A New Spin on Galactic Evolution

Article written: 2 May , 2011
Updated: 24 Dec , 2015
by

 

There’s a new concept in the works regarding the evolution of galactic arms and how they move across the structure of spiral galaxies. Robert Grand, a postgraduate student at University College London’s Mullard Space Science Laboratory, used new computer modeling to suggest that these signature features of spiral galaxies – including our own Milky Way – evolve in different ways than previously thought.

The currently accepted theory is as spiral galaxies rotate, the “arms” are actually transient structures that move across the flattened disc of stars surrounding the galactic bulge, yet don’t directly affect the movement of the individual stars themselves. This would work in much the same way as a “wave” goes across a crowd at a stadium event. The wave moves, but the individual people do not move along with it – rather, they stay seated after it has passed.

However when Grand researched this suggested motion using computer models of galaxies, he and his colleagues found that this was not what tended to happen. Instead the stars actually moved along with the arms, rather than maintaining their positions.

Also it was observed in these models that the arms themselves are not permanent features, but rather break up and reform over the course of 80 to 100 million years. Grand suggests that this may be due to the powerful gravitational shear forces generated by the spinning of the galaxy.

“We simulated the evolution of spiral arms for a galaxy with five million stars over a period of 6 billion years. We found that stars are able to migrate much more efficiently than anyone previously thought. The stars are trapped and move along the arm by their gravitational influence, but we think that eventually the arm breaks up due to the shear forces.”

– Robert Grand

Snapshots of face-on view of a simulated disc galaxy.

The computer models also showed that the stars along the leading edge of the arms tended to move inwards toward the galactic center while the stars lining the trailing ends were carried to the outer edge of the galaxy.

Since it takes hundreds of millions of years for a spiral galaxy to complete even just one single rotation, observing their evolution and morphology is impossible to do in real time. Researchers like Grand and his simulations are key to our eventual understanding of how these islands of stars formed and continue to shape themselves into the vast, varied structures we see today.

“This research has many potential implications for future observational astronomy, like the European Space Agency’s next corner stone mission, Gaia, which MSSL is also heavily involved in.  As well as helping us understand the evolution of our own galaxy, it may have applications for regions of star formation.”

– Robert Grand

The results were presented at the Royal Astronomical Society’s National Astronomy Meeting in Wales on April 20. Read the press release on the Royal Astronomical Society’s website here.

Top image: M81, a spiral galaxy similar to our own Milky Way, is one of the brightest galaxies that can be seen from Earth. The spiral arms wind all the way down into the nucleus and are made up of young, bluish, hot stars formed in the past few million years, while the central bulge contains older, redder stars. Credit: NASAESA, and The Hubble Heritage Team (STScI/AURA)

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30 Responses

  1. Member
    Jon Voisey says

    Doesn’t sound “new” to me considering I wrote on alternative methods to form spiral arms in 2009: http://www.universetoday.com/47083/spirals-tides-and-m51/

    • This is more about the motion of the arms relative to the stars themselves, rather than the initial development of the arms and what happens to them over time. I was surprised to find out that the currently-held belief is that the arm structure moved across the disc of stars, rather than the stars moving with them (as this new hypothesis suggests.)

      • Member
        Jon Voisey says

        Tidal formation would mean the arms were the movement of the stars themselves too. This study just seems to say such things are possible, but not even care how they got started in the first place.

        And while it’s the “currently-held belief”, don’t think it’s to the exclusion of other forms. It seems astronomers have long recognized that some spiral structure may be due to other mechanisms.

      • I think it would seem that it makes more sense that the stars move along with the arms, rather than have arm structure ripple through them.

        Perhaps this is just the latest computer modeling that supports these alternative ideas that makes this significant.

    • Member

      Me too! I wrote on a model in 2010:
      http://www.universetoday.com/81814/astronomy-without-a-telescope-secular-evolution/
      …where the inner stars move faster than the arms – while the outer stars move slower (so the arms do plough through the outer stars, though these stars get speeded up a bit in the process). This model provided a mechanism for an outward transfer for angular momentum.

