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Entertain an idea from an ignorant student for a minute

  1. Jan 15, 2007 #1
    Hiho, long time since I've posted here.

    So I've been thinking about 1 or 2 things. Globular clusters are assumed to be the earliest structures formed in the universe because they are roughly the size of the Jeans length. But I'm thinking that if the universe was very nearly isotropic the areas with slightly different densities must be very large. When the gas in these areas condenses surely the gas clouds, and later on stellar nurseries would be similarly very large?

    Anyway, large quanities of gas = large stars, so surely the first early stars would be massive beasts? If globular clusters are the first structures that formed in our universe is it possible that they could originally have been inhabited not by small 1 solar mass stars we see nowadays but giant O/B main sequence stars? These stars would die farily soon after their creation so no evidence of their existence would remain except for a black hole.

    This would mean that our estimation of the masses of globular clusters is far below their true masses. Possibly entire clusters only contained massive stars and no low mass ones so that nowadays nothing is left except a cluster of black holes.

    What I'm trying to get at is that globular clusters are found, if I remember correctly, almost exclusively in galaxies and more importantly in the halo of galaxies. The average mass of a galaxy is something like 10 billion solar masses. Globular clusters contain about a million visible stars. If massive stars were a lot more common in the early universe these could have been outnumbered by a large factor by massive stars, bumping the mass of a globular cluster up to something like 100 to 1000 million solar masses. Add to this the possibility of completely 'dark' globular clusters containing only black holes and neutron stars then could this account for the invisible dark matter that appears to be in every galaxy's galactic halo?

    I know this argument is held together with string and 'what if's', I'd just like to know why this situation was unlikely/impossible to occur. Would the gravitational lensing from dark globular clusters be so large that we would have had to be blind not to notice it? Even if they resided on the edge of the dark matter halo? (which is iirc about 10 times the radius of the visible halo, at least for our galaxy)

    Just some guy.
  2. jcsd
  3. Jan 15, 2007 #2
    Hi Just some guy,

    You have a pretty creative idea here but there are some holes in your chain of logic.

    The typical size of the areas with different densities (patches) CANT be inferred from the fact that the Universe was very nearly isotropic. It can be measured though. If you expand the WMAP data in an infinite series of spherical harmonics then you see a peak around l=200 (or about half a degree on the sky). This is the typical size of an overdense region.

    It is also true that the volume of an entire star forming region is very large compared to the volume of gas that collapses to form a single star. As far as i know this is always trure.

    It is NOT true that large quantities of gas = large stars. The first generation of stars were massive because of their composition. There were no heavy metals in these stars. There was nothing except H and He. Heavy elements and dust grains help cool a gas down and allow it to collapse (reducing the Jeans Mass). The first stars were massive because you need a lot of mass to collapse these warmer clouds of gas.

    These pop III stars, as they are called, do burn their fuel much quicker than stars like the sun. Some may even collapse directly to black holes. The detailed statistics about these stars composition, abundance, spectra, rate of black hole formation... is awaiting observations that are more sensitive and detailed simulations. So the extrapolation to pop III remnants in globular clusters is a stretch.

    I dont think you can go from "pop III stars were more common in the early universe" to globule clusters have more mass in pop III remnants than in sun like stars. pop III stars were probably much rarer objects than the current generation of stars due to the trouble with cooling. One can put an upper limit on their abundance by looking at the cosmic infrared background light which is starlight that has been red shifted out of the visible spectrum.

    and as you've pointed out, black holes as dark matter would be in the MACHO (MAssive Compact Halo Objects) category and would have been seen in microlensing observations if they constitute all of the dark matter. There still could be a population of pop III black holes that are at the edges of galaxies (or in the IGM) where there is not much gas to accrete and therefore generate observables.

    Last edited: Jan 15, 2007
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