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Our Galaxy Spin

  1. Dec 17, 2007 #1


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    We call it home, but the Milky Way can still surprise us. It does not have just one halo of stars, as we thought, but two. The finding calls into question our theories for how our galaxy formed.

    Daniela Carollo at the Torino Observatory in Italy and her colleagues were measuring the metal content and motion of 20,000 stars in the Milky Way, observed by the Sloan Digital Sky Survey, when they made their discovery.

    They found that the halo can be divided into two distinct regions, rotating in opposite directions, and containing stars of different chemical composition. "We really weren't expecting to see anything like this," says Carollo.

    The team found that the inner halo is flattened and extends out to about 4.6 x 1017 kilometres from the galactic centre, rotating at 20 kilometres per second, in the same sense that the Sun travels round the galactic centre. The outer halo is spherical, stretching out to over 6.0 x 1017 kilometres and spinning in the opposite direction at about 70 kilometres per second.

    It seems odd that no one noticed this in the past, but Carollo points out that while astronomers had found a few stars that appeared to be moving in the "wrong direction", they did not have enough data to conclude that the halo was split into two parts.

    Odd is the word, intuitively it seems wrong to me, would not all gravitating bodies lock after millions of years?
    Last edited: Dec 17, 2007
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  3. Dec 17, 2007 #2


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    That does seem quite odd.
    Here's the Nature abstract:

    Maybe someone with a good grasp of dissip'al and dissip'less coagulation can give us an intuitive notion of how they differ and why the earlier formation process apparently involved less of the astrophysical analogs of friction and viscosity.

    for instance it could be that the earlier process of formation took place when the whole system was more thinned-out--- more rarified ---so not as much chance for encounters that dissipate energy (analogous to aerodynamic drag).

    I will look for something by Daniela Carollo in arxiv. The abstract says that the suspicion of a two phase halo has been growing over the years, so there may be some earlier articles about it that offer possible reasons for it.

    Several people at PF may already know about this. I don't, but it presents an interesting puzzle.
    Last edited: Dec 17, 2007
  4. Dec 17, 2007 #3


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    we lucked out!


    the article in Nature (where things are usually pay-per-view) was already posted on arxiv.

    now we can see if the authors themselves speculated as to possible mechanisms that might have caused this two-stage sorting-out of stars

    =======quote from Carollo et al, around page 8=======
    An early model for the formation of the Galaxy, based on the rapid (a few hundred million years) monolithic collapse of a gaseous proto-Galaxy21, has yielded to the more recent idea that the halo of the Galaxy was assembled, over the span of several billion years, from smaller proto-Galactic clumps22. This hierarchical assembly model has received close attention in recent years, in part because it fits well with the prevailing theory for the formation and evolution of structure in the Universe, based on the early collapse of ‘mini-haloes’ of cold dark matter (CDM)23,24. Modern numerical simulations for the assembly of large spirals based on CDM cosmogonies predict that the stars in the haloes of galaxies like the Milky Way might be comprised of the shredded stellar debris of numerous dwarf-like galaxies that have been torn apart by tidal interactions with their parent galaxy24--27. Recent quantitative analysis of the amount of structure visible in the halo of the Galaxy from SDSS28--31 imaging provides compelling additional evidence32. Others have argued that some combination of a monolithic collapse and a hierarchical assembly model may be necessary to fully explain the observed data2,15,33 . Within the context of the CDM model, the formation of the inner halo may be understood in the following manner. Low-mass sub-Galactic fragments are formed at an early stage. These fragments rapidly merge into several (in many simulations, two26,34) more- massive clumps, which themselves eventually dissipatively merge (due to the presence of gas that has yet to form stars). The essentially radial merger of the few resulting massive clumps gives rise to the dominance of the high-eccentricity orbits for stars that we assign here to membership in the inner halo. Star formation within these massive clumps (both pre- and post-merger) would drive the mean metallicity to higher abundances. This is followed by a stage of adiabatic compression (flattening) of the inner halo component owing to the growth of a massive disk, along with the continued accretion of gas onto the Galaxy34,35 .

    The fact that the outer-halo component of the Milky Way exhibits a net retrograde rotation (and a different distribution of overall orbital properties), as found here, clearly indicates that the formation of the outer halo is distinct from that of both the inner-halo and disk components. We suggest, as others have before, that the outer-halo component formed, not through a dissipative, angular-momentum-conserving contraction, but rather through dissipationless chaotic merging of smaller subsystems within a pre-existing dark-matter halo. These subsystems would be expected to be of much lower mass, and subject to tidal disruption in the outer part of a dark-matter halo, before they fall farther into the inner part. As candidate (surviving) counterparts for such subsystems, one might consider the low-luminosity dwarf spheroidal galaxies surrounding the Galaxy, in particular the most extreme cases recently identified from the SDSS36,37. Subsystems of lower mass, and by inference, even lower metallicity, may indeed be destroyed so effectively that none (or very few) have survived to the present day. If so, the outer-halo population may be assembled from relatively more metal-poor stars, following the luminosity-metallicity relationship for Local Group dwarf galaxies38. The net retrograde rotation of the outer halo may be understood in the context of the higher efficiency of phase mixing for the orbits of stars that are stripped from subsystems on prograde, rather than retrograde orbits39,40.

