The Sky Glows With The Lyman-Alpha Emission

Deep observations made with the MUSE spectrograph on ESO’s Very Large Telescope have uncovered vast cosmic reservoirs of atomic hydrogen surrounding distant galaxies. The exquisite sensitivity of MUSE allowed for direct observations of dim clouds of hydrogen glowing with Lyman-alpha emission in the early Universe – revealing that almost the whole night sky is invisibly aglow.

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I spent around 3 months as a postdoc trying to generate Lyman-alpha (122 nm), it’s not fun. It is very difficult to detect because it’s vacuum UV. I was very surprised to see large-scale observations of it via a telescope. At the very end of the article I found out why:

The Lyman-alpha radiation that MUSE observed originates from atomic electron transitions in hydrogen atoms which radiate light with a wavelength of around 122 nanometres. As such, this radiation is fully absorbed by the Earth’s atmosphere. Only red-shifted Lyman-alpha emission from extremely distant galaxies has a long enough wavelength to pass through Earth’s atmosphere unimpeded and be detected using ESO’s ground-based telescopes.

That’s one heck of a red shift to get from 122 nm to ~ 300 nm to get it through the atmosphere. Astronomical spectroscopy is very interesting and weird to me.


Well the red shift is due to the expansion of the universe. This hydrogen gas is from very long time ago and very far away.

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Sure. I suppose they look at the pattern of wavelengths or something to determine where the lyman-alpha line is. As a “normal” spectroscopist I just find it incredible to think about doing things with wavelengths that shift around based on how far away they are from the observation. It’s a bit backwards to me since I usually identify something because it’s wavelength doesn’t shift.

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It is a simple calculation from red shift to distance. Realize that it is space that is expanding. So the wavelegth gets longer as wave travels through expanded space. Here is how the calculation is done.

Ethan explains it well:


No, I get the math, that’s not a problem. I’m just surprised you can identify the red-shifted lyman-alpha line as lyman-alpha. In almost all spectroscopy work I’ve interacted with you identify the line based on the wavelength because there are a bunch of other lines around so figuring out what’s what is non-trivial. To find the redshift you have to be able to say “ah hah, there’s lyman-alpha” in order to calculate redshift and distance. It’s seems utterly backwards to me since I’m usually dealing with a bunch of peaks and I’m trying to figure out where the heck they came from.


Yes, I understand what you are saying. The basic assumption is that the lyman-alpha line at 122 nm is the same everywhere in the universe and the same for all time. So by Hubble’s work you get the linear relationship between red shift and distance away. A question I have for you is, what could these spectral lines be if they are not red shifted light from distance hydrogen gas?


Well, I don’t work in astronomy. In my world there are all kinds of possibilities. I’ve done both gas phase and condensed phase spectroscopy and usually you have to know where you are looking for a molecule to show up in the spectrum pretty precisely. So if I’m looking for the doubly forbidden O_2 electronic transition in the visible, the first calibration experiment for the technique I developed in grad school, I look at 627 nm. There are other peaks around there but I know those are not O_2 because I know where to look fairly precisely. I don’t expect to see it a 750 nm :grinning:.

So to answer you question more specifically, I imagine there are many other peaks in the spectrum (from hydrogen or other gases?), any of which could be red-shifted to the wavelength I’m observing. I’m clearly not an astronomer but I’m guessing they do something like looking at the Balmer series in addition to Lyman-alpha and fit several peaks to find the particular redshift.


So you would expect that line at 627 nm if you were at the place and time it occurred, correct. Now what if you saw that line shifted in wavelength proportional to how far away and how long ago that transition too place. That is what Hubble did and now is being confirmed for every transition line including the Cosmic Background Microwave Radiation which was emitted 13.8 billion years ago and is at a red shift of z = 1080.


@Patrick, I understand how it works. I’m just saying that I think, from my experience of non-astronomical spectroscopy, that it’s rather incredible that it can be done. I would think the spectra would often be too crowded and weak to be able to tell the red shift with any level of certainty. I find it amazing.

