The galactic halo is filed by high energy plasma. This clouds of the plasma are very close each other in a thermal equilibrium . There are the globular clusters of the old and relatively cold stars too.

1. There might be a lot of Jupiter like planets and gaseous bulbs. Such a bulb and planet or brown dwarf is relatively cold and may consist or be surrounded by neutral hydrogen. It is natural and obvious, I think.
2. There is a double layer around such a bulb (cell).
3. The double layer around a cell cause a charge separation.
4. The neutral , cold hydrogen cell is tight surrounded by hot plasma.
5. The neutral hydrogen emits radio frequency photons.
6. The radio frequency photons are absorbed by the surrounding plasma electrons and emitted according to inverse Compton scattering.

The question is – how can we detect this neutral hydrogen if its radiation is absorbed and emitted by high frequency electrons.

Is it possible to detect a neutral hydrogen by its radiation in this case ?
Or is it possible to detect it by gravitationally weak lensing only?
A Dark Matter could be just unseen neutral hydrogen in the cold globular cluster’s space ?

I think it would be possible. You would get redshifted radiation (by Compton scattering), but besides that I don't know what the spectrum would look like. Although, I don't know how much those electrons would dim the radiation, but I think it would be possible to detect the radiation at least in theory.

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What happened to the radio wave photons of the neutral hydrogen coming through the plasma cloud ? Could you send me a link about it ?

I just realized that what I said in my previous post is not entirely accurate. You are talking about inverse Compton scattering, so some of the photons can also be blueshifted (electrons are also moving, so they can "kick" some more energy to the photons). I would quess that the received spectrum would be just like the spectrum of neutral hydrogen, but the emission lines would be spread.

They are still radio wave photons, only red/blueshifted.

Sorry, I don't have any links for this.

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If the photons are absorbed by high energetic electrons they will be blueshifted according to electron’s frequency. How can we say then , the photons are coming from a neutral hydrogen or from high energetic plasma electrons ? Is it possible to estimate a neutral hydrogen’s emission behind the high energetic plasma cloud ?

As far as I know, the photons are not actually absorbed and re-emitted, instead they are deflected by the collisions with electrons. So they are same photons, but their direction and energy have changed.

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Yes, but if their direction and energy have changed we do not see the real image. The radiation of the neutral hydrogen could seem like radiation of the colder plasma. Many radio waves are deflected and reflected by a surrounding plasma like around our Earth. It is not possible to see the whole radiation of the neutral hydrogen covered by plasma, I think.

That's true, but I think it might be possible to see the real spectrum of the neutral hydrogen.

Let's assume that you know two things:

1. you know what kind of spectrum the colder plasma should have
2. you know what kind of spectrum the neutral hydrogen should have

For the sake of the argument, let's also assume that they both have only one emission line, emission line of colder plasma has a frequency of fp and emission line of neutral hydrogen has a frequency of fh.

Next you measure the spectrum of your object, which is a cloud of cold plasma in front of a cloud of neutral hydrogen. You should see two emission lines, one with a frequency of fp and one spread out line (due to red/blueshifting of scattering process) which has a frequency roughly equal to fh. But there is one situation, when you only see one spread out line. It happens when fp is roughly the same as fh.

So, while you might not get the accurate picture of what your hydrogen cloud looks like, you should be able to quite accurately revive the spectrum of the hydrogen cloud, unless the hydrogen cloud's spectrum is the same as the spectrum of the plasma (and even in that case we might be able to say that there is something behind the plasma (?) ).

Well, that's what I think, but I can't be sure, since I'm no expert on this. It would be nice if one of our resident experts would say if I'm wrong or right. (Hint, hint. )

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Thank you Ari. I thought about it too. If there are differences in the frequencies , it might suggest there is something behind.
I found a link about Milky Way - http://physics.njit.edu/~dgary/202/Lecture22.html
Molecular clouds, including hydrogen but also CO and other molecules, radiate in the mm-wavelength radio range. The dust, of course, radiates mostly in the IR. Here is a view of the Milky Way in various wavelengths. http://adc.gsfc.nasa.gov/mw/mwpics/mwmw_8x10.jpg

I have special interest on the globular clusters in the galaxy halo. Why there is so little neutral hydrogen detected ? Is there an ionized hydrogen only ? The stars in the halo are very old and relatively cold (brown dwarfs, planets). This gas in among the Globular Clusters should be cooled too. There have to be much more neutral hydrogen then we can see it in radio wave radiation.

If the radio waves are reflected by surrounding plasma we can not see it. There could be a kind of radio heating and most photons might be lost for our sight, this way. The weak lensing suggests a Dark Matter around Globular Clusters in the galaxy halo. There could be a problem to measure exactly their hydrogen mass, I think.

In general, neutral hydrogen clouds are observed using the specific 21 cm emission line.

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Yes. there is much neutral hydrogen in the hot space near galactic centre. Why is it so little in relative cold galactic halo ? Neutral hydrogen is colder then this ionized. Do we see all the neural hydrogen ? The radio wave may be closed by surrounding plasma and there would be a kind of radio heating in such a closed hydrogen cloud. An ionized plasma reflects the radio waves around our Earth. I do not exactly how but it is possible, I think.