten out and take up the shape of a disc? After all, these dark matter particles will experience a centrifugal force. But this is not observed for the galactic halo. The halo retains its spherical shape even though the galaxy is rotating. What is wrong with my reasoning? Isn't the halo also expected to rotate like the visible matter in the galaxy?






      cosmology astrophysics dark-matter







      share|cite|improve this question











      $endgroup$





      • Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below below


      • Assuming that the halo is purely made up of dark matter which has only gravitational interaction and nothing else. Therefore, it cannot dissipate energy with time.


      • Now imagine the system is given a uniform rotation at $t=0$ about the axis shown in the figure.



      Wouldn't such a motion cause the dark matter distribution to flatten out and take up the shape of a disc? After all, these dark matter particles will experience a centrifugal force. But this is not observed for the galactic halo. The halo retains its spherical shape even though the galaxy is rotating. What is wrong with my reasoning? Isn't the halo also expected to rotate like the visible matter in the galaxy?






      cosmology astrophysics dark-matter




      cosmology astrophysics dark-matter






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited 18 hours ago









      Kyle Oman

      15.4k955111




      15.4k955111










      asked 19 hours ago









      SRSSRS

      6,561433123




      6,561433123






















          1 Answer
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          active

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          3












          $begingroup$

          The premises of your question are false, according to current theory and measurements. Halos do rotate and their shapes are not spherical. Some references:




          • Current constraints on the Milky Way dark halo shape from the orbits of globular clusters suggest an axis ratio of $sim 1.3$: Posti & Helmi (2019).

          • The halo angular momentum has been theoretically well motivated for a long time: Hoyle (1949), Efstathiou & Jones (1979).

          • The dimensionless halo spin parameter is $lambdasim0.035$, indicating that dark halos are mostly supported by dispersion (random motions) rather than rotation, so flattening into a disc is not expected, but a slight elongation is : Mo, van den Bosch & White (2010), p. 358-359 - this is a textbook reference, there are also many articles on this well-studied topic.

          • There are also theoretically derived constraints on discs of dark matter that could in principle form by collecting within the stellar discs of galaxies. This seems not to be a very prevalent phenomenon, though: Schaller et al. (2016) - disclaimer, I am one of the authors.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Then, my question would be: why isn't the flattening as serious as that of the visible matter? I would like a simple argument, if possible.
            $endgroup$
            – SRS
            18 hours ago








          • 3




            $begingroup$
            @SRS ordinary matter radiates energy electromagnetically, allowing it to collapse much more than dark matter can. As it collapses, angular momentum is conserved and it spins faster and flattens out.
            $endgroup$
            – Kyle Oman
            18 hours ago












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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          3












          $begingroup$

          The premises of your question are false, according to current theory and measurements. Halos do rotate and their shapes are not spherical. Some references:




          • Current constraints on the Milky Way dark halo shape from the orbits of globular clusters suggest an axis ratio of $sim 1.3$: Posti & Helmi (2019).

          • The halo angular momentum has been theoretically well motivated for a long time: Hoyle (1949), Efstathiou & Jones (1979).

          • The dimensionless halo spin parameter is $lambdasim0.035$, indicating that dark halos are mostly supported by dispersion (random motions) rather than rotation, so flattening into a disc is not expected, but a slight elongation is : Mo, van den Bosch & White (2010), p. 358-359 - this is a textbook reference, there are also many articles on this well-studied topic.

          • There are also theoretically derived constraints on discs of dark matter that could in principle form by collecting within the stellar discs of galaxies. This seems not to be a very prevalent phenomenon, though: Schaller et al. (2016) - disclaimer, I am one of the authors.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Then, my question would be: why isn't the flattening as serious as that of the visible matter? I would like a simple argument, if possible.
            $endgroup$
            – SRS
            18 hours ago








          • 3




            $begingroup$
            @SRS ordinary matter radiates energy electromagnetically, allowing it to collapse much more than dark matter can. As it collapses, angular momentum is conserved and it spins faster and flattens out.
            $endgroup$
            – Kyle Oman
            18 hours ago
















          3












          $begingroup$

          The premises of your question are false, according to current theory and measurements. Halos do rotate and their shapes are not spherical. Some references:




          • Current constraints on the Milky Way dark halo shape from the orbits of globular clusters suggest an axis ratio of $sim 1.3$: Posti & Helmi (2019).

          • The halo angular momentum has been theoretically well motivated for a long time: Hoyle (1949), Efstathiou & Jones (1979).

          • The dimensionless halo spin parameter is $lambdasim0.035$, indicating that dark halos are mostly supported by dispersion (random motions) rather than rotation, so flattening into a disc is not expected, but a slight elongation is : Mo, van den Bosch & White (2010), p. 358-359 - this is a textbook reference, there are also many articles on this well-studied topic.