      • Andrew James says

        Quite an interesting idea, but nearly almost impossible to prove. One of the more difficult constraints is the orbits of many stars don’t just circle around the galaxy, but do so in elliptical orbits, or orbit by moving through the galactic disk (twice per orbit.) They might start that way in nice circular orbits when born, but many of the stars become scrambled, probably mostly from the slow disintegration of star clusters or stars consumed by galaxy interactions or companion dwarf galaxy absorptions. Worst, is that this only really work for the new Pop I stars. Most of the established Pop II will just ignore the density wave altogether; especially important if the Pop II makes up >85% of ‘active’ stars in galaxies.
        (At least with galaxy evolution, there is certainly plenty of different parameters to play around with. That certainly badly dogged theorists in the 80s and 90s, let alone. not knowing which ones we important.)

    • Andrew James says

      Jason and Jon,
      You might like to read the following paper that is available on the ADS. This was written by Bruce Elmegreen, who in the late 1980s did much work on the evolution of galaxies and star formation within galaxies. I.e. Large scale star formation – Density waves, superassociations and propagation.

      Whilst it is a bit out of date, this paper gives an excellent summary of the history on density waves in galaxies and there significance (or lack thereof) — enough for you to fill in the blanks. (Elmegreen, for instance, strongly argues against density waves, but this argument was mostly resolved by the early 1990s, as the likely cause of star formation processes in galaxies. I.e. V.I. Korchagin & A.D. Ryabtsev, “Coherent waves of star formation in a galactic disc.“, MNRAS., 255, 19 (1992) The argument that changed it was in discarding the idea of using just a “rigid rotation disk”, which was better replaced by the influence of spiral shock waves generated by supernovae, etc.)

      (I once wrote a paper/presentation on this many years ago. If you want to search on the early computer simulations of density waves and star formation you might like to search through some of the papers about the same time. All the basic groundwork and mathematics for the general theory was really formulated then. Works by Joseph Silk would be a great start.)

      Cheers.

  2. Torbjorn Larsson OM says

    I believe I see how that works. Despite the DM halo since it would tend to be equipotential at every radius.

    Wonder how the “gravitational sorting” (which is nothing of the kind) affects star populations. Will the outskirts be populated with more older stars than a naive non-sort model would suggest? If so it may be one test of this.

  3. Question says

    i can’t believe i’m writing this but i’d be interested to read the explanation of the “electric” posters as to why stars at the leading and trailing edges of the arms migrated in such a fashion.

    • Mr Mike says

      @Question

      Well, there was not a nibble on your hook by EU/PU posters or by any mainstream posters. I might be firmly closed mouth at times but in this I choose to bite. I shall take a stab at this if I may. The following is not my pet belief or theory, not even a static thought in my mind, it is just an occasional whimsy if you would.

      If the Galatic Cuisinart theory is to be advanced it must be tested and address the following items…
      The inward migration for those stars on the leading edge; the outward migration for those stars on the trailing edge. The size (average) of the arms in width and length along with the degree of twist for each, the number of arms and the speed of any pseudo forward motion.

      If the Galactic Wagon Wheel theory is to be advanced it must be tested and address the following items…
      The inward migration for those stars on the leading edge; the outward migration for those stars on the trailing edge. The size (average) of the arms in width and length along with the degree of twist for each, the number of arms and the speed of any pseudo forward motion.

      Does anyone see where this is heading? There is also the Galactic Turntable, the Galactic Frisbee, the Galactic Whiffle Ball, the list just goes on. It could boggle the mind to dream up and test all these theories.

      All these address a firm disc model though and therein lies the rub, literally, since the density waves can be both independent and causal to the formation of stars and the evolution of prior stars can be the source of this multiple density wave phenomena.

      This model can be thought of in terms of what it strongly resembles — a gaseous mass which has peculiar initial conditions and which conditions change over time in known and testable ways.

      In a 3D reduction there exists spacetime, within and upon this there exists — the remainder is left as an inducement, an exercise, for the rest of us.

