Questions about Lyman-alpha forest

In the web site:
https://medium.com/starts-with-a-ban...r-1b2f992ac353

Astronomer Ethan Siegal writes:

quote
Ask Ethan #66: Did we just find dark matter?

Not a chance. What we've found may be a mystery, but it’s definitely not
our Universe's missing mass.
end quote

Then, he comes to the proof. He says:

quote
By looking at how deep these 'forest lines' are, especially early on, we
can constrain how light dark matter is allowed to be. Even under the
most liberal of circumstances, we can see that the absorption lines are
incredibly strong --- consistent with dark matter being incredibly
cold --- which means it has to be at least above a certain mass threshold.

Well, what is that threshold? It has to be, at this point in time,
heavier than about 10 keV, based on the strength of the observed
absorption lines. In other words, about a factor of 3 heavier (or 50%
heavier, for a decaying particle) that this supposed 'dark matter
signal' is!
end quote

I would like to have a reference (or book, whatever) where I could
understand

(1) why "deep" lines imply cold matter
(2) why cold matter imposes a mass threshold.

Actually, a book would be better than a paper, if possible.

Thanks in advance

[Mod. note: non-ASCII characters removed. Please, please, please,
learn to do this yourself -- mjh]

In article , jacob navia
writes:

In the web site:
https://medium.com/starts-with-a-ban...r-1b2f992ac353

Astronomer Ethan Siegal writes:

quote
Ask Ethan #66: Did we just find dark matter?

Not a chance. What we've found may be a mystery, but it's definitely not
our Universe's missing mass.
end quote

Then, he comes to the proof. He says:

quote
By looking at how deep these 'forest lines' are, especially early on, we
can constrain how light dark matter is allowed to be. Even under the
most liberal of circumstances, we can see that the absorption lines are
incredibly strong --- consistent with dark matter being incredibly
cold --- which means it has to be at least above a certain mass threshold.

Well, what is that threshold? It has to be, at this point in time,
heavier than about 10 keV, based on the strength of the observed
absorption lines. In other words, about a factor of 3 heavier (or 50%
heavier, for a decaying particle) that this supposed 'dark matter
signal' is!
end quote

I would like to have a reference (or book, whatever) where I could
understand

(1) why "deep" lines imply cold matter
(2) why cold matter imposes a mass threshold.

Apparently he is talking about absorption lines in a spectrum. If the
material is hot, Doppler broadening will make the lines less "deep". He
seems to be saying as well that the particles have a minimum mass which
is heavier than that of putative dark-matter particles.

On 12/14/14, 3:35 AM, jacob navia wrote:
[Mod. note: entire quoted article removed -- mjh]

"deep lines" may relate to spectral absorbance as related to hydrogen
species concentration. Ethan may be using the term 'deep' representing
the area under absorbance curve related to a particular species
concentration (or density). The deeper the absorbance, the greater the
concentration (or density). Then density~1/Temperature is used to
conceptually indicate a mass (density?) threshold.
This concept is used in chemistry instrumental analysis
but there is always a standard,
something that is not obvious here.

Richard D Saam