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A recent article from a popular astronomy website tells of discovery of missing mass (not dark matter) that has puzzled astronomers for some time. Apparently, the discovery involves enhanced electron density in filaments associated with superclusters of galaxies. How were astronomers able to determine that this baryonic mass was missing in the first place, and what percentage of total baryonic mass did it entail?

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great question. – lurscher May 27 '11 at 21:09
To be horribly pedantic, electrons are not baryons. Or is there an implication of unseen nucleons, too? //don't know of a word that distinguishes weido dark matter from ordinary stuff matter. – dmckee May 27 '11 at 23:13
To answer my own question the article is discussing diffuse, hot plasma, so yes the electrons are expected to be associated with baryons. – dmckee May 27 '11 at 23:16
Cosmologists generally use "baryonic matter" to mean matter made of protons, neutrons, and electrons. Often the electrons are what we see, but we impute the presence of nuclei and call the whole mess "baryonic." Astronomy's full of odd terminology. This one isn't as bad as the way we use the word "metals"! (For those who don't know, in astronomy all elements other than hydrogen and helium are called "metals." Phrases like "metals such as neon" occur all the time in astronomy talks, and people think nothing of it.) – Ted Bunn May 28 '11 at 15:52
@Ted: Thanks for the clarification. I was aware of the connotation of metals as I took a class on the Structure and Evolution of Star in school, but I have never studied cosmology. – dmckee May 28 '11 at 21:39
up vote 2 down vote accepted

The article answers the question, though it does not lean on it:

His evidence came from the orbital velocities of galaxies in clusters, rotational speeds, and gravitational lensing of background objects.

Nor is this a surprise, orbital velocities and lensing are really the only reliable tools for weighing things at a very long distance.

Another thing to note is that the dark matter (what ever it may be) interacts gravitationally, too. So just seeing evidence of more mass than you can account for does not distinguish between some unexpected ordinary matter and dark matter, which is why the x-ray band observation is so interesting.

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dmckee: I understand that "His evidence came from the orbital velocities of galaxies in clusters, rotational speeds, and gravitational lensing of background objects.", but dark matter has the same affect. Unless it's quite massive, I wonder how one can distinguish the effect between dark matter and this baryonic discovery. – Michael Luciuk May 28 '11 at 0:04
@Michael: To the extent that you can believe the models you have an estimate of the geometry that might be associated with dark matter. Normally interacting matter will generally follow a different patter according to it's different physics. Plus, you guess, you write your grant request, and if funded you make the observations. – dmckee May 28 '11 at 0:12
dmckee: The article mentions that the predictions were that "the mass would be low in density, but high in temperature." Wouldn't these filaments be subject to Coulomb forces that would disrupt such a structure? I'm missing something important. – Michael Luciuk May 28 '11 at 3:03
@Michael If it is ionized from neutral matter (i.e. there are baryons in there) it will be bulk neutral. Certainly Coulomb forces are important in magnetohydrodynamics (plasma dynamics). Problem for the student: why are emissions only observed from the electrons? – dmckee May 28 '11 at 3:07
Thanks dmckee for suggesting I do some research. Ionization in the 10^6 K plasma created thermal bremsstrahlung from the accelerating electrons affected by Coulomb forces. This was the x-ray radiation observed in the study. Since electron mass << than ion mass, bremsstrahlung from the much slower accelerating ions was negligeable. The resultant x-ray spectrum was used to determine the plasma temperature. – Michael Luciuk May 28 '11 at 5:30

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