The Sky Glows With The Lyman-Alpha Emission

I don’t usually make Oh WOW! comments, but …

OH WOW! That is SO cool!

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Not exactly, I’m wondering why the “forest” is < 1210 Ang. when I expected the whole thing to be redshifted like in the last figure (where red-shifted Lyman-alpha is ~ 8500 - 9000 Ang.) so I’m assuming that it’s been shifted back so that it’s centered at 121 nm for the original quasar emission. So as the original 121 nm light travels towards Earth the whole spectrum is shifting while every time it runs into neutral hydrogen the “current” 121 nm wavelength is absorbed, leading to the forest at lower wavelengths.

Ah I see. Yes, you are absolutely correct! This is why the x-axis in the first 2 plots are labelled “Emitted Wavelength”. This is the wavelength as seen by an observer in the frame that is at rest with the quasar, also called the “emission frame”.

In contrast, the x-axes of the other plots are something we call the “observed wavelength”, which is the wavelength seen from the rest frame of the Earth.

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Awesome!

Interesting, that is definitely different from the “terrestrial” spectroscopy I do. So I read the Science Daily article that @Patrick posted to start this thread and I’m still trying to catch some of the significance and uncertainty. So, if I get this right, the astronomers detected not a new Lyman-Alpha Forest (Lyman-alpha absorption of red-shifted emission) but instead a new (very weak) red-shifted emission from far distant (and therefore old) hydrogen gas clouds? I guess I’m not sure where the emission is coming from. They make it sound like it’s the H gas clouds themselves but that doesn’t seem right (where’ the energy coming from).

The significance is that it’s widespread? And because of the large red-shift it’s relatively early in the universe’s history?

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Warning: I have not read the actual paper. However, detecting the emission of these diffuse hydrogen was a longstanding problem in observational astronomy, so I know a little bit about it.

That’s right. Note though that these hydrogen gas clumps that this observation saw were the same types as the ones that were previously seen in quasar absorptions. However, this time they are seen in emission.

Yes, it’s the hydrogen gas themselves. Some of these hydrogen clumps are quite warm (~10,000K). This is less of a statement of them being heated up but more a statement that it is difficult to cool rarerified gas.

Well that, and the fact that we know from quasar absorption spectra that they should be there but no one were able to see them in emission before.

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Ah right. Again, I’m used to gases mostly being room temp or less to freeze out degrees of freedom. I’m used to having to provide energy (i.e. a laser) to look at electronic transitions. 10,000K quite warm indeed.

I get it, very cool!

@PdotdQ , thanks so much for all the help. It’s so nice to have an expert to interact with as a scientist exploring other fields than what I was trained in.

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