      Mr Mike

  4. @Question,

    “i can’t believe i’m writing this but i’d be interested to read the explanation of the “electric” posters as to why stars at the leading and trailing edges of the arms migrated in such a fashion.”

    Plasma Cosmology like the Big Bang model does not speak with a single voice therefore there is more than one explanation for a number of observations consistent with the Plasma Cosmology perspective but not necessarily consistent with each other. Spiral galaxy rotation curves are one of these explanations that have more than one explanation in Plasma Cosmology.

    One version is that a spiral galaxy is build by lines of magnetic forces that generate outward like the spokes of a wagon wheel where the hub would be the galactic core and the motion of the spokes motivate the disk. The idea is that lines of magnetic influence are generated by the galaxy’s central black hole and move out generally in a linear fashion from this central black hole. As the black hole rotates these lines of influence usher around the electrons and protons of the galaxy which dominate galactic matter not tied up in stars. These clouds of material act as a wind with gravitational influence concerning adjacent stars whereby like the spokes of wheel the outside stars can move faster than the inside stars or generally just as fast. If this explanation were valid dark matter would not be needed to explain galaxy rotation curves.

    Although I do not ascribe to this model or any Plasma Cosmology model, based upon present observations I believe there is a lot more magnetic influences within galaxies than the present theory of galaxy mechanics would allow.

    If magnetic influences are a major player in spiral galaxies rather than just a minor factor, then at least this facet of the standard model would seemingly need to be replaced.

    • Andrew James says

      Giggle.

      One version is that a spiral galaxy is build by lines of magnetic forces that generate outward like the spokes of a wagon wheel where the hub would be the galactic core and the motion of the spokes motivate the disk.

      No. There are half-a-dozen reasons why this is wrong. Nearly every paper on magnetic field in galaxies between 1950 and 1970 has been rejected as improbable and highly unlikely. I.e. Bostick onwards.

      “The idea is that lines of magnetic influence are generated by the galaxy’s central black hole and move out generally in a linear fashion from this central black hole. As the black hole rotates these lines of influence usher around the electrons and protons of the galaxy which dominate galactic matter not tied up in stars.”

      Plainly silly and just so wrong. Actually, the accretion disk generates the field, not the black hole. The field created is far too small to influence the centre of the galaxy, let alone as far out as the spiral arms! Funny too, as most of those following plasma cosmology pretend black holes don’t exist!)

      These clouds of material act as a wind with gravitational influence concerning adjacent stars whereby like the spokes of wheel the outside stars can move faster than the inside stars or generally just as fast. If this explanation were valid dark matter would not be needed to explain galaxy rotation curves.

      Even more silly. “wind with gravitational influence concerning adjacent stars.” What the hell does that even mean? Also the outside stars move fast than the inner ones. Eh? They all move at around the same velocity. If it were true, the spiral arms would be back the front! Giggle!

      As for; “If magnetic influences are a major player in spiral galaxies rather than just a minor factor, then at least this facet of the standard model would seemingly need to be replaced”

      Now your just making things up, “Magnetic influence” for most galaxies are already rather minor factors. (Just look at the incoherent fields in Steve’s recent article “Astronomy Without A Telescope – Cosmic Magnetic Fields.”

      Plasmas cosmology in this story is plainly a laughable notion here. Absolutely no proof, and much of it has show to be dead wrong.

      Nice try though!

      • Torbjorn Larsson OM says

        Actually I was more like ROTFL then PU now backs BHs because it is convenient. Theology is such a slippery thing.

    • Andrew James says

      Oh! If this is all so true, can you possibly explain why this article completely fails to mention any magnetic fields at all?

      Also please explain what is THE “standard model” of galactic evolution?
      (I actually don’t even know of one, BTW!)

      • Torbjorn Larsson OM says

        I read that as gravity being important and so a connection to the standard cosmology.

        [Otherwise “the standard model” in the context of cosmology would more precisely be the QED/QCD/Higgs particle model.]

        I don’t think there are any ties between standard cosmology and the exact outcome of galactic evolution though. It is the pattern recognition business running rampant again, only having gravity as target.

