I'm not exacty sure why but Scitech Daily's Seeing Dark Energy’s True Colors: DESI Creates Largest 3D Map of the Cosmos includes the graphic and caption below.

It indicates a quasar in a Hubble image and included is an annotated spectrum from 5000 to 9800 Angstroms of a "new DESI Quasar at z = 6.53" , which shows what looks like flat noise between 5000 and 9100 Angstroms and then a sudden jump up at about 9150 Angstroms and higher.

The article talks about using DESI data to get redshifts and therefore distances, but there's no indication of how this spectrum and its pronounced feature is used to do this.

So I'd like to ask:

Question: How to interpret this spectrum of the "new DESI Quasar at z = 6.53"; what causes the big edge at about 9150 Angstroms?

enter image description here

A new quasar discovered using DESI gives a glimpse of the universe as it was nearly 13 billion years ago, less than a billion years after the Big Bang. This is the most distant quasar discovered with DESI to date, from a DESI very high-redshift quasar selection. The background shows this quasar and its surroundings in the DESI Legacy imaging surveys. Credit: Jinyi Yang, Steward Observatory/University of Arizona


2 Answers 2


The jump occurs at the redshifted wavelength of the Lyman-$\alpha$ line, so this is the Gunn-Peterson trough, which is caused by neutral hydrogen in the intergalactic medium suppressing any radiation with shorter wavelengths for sufficiently distant quasars.

This is a limiting case of the Lyman-$\alpha$ forest formed by the absorption lines caused by all the hydrogen clouds the light passes through on its way to us, with the absorption lines becoming so numerous due to the distance that there is no light left anymore (you could say that in this case we don't see the forest for all the trees anymore).

  • $\begingroup$ Thanks, it will be informative for me to read those links. For your Stack Exchange answer here can you address a little more "How to interpret this spectrum...?" Does the location of the edge at about 9150 Angstroms tell us $z = 6.53$? If so, how? If not, then what (if anything) can we learn from this spectrum besides "it's a quasar with $z \ge 6$? Thanks! $\endgroup$
    – uhoh
    Commented Jan 16, 2022 at 11:02
  • 1
    $\begingroup$ The peak in the spectrum is Lyman $\alpha$ emission from the quasar. At $z = 6.53$, Ly $\alpha$ (rest wavelength = 1216 Å) is redshifted to 9150 Å. $\endgroup$ Commented Jan 16, 2022 at 12:58
  • 1
    $\begingroup$ @uhoh Yes, 1216*(1+6.53)=9156 Angstrom. I don't quite follow Wikipedia though that the cutoff shortward of the Ly-$\alpha$ would be due the higher density of neutral hydrogen if z>6 ( i.e. before reionization), as hydrogen clouds affecting the left part of the spectrum (5000-6000 A) would only be located around z=3. I have to think about this further. In the meanwhile you might want to have a look at astro.ucla.edu/~wright/Lyman-alpha-forest.html for a basic explanation of the Lyman-$\alpha$ forest (note that the spectra there are scaled back to the original wavelenghts) $\endgroup$
    – Thomas
    Commented Jan 16, 2022 at 17:21
  • $\begingroup$ Okay I will take a look, thanks! Perhaps the assumption is that of the hydrogen between us and the quasar, it is the earliest hydrogen whose edge is most red-shifted that is associated with the long-wavelength limit of the forest. $\endgroup$
    – uhoh
    Commented Jan 16, 2022 at 21:27
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    $\begingroup$ @uhoh The left half of the peak (the Ly-$\alpha$ emission of the quasar) end everything shortward of it is practically chopped off by the intergalactic Ly-$\alpha$ absorption (the line width of the latter being much smaller). That's why it looks like an edge. $\endgroup$
    – Thomas
    Commented Jan 17, 2022 at 21:53

Thomas' answer is completely correct, I'd just like to elaborate a little on the reason for such a spectrum.

Quasar spectra generally have broad emission lines, in particular Lyman $\alpha$ at a rest wavelength of $1216\,\mathrm{Å}$, originating from hydrogen being ionized by the quasar's strong UV continuum. The hydrogen quickly recombines, and roughly ~2/3 of each recombination results in the emission of a Ly$\alpha$ photon.

Nearby quasars

A typical spectrum of a nearby quasar (3C 273; the first ever to be identified) is seen here:

3C273 Credit: Jannuzi (1996).

3C 273 lies at a distance of 2.2 billion lightyears and consequently has a redshift of $z=0.158$, so we observe the Ly$\alpha$ emission line at $\lambda_\mathrm{obs} = (1+z)\lambda_\mathrm{em} = 1408\,\mathrm{Å}$. A broad absorption line is seen at $\lambda=1216\,\mathrm{Å}$; this features is not from the quasar, but is imposed on the spectrum by neutral hydrogen inside our Milky Way, as the light completes its final part of its journey.