          • There are also theoretically derived constraints on discs of dark matter that could in principle form by collecting within the stellar discs of galaxies. This seems not to be a very prevalent phenomenon, though: Schaller et al. (2016) - disclaimer, I am one of the authors.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Then, my question would be: why isn't the flattening as serious as that of the visible matter? I would like a simple argument, if possible.
            $endgroup$
            – SRS
            18 hours ago








          • 3




            $begingroup$
            @SRS ordinary matter radiates energy electromagnetically, allowing it to collapse much more than dark matter can. As it collapses, angular momentum is conserved and it spins faster and flattens out.
            $endgroup$
            – Kyle Oman
            18 hours ago














          3












          3








          3





          $begingroup$

          The premises of your question are false, according to current theory and measurements. Halos do rotate and their shapes are not spherical. Some references:




          • Current constraints on the Milky Way dark halo shape from the orbits of globular clusters suggest an axis ratio of $sim 1.3$: Posti & Helmi (2019).

          • The halo angular momentum has been theoretically well motivated for a long time: Hoyle (1949), Efstathiou & Jones (1979).

          • The dimensionless halo spin parameter is $lambdasim0.035$, indicating that dark halos are mostly supported by dispersion (random motions) rather than rotation, so flattening into a disc is not expected, but a slight elongation is : Mo, van den Bosch & White (2010), p. 358-359 - this is a textbook reference, there are also many articles on this well-studied topic.

          • There are also theoretically derived constraints on discs of dark matter that could in principle form by collecting within the stellar discs of galaxies. This seems not to be a very prevalent phenomenon, though: Schaller et al. (2016) - disclaimer, I am one of the authors.






          share|cite|improve this answer









          $endgroup$



          The premises of your question are false, according to current theory and measurements. Halos do rotate and their shapes are not spherical. Some references:




          • Current constraints on the Milky Way dark halo shape from the orbits of globular clusters suggest an axis ratio of $sim 1.3$: Posti & Helmi (2019).

          • The halo angular momentum has been theoretically well motivated for a long time: Hoyle (1949), Efstathiou & Jones (1979).

          • The dimensionless halo spin parameter is $lambdasim0.035$, indicating that dark halos are mostly supported by dispersion (random motions) rather than rotation, so flattening into a disc is not expected, but a slight elongation is : Mo, van den Bosch & White (2010), p. 358-359 - this is a textbook reference, there are also many articles on this well-studied topic.

          • There are also theoretically derived constraints on discs of dark matter that could in principle form by collecting within the stellar discs of galaxies. This seems not to be a very prevalent phenomenon, though: Schaller et al. (2016) - disclaimer, I am one of the authors.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered 18 hours ago









          Kyle OmanKyle Oman

          15.4k955111




          15.4k955111












          • $begingroup$
            Then, my question would be: why isn't the flattening as serious as that of the visible matter? I would like a simple argument, if possible.
            $endgroup$
            – SRS
            18 hours ago








          • 3




            $begingroup$
            @SRS ordinary matter radiates energy electromagnetically, allowing it to collapse much more than dark matter can. As it collapses, angular momentum is conserved and it spins faster and flattens out.
            $endgroup$
            – Kyle Oman
            18 hours ago


















          • $begingroup$
            Then, my question would be: why isn't the flattening as serious as that of the visible matter? I would like a simple argument, if possible.
            $endgroup$
            – SRS
            18 hours ago








          • 3




            $begingroup$
            @SRS ordinary matter radiates energy electromagnetically, allowing it to collapse much more than dark matter can. As it collapses, angular momentum is conserved and it spins faster and flattens out.
            $endgroup$
            – Kyle Oman
            18 hours ago
















          $begingroup$
          Then, my question would be: why isn't the flattening as serious as that of the visible matter? I would like a simple argument, if possible.
          $endgroup$
          – SRS
          18 hours ago






          $begingroup$
          Then, my question would be: why isn't the flattening as serious as that of the visible matter? I would like a simple argument, if possible.
          $endgroup$
          – SRS
          18 hours ago






          3




          3




          $begingroup$
          @SRS ordinary matter radiates energy electromagnetically, allowing it to collapse much more than dark matter can. As it collapses, angular momentum is conserved and it spins faster and flattens out.
          $endgroup$
          – Kyle Oman
          18 hours ago




          $begingroup$
          @SRS ordinary matter radiates energy electromagnetically, allowing it to collapse much more than dark matter can. As it collapses, angular momentum is conserved and it spins faster and flattens out.
          $endgroup$
          – Kyle Oman
          18 hours ago


















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