    • Question says

      forrest noble,

      that particular example does not sound like it would produce similar results in a sim. it’s mostly just a “for fun” challenge really because this is only a sim.

  5. Andrew James says

    This news story, and the reportage of it, to me is just a little confusing.

    Our ‘standard’ picture of galaxies is a different one than many imagine.
    Firstly, we have three main components in galaxies. New Population I stars (along the spiral arms), Population II (a haze predominating the core, and spread relatively evenly throughout the flat disk); and the gaseous nebulosity (pervading the whole disk.)
    The central hub or core is mostly older stars, similar to the globulars, where little exists of any star-bearing nebulosity. On the boundary between the central core and the flat disk are several structure (bluish haze in the galaxy depicted in this story); comprising closest dust clouds, then the little larger optical dust clouds of greater density; and the much larger depository of neutral hydrogen (H I). Further out into flat disk, these dust and the neutral hydrogen peter out, but extend past the optical disk.
    Now Pop I stars are newly born stars that are created in the nebulosity of the spiral arms (blue stars in the galaxy depicted above.) These are usually big massive OB stars that have short lives. They are formed by the spiral density wave colliding against the relatively fixed dark and bright nebulae, and quickly condensate out as stars. (Smaller massed stars are also produced, where the range of masses depend on the process of the nebula collapse.) Stars are not produced in ones or two but in open star clusters. The distribution of all galactic open clusters (young and old) are interspersed in both the spiral arms and the gaps between them, and there is no large variance in the distribution. Once the stars are formed, they disperse their own nebulosity’s by thermal pressure, and fall into a lull between the spiral arms. In time, the smaller long lasting stars, around and less than one solar mass, become the Pop II stars mixed into the whole flat galaxy disk.
    Meanwhile the density wave moves on triggering star formation as it goes, and the cycle of stellar evolution moves on as it has for billions of years. For the galaxy, it also chemically evolves, and as the big stars go supernova and the aged stars blow off their outer regions into interstellar space; they ‘pollute’ the disk full of ‘metals.’ (all the elements that are heavier than Helium) In turn, this material is recycled, forming in new star manufactured in the spiral arms, and this slightly changes the evolution of the stars.
    Now what this story alludes to is that the stars formed in the spiral arms move along with the density wave. I think this is a likely the case, as the fixed or near fixed nebula is partly swept along with the wave as it collapses as a gas, but when it forms it stars, the density wave no longer affects it. Gradually the stars fall behind it. Star surviving more than about 80-100 million years will remain intermixed in the galactic disk, while the short-lived blue stars destroy themselves as supernovae, and disperse any of the remaining nebulosity surrounding them.
    The key to this story is how fast the nebulae are moving in relation to the stars. Most theories assume the motion of them is fixed, but this simulation asserts they might be in motion, and therefore, the stars they make retain that same motion. Another possibility is that the density wave has a significant mass, and that mass acts on the region through which it moves. Hence, all these new stars are attracted to that whole mass and are dragged along with it. (If the big stars go supernovae, this would assist the “breaks up due to the shear forces,…” stated in the press release.
    I haven’t read the paper (hope it comes out in arXiv!), but I’d assume that BOTH the spiral density wave in the older accepted theory AND the motions described here might by simultaneously happening together. This would have to be the case to explain the distribution of open star cluster seen in the galactic disk (a prerequisite to test if this idea was true.

    Hope this explains this story from a slightly different perspective.

    [One point I should have said, is how the neutral hydrogen clouds are distributed. It is sometimes thought that the boundary of the galactic bulge and the disk somehow is feed from this region; hence, the spiral density wave continues to have something to collapse the new stars. Another issue are the elliptical galaxies, which do not seem to have any gas left to form stars. I.e. Do ‘density waves’ of sorts still move through elliptical galaxies, is probably an interesting question, too?]

  6. @Andres James

    quote:

    No. There are half-a-dozen reasons why this is wrong. Nearly every paper on magnetic field in galaxies between 1950 and 1970 has been rejected as improbable and highly unlikely. I.e. Bostick onwards.