Moderately distant quasars

The next figure shows the spectrum of the more distant quasar PG 1634+706:

PG1634+706 Credit: Jannuzi (1996).

The spectrum looks like the one above, except that 1) the Ly$\alpha$ line is redshifted to $2837\,\mathrm{Å}$ (because the quasar lies at $z=1.334$), and 2) the part of the spectrum blueward (left) of the Ly$\alpha$ lines display a bunch of little absorption lines.

Before investigating this, let's take a look at an even most distant quasar, Q1422+2309 at $z=3.63$:

Q1422+2309 Credit: Jannuzi (1996).

Again, we see the Ly$\alpha$ emission, but now the blue part of the spectrum shows even more, narrow absorption line.

The Lyman $\alpha$ forest

What going on here? This is the so-called Lyman $\alpha$ forest: As the spectrum of the quasar leaves the host galaxy and begins its journey toward Earth, it is slowly redshifted. After a while, the light meets a diffuse cloud of neutral hydrogen, causing an absorption line$^\dagger$, just like the Milky Way did in the case of 3C 273 above, except that this cloud is not a full-blown galaxy like the Milky Way, but a much small, intergalactic cloud of gas.

When the light meets this cloud, it has already been redshifted a little bit. Hence, although the absorption takes place at a rest wavelength of $1216\,\mathrm{Å}$, the absorbed light started out with a slightly bluer wavelength of, say, $121\mathbf{5}\,\mathrm{Å}$. Shortly after, the light meets a new cloud, causing absorption at a wavelength that started out even bluer.

The result is that all the light that started out bluer than the emitted Ly$\alpha$, and that happens to meet a hydrogen cloud at a point in space where it has been redshifted to $1216\,\mathrm{Å}$, experiences an absorption line.

This feature — the wealth of narrow absorption lines blueward of a Ly$\alpha$ emission line — is called the Lyman $\alpha$ forest (LAF) due to its resemblance of… um… trees… -ish…

Evolution of the intergalactic medium

So why does 3C 237 show almost no LAF, PG 1634+706 shows a little, and Q1422+2309 is full of "trees"?

The reason is that 1) the Universe was more neutral in the past so there were more clouds to absorb the light, and 2) the Universe was smaller so there clouds were closer together.

What then happens if we observere even more distant quasars?

The Gunn-Peterson trough

As we approach $z\sim6$, what we see is that the LAF lines eventually begin to become so numerous and overlap, that in the end all light is absorbed, as seen in this spectrum of J2315–0023:

J2315–0023 Credit: Becker et al. (2015).

Predicted by Gunn & Peterson (1965), and hence named the Gunn-Peterson trough, this effect was only observed much later (Becker et al. (2001).

In other words, the answer to your question is "The interpretation of the DESI spectrum is that it is emitted by a quasar so distant that we see it at a time when the Universe was much more dense and neutral than it is today".

The Epoch of Reionization

So, if the Universe was so neutral in the past, why is it so ionized today? What happened?

The culprit is the first stars and galaxies, emitting an abundance of hard, ionizing radiation that started to fry the Universe (and to a lesser extent, quasars, although we now believe that their impact was sub-dominant). This process lifted the Universe out of its Dark Ages and re-ionized it ("re-", because it had been ionized before, until it recombined 380.000 years after the Big Bang).

Although reionization is caused by the first luminous sources, appearing 100–200 million years after the Big Bang at $z\sim20\text{–}30$, it took a while to "penetrate" the intergalactic medium. The exact course of the process is a hot topic, but in the current paradigm reionization is believed to begin as "bubbles" of ionized hydrogen around galaxies that eventually overlapped and rendered the Universe largely ionized by $z\sim6$.

The last figure shows a bunch of observations and models of how the global neutral fraction of hydrogen decreased from $x\mathrm{HI}\sim1$ (i.e. mostly neutral) at $z\simeq12$ when the Universe was ~400 million years old, to $x_\mathrm{HI}\sim0$ (i.e. mostly ionized) at $z\simeq6$ when the Universe was ~900 million years old.

EoR Credit: Naidu et al. (2020).

As discussed in this answer, however, residual neutral hydrogen was enough to cause much absorption for several billion years after the Epoch of Reionization.

$^\dagger$In fact it is not really absorption, but scattering. The photon which has been redshifted to $1216\,\mathrm{Å}$ excites a neutral hydrogen atom, which de-excites after some $\sim10^{-9}\,\mathrm{s}$, emitting another Lyα photon in another direction. But since the photon hence is removed from our line of sight, in effect this is the same as absorption.


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