    I've bolded what I think is the key idea. We know that the space around Milky is peppered with blobs of stars (globular clusters like a spherical cloud of gnats, dwarf galaxies, fragments like the Magellans). We think that Milky formed in a progressive hierarchic way by gradually gobbling up these smaller fish that had been gobbling up even smaller ones. A process of merging subassemblies----like putting an engine together.

    After the disk formed, these blobs had to FALL THRU THE DISK periodically because they were all orbiting the center. Each blob has its orbit and the orbit typically requires it to dive thru the disk every now and then.

    OK, the blobs are of two kinds. the prograde ones which slantwise partly agree with the majority disk rotation. the retrograde ones which slant against the majority as they dive thru.

    as a blob dives thru it is subject to TIDAL DISRUPTION which is more efficient if it is going slower relative to the disk objects and spends more time fraternizing with passerbys on the way thru.

    so the disruption is more efficient operating on the prograde blobs! so they get DIGESTED and assimilated preferentially by the disk.

    the retrograde blobs go roaring thru the disk OPPOSITE TO FLOW OF TRAFFIC so they don't chum up with disk objects long enough to get disrupted, so the disruption is not so efficient and they don't get digested.

    after a while the remaining blobs show a statistical bias in favor of retrograde, because fewer of them have been eaten.

    I think this could be what they are saying. I could be paraphrasing wrong. And their explanation could be wrong. Must say it is a fascinating puzzle and their proposed answer is a nice one. They cite two 1989 sources for it---references 39, and 40. So the idea has been around for a while.
    Last edited: Dec 17, 2007
  5. Dec 17, 2007 #4
    Marcus, your explanation sounds right to me.

    In addition to prograde and retrograde rotational motion, is their supposed to be any appreciable amount of randomized virial motion in the galaxy, or does virial motion primarily occur at the level of galaxies within clusters?

  6. Dec 17, 2007 #5


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    Hi Jon. As I picture it, all the motion here is randomized.

    For a given blob, the prograde or retrograde component of its velocity vector would typically be a small part.

    This thread is in astrophysics now I guess. We ought see what people here say.
    As I picture it, there is random motion of stars within blobs, and random motion of the blobs themselves.

    So the general character of the motion at each subassembly level is rather similar to what you picture for individual galaxies within clusters (although we arent talking about that)

    Just as a matter of terminology, I don't think virial means random (and I don't understand your post well enough to know what your view is, so this is not in reply to you: it is a separate semantic aside.)

    a bunch of planets in perfect concentric circle orbits can satisfy the virial theorem without their motion being in any sense random.

    in that sense the Earth's motion around the sun is virial.

    It doesnt mean random IMO, it means that the kinetic is half the potential.

    When N bodies have been together in a blob or swarm for enough time then their motion gets virialized, which means that the kinetic is half the potential EVEN THOUGH they may not have gotten all arranged in a solar system pattern.

    Being virial is something that both orderly-looking and disorderly-looking systems can share.

    This is just my own opinion, I defer to Wallace or Pervect or various others on this. It is how I've always used the word.

    I would not equate virial with random. Although random can often BE virial.
    Last edited: Dec 17, 2007
  7. Dec 17, 2007 #6


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  8. Dec 17, 2007 #7
    Hi Marcus,

    Yes I agree, virial does not imply random. My mistake.

  9. Dec 18, 2007 #8


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    Again, I would defer to someone who is more familiar with galaxy dynamics. I'm just trying to understand this as best I can, in the absence of someone versed in it.

    I would say, in answer to your question, in principle NO. Gravity affects everything. Nobody gets off free.

    but effectively, on the average and for the most part YES because of this reasoning.
    Galaxies and smaller star clusters are very sparse. so even when one is passing thru the other, the typical distances between stars are large. So the forces between the stars in the cluster, and the stars in the host galaxy amongst which the cluster is passing are WEAK.

    A weak force, in order to have significant effect, must act for a comparatively long time.
    But if the blob of stars is going thru against the flow of traffic it only sees a given set of neighbors for a short time----they don't have time to affect him before he has passed on to a different neighborhood with different stars pulling in different directions

    so all the host galaxy effort to deflect the stars in the blob and pull the blob apart by tidal, and disrupt it, tend to AVERAGE OUT AND CANCEL.

    Because no one patch of host stars spends enough time near the blob to take effect.
    So he gets thru intact. Like a guy who flirts with all the women at a party in rapid succession.

    So the answer is in effective sense YES you can think of the blob as orbiting the center of the host galaxy on an inclined orbit and passing repeatedly thru the densely populated plane without losing it's integrity (as if it did not feel the local gravity anywhere but only felt the overall pull of the host galaxy that it is orbiting.

    but to be scrupulously exact about it, you can't neglect anything---the web of forces is really there, only that a lot of it cancels and washes out, so the host doesn't have enough time to compromise the blob and start assimilating it.
    Last edited: Dec 18, 2007
  10. Dec 18, 2007 #9


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    Thank you Marcus, that is interesting.
  11. Dec 22, 2007 #10
    so how does the claimed dark matter effect this motion?
    shouldnot this be a test of dark matter's effects?
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