Hi I am curious to know why dark matter forms a halo? Or why doesn't normal matter form a halo.What is the difference between the two
My level is amateur
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1$\begingroup$ Possible duplicate of If dark matter only interacts with gravity, why doesn't it all clump together in a single point? $\endgroup$– John RennieOct 2, 2017 at 7:03
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Baryonic (normal) matter also forms a halo around the galaxy, it is formed out of hot gas, too hot to collapse, but still gravitationally bound to the galaxy. This gas is usually measured using its X-ray emission, below there's an example
It shows the Abell 2029 cluster in X-ray and in optical side by side at the same scale, you can immediately see that the X-ray image extends out in a smooth halo. As matter of fact, most of the baryonic mass in clusters is contained in the form of this diffuse hot halo: 1% is in the form of stars, 10% in hot gas and the remaining 89% in dark matter.
More modest galaxies also have a baryonic halo, which include stars.
This image shows the stellar halo of our neighbor M31, again, it is a diffuse structure around the disk, which interestingly shows all the scars left by interacting with smaller galaxies in the past.
In these two cases the problem is that detecting the baryonic halo around a galaxy is not simple, mainly because it is a diffuse, extended, faint structure