    Of course you are correct that such papers are not accepted by the mainstream. This is why Plasma Cosmology is not the mainstream view — because most practitioners believe that reality is explained by the mainstream view.

    quote:

    Plainly silly and just so wrong. Actually, the accretion disk generates the field, not the black hole. The field created is far too small to influence the centre of the galaxy, let alone as far out as the spiral arms! Funny too, as most of those following plasma cosmology pretend black holes don’t exist!

    This is a point of contention. According to the mainstream model your opinion is correct. Realize that Plasma Cosmology is an alternative model. There is no proof that the rotation of the central black hole in conjunction with its surrounds creates such a field. This is the theory just like redshifts equate to the expansion of the universe according to the mainstream model, but there may be another explanation for them unrelated to the mainstream version or that of Plasma Cosmology. All is simply theory.

    Plasma Cosmology in general does not deny black holes, only in the mathematical form of zero dimensional points. The central black holes of galaxies from afar cannot influence the outer galaxy but in a linear fashion of magnetic influence involving unbroken chains of influence, such influence could extent to the next galaxy or beyond according to the theoretical model.

    quote:

    Even more silly. “wind with gravitational influence concerning adjacent stars.” What the hell does that even mean? Also the outside stars move fast than the inner ones. Eh? They all move at around the same velocity. If it were true, the spiral arms would be back the front! Giggle!

    “They all move at around the same velocity.”

    This is what current observations profess and what the present understanding seems to suggest. This version of Plasma Cosmology however would suggest that for the most Spiral Galaxies the outer stars move at a faster rate than the inner stars. This is their explanation of how over long periods of time that spiral galaxies can maintain a similar form. Realize that for a wagon wheel to maintain its form the outside or the spoke must move at a much faster rate than the inside of the spoke.

    quote:

    Now your just making things up, “Magnetic influence” for most galaxies are already rather minor factors. (Just look at the incoherent fields in Steve’s recent article “Astronomy Without A Telescope – Cosmic Magnetic Fields.”

    Realize that this is another Cosmological model. Current observations give little credence to galaxies having major magnetic fields. This is why this is called an alternative model. If the model were correct than it would explain the rotation of spiral galaxies and dark matter would not be needed for such an explanation.

    Do I ascribe to this model, no. Do I think it has merit, if it does have merit I think such merit will be found in the magnetic influences of galaxies, not in the models dependence on anti-matter which I believe is its primary failing as an overall cosmological model.

    • Andrew James says

      Easy.
      Realize that Plasma Cosmology is an REJECTED alternative model.
      So why mention it, eh?

    • Olaf says

      —This is why Plasma Cosmology is not the mainstream view — because most practitioners believe that reality is explained by the mainstream view.—

      Believe??? Where the hell does “believe” come in science?
      Believe is for creationists, not scientists.

  7. @Torbjorn Larsson OM

    quote:

    I don’t think there are any ties between standard cosmology and the exact outcome of galactic evolution though. It is the pattern recognition business running rampant again, only having gravity as target.

    My answer:

    I agree with you. I believe the standard model could endure much change in galactic evolution theory without the changing of General Relativity.

    I do not think the standard BB model could endure if it were somehow proven that dark matter does not exist. Dark matter like any other theoretical entity cannot be disproved. Instead if it were invalid theory, I think only a “better explanation” could replace it as well as the BB model or any other theory.

    • Greg says

      Does any of this deserve a response? This was a very useful and erudite discussion. Please inculde quotes and references directed to studies with meaningful measurments and observations as the others have done. This enables other readers such as myself to validate the arguments being made independently. Otherwise what you are saying amounts to inconsequential side comments, such as this one, rather than discussion of a valid counter-argument.

    • Torbjorn Larsson OM says

      I’ll have to agree with Greg, but respond by noting that both standard cosmology and DM has survived independent tests. They could have failed, in which case DM had been rejected.

  8. Olaf says

    It appears that Einstein is right again.
    There is evidence of the space-time vortex measured by Gravity-B probe.

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