So a very rough example might be, how do you distinguish a significant red shift from an Lyman-alpha from a very long distance from a relatively close Balmer-alpha. They may show up at the same wavelength on the observed spectrum. So how do we know that the Lyman-alpha is the Lyman-alpha. I think you’d have to instead compare sets of “fingerprint” lines, which is pretty cool.


@Jordan as a spectroscopist, perhaps you might find this interesting:

These are the spectra of light beams emitted by quasars (supermassive black holes) located extremely far from us. Note that a quasar has a known broadband spectrum that is nice and smooth:

Image result for lyman alpha forest

Almost all of those spikes are due to Lyman-Alpha absorption lines! As the beam travels through intergalactic space, it meets clumps of neutral hydrogen gas, which absorbs the emission at the Lyman-Alpha wavelength. However, because the beam of light is redshifted, these clumps absorb the broadband spectrum at different wavelengths.

This spiky pattern is likened to a forest, and this phenomena is thus called the Lyman-Alpha Forest. It contains information about the population, density, and position in spacetime of these neutral hydrogen clumps.

If you observe quasar that is even further away from us (which is equivalent to observing a quasar further back in the past), you see an additional feature:

Forget about the blue and red curve. The black curve shows the observed spectrum of a quasar. Under a certain wavelength (equivalent to far away and further back in time), the entire spectrum is extinguished.

If the Lyman-Alpha Forest is caused by the light beam traveling through clumps of neutral hydrogen, this trough is due to the fact that this far back in time the entire Universe is neutral (well, aside of small ionized pockets).


@PdotdQ it is great to have a Astrophysics postdoc here! Thanks.


It seems it is a nearly unique experience to be in such sustained technical conversation, just for the fun of it, with people of such varied fields. That includes the scientists here, of course, but also the philosophers, historians, exegetes, and theologians. I’m not sure I’ve had a social experience quite this deep and sustained before.


Just out of curiosity, does it get into the visible spectrum?

For very far away light from galaxies probable does get into the visible but will be very dim.

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Clear not visible to the naked eye. Just curious if it would be visible range.

Even a Catholic one :stuck_out_tongue_winking_eye: ? Just kidding, you’re very welcome, Patrick.

It goes to the visible spectrum and even beyond to the near infrared.


Of course. Some of the best astronomers were Catholic, Copernicus, Galileo, Lemaitre and I not too sure, perhaps Penzias was Catholic. The Final results of Planck papers had over a thousand authors. I am sure there were a lot of Catholics among them. But I am glad you are working on the physics of the early universe as opposed to the physics of transubstantiation. :rofl:


Wow! Truly a forest there. I was thinking something like that but that’s even more crowded. Are the x-axes of the top two quasar spectra “shifted” back? 1210 Angstroms = 121 nm which is where Lyman-alpha normality is.

So how do we know that all (most) of those lines are from Lyman-alpha and not something else?

So in the last figure, there is so much neutral hydrogen around that a whole swath from Lyman-alpha is completely absorbed? Amazing!

The quasar itself emits brightly in Lyman-Alpha. This is the large emission peak at around 1210 Angstrom. This is the only feature at exactly this wavelength because the structure that causes this (the quasar itself) is at the exact same redshift as the quasar (obviously), and there is no neutral hydrogen clump close to the quasar (because the quasar is super hot and produces a lot of UV).

Are you saying that the center of the emission peak is not exactly at 1210 Angstroms? It does look a bit off center. I don’t know why that is. I think it is contaminated by the broadband emission from the quasar that makes it look a bit off center.

In typical astronomy fashion, it’s more that we start with the assumption that all of these are Lyman-Alpha because we expect neutral hydrogen clumps to produce the Forest. After the data is taken, we can compare the relative positions/heights of different absorption troughs to see if any of them are actually NOT Lyman-Alpha. For example, here are quasar spectra with some other elements/transitions identified:

So the strategy is not “find Lyman-Alpha”, but rather “Lyman-Alpha unless proven otherwise” :rofl:

Yup, that’s absolutely correct!