I read that in the early Universe dark matter forms halos, and dust and gas inside those halos formed the first ever galaxies. I am curious to know whether the dust and gas formed the galaxy inside a previously formed halo, or whether the halo formed around the dust and gas.
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$\begingroup$ Note that you have accepted an answer that consists of non-mainstream claims that don't even really answer your question. Please consider unaccepting it. $\endgroup$– ACuriousMind ♦Commented Oct 13, 2017 at 16:37
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$\begingroup$ @ACuriousMind What answer are you referring to? $\endgroup$– xavier raeCommented Oct 16, 2017 at 8:53
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$\begingroup$ Ah, you had accepted a wrong answer that has since been deleted for reasons unrelated to its content. Never mind. $\endgroup$– ACuriousMind ♦Commented Oct 16, 2017 at 9:37
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1 Answer
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In the 'standard' model of galaxy formation in a $\Lambda$CDM cosmology (which admittedly has it's fair share of glitches), the halo forms first. This is because in the early Universe, most of the energy density is in the form of radiation (most of the energy is in the form of photons). The gas that will eventually form stars and galaxies is still so hot that it is a plasma, i.e. made of charged particles, which interact strongly with the photons. The result is that any bit of gas that tries to cool and collapse is immediately re-heated by the bath of photons. In the meantime, the dark matter, which feels no such radiation pressure, begins to collapse and form halos. Eventually the Universe expands and cools enough that electrons can bind to protons and make (neutral) hydrogen, which is transparent to the photons and can finally begin to cool and collapse into the now (partially) formed dark matter halos.
You mention dust, but until the first stars have had time to do some fusion reactions, there is nothing in the Universe to make dust out of: there is only H, He, and trace amounts of Li.
answered Oct 10, 2017 at 7:38
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$\begingroup$ But are the dark matter objects before galaxies formed in the shape of halos? Seems to me like the halos themselves are because the galaxy or does dark matter naturally collect into a halo shape? $\endgroup$ Commented Oct 10, 2017 at 21:14
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$\begingroup$ A dark matter halo really isn't donut-shaped, which you seem to think it is. $\endgroup$ Commented Oct 10, 2017 at 21:16
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$\begingroup$ @C.A.C. as Peter Shor points out, DM halos are thought to be approximately spherical, with a central concentration (dense in the middle). $\endgroup$ Commented Oct 10, 2017 at 23:36
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$\begingroup$ That makes sense. I always wondered why dark matter wouldn't be shaped like a sphere. $\endgroup$ Commented Oct 10, 2017 at 23:50
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$\begingroup$ Thanks for your answer it's helped me considerably. You say the Cold Dark Matter model has it's fair share of glitches,I am curious if there's any specific fault in the theory that other theories might better explain? $\endgroup$ Commented Oct 11, 2017 at 22:04