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I'm studying Astrophysics and I have some video's from a past course. I'm currently studying the reionization of the universe. At the current point in the class, the instructor is talking about quasars and the redshift at which the Lyman-Alpha forest disappears. This appears to occur at a redshift greater than 6. The instructor goes into finding quasars with z > 6.5. He states that quasars that far away, "They're only emitting infrared light.". I don't understand that statement. Does the instructor mean they are only visible in IR or those very old quasars only emit in IR? Why would quasars only emit in IR? I understand why we'd only see IR from them.

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The statement "They're only emitting infrared light" is wrong, or at least poorly phrased.

The Ly$\alpha$ forest

The Ly$\alpha$ forest (LAF) is caused by the spectrum of the quasar being redshifted along its way; wherever a cloud of neutral hydrogen is located, the part of the spectrum that at that at that position has been redshifted to $\lambda_0 = 1216\,\mathrm{Å}$ has a high probability of being scattered. As the probability of being scattered back into the line of sight is negligibly small, the spectrum effectively suffers absorption, but in fact the light is not absorbed.

Because the lines are caused by the Ly$\alpha$ line, it only affects the part of the spectrum blueward of the quasar's "intrinsic" Ly$\alpha$ emission (well, similar but weaker spectral featured are seen e.g. at Ly$\beta$, and for very strong absorbers such as damped Ly$\alpha$ absorbers you also see metal absorption).

The Gunn-Peterson trough

The further back in time you go — i.e. the higher the redshift you observe — the higher the fraction of the gas of the Universe was neutral, and the denser the Universe was. As you approach the Epoch of Reionization, the LAF "absorption" lines begin to overlap, until eventually, around $z\sim6$, everything blueward of the quasar's Ly$\alpha$ line is erased. This is the so-called Gunn-Peterson trough.

A quasar seen at $z \geq 6$ has its Ly$\alpha$ line redshifted to $$ \lambda_\mathrm{obs} = (1+z) \lambda_0 \geq 8\,500\,\mathrm{Å}, $$ which is in the infrared. But it is incorrect to say that the quasar only emits in the infrared.

Reionization

The reionization started with the first light-emitting sources, most likely hard UV from the first galaxies; a non-negligible fraction may also be from quasars, but this is probably only significant at later epochs. That is, it started already at $z\sim12$ or higher, and by $z\sim6\text{–}10$ the Universe was almost fully ionized.

So if most gas is ionized at $z=6$, why is all light blueward of Ly$\alpha$ erased? This is because the Ly$\alpha$ cross section is so large: it doesn't take more than a neutral fraction of $10^{-4}\text{–}10^{-3}$ to create the GP trough.

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    $\begingroup$ Thanks, Pela. This is pretty much what I expected. I was just confused by the wording. Thank you for such a complete answer. Your answer advanced my understanding. $\endgroup$ – Natsfan Jan 31 '20 at 16:36
  • $\begingroup$ @jmh My pleasure :) $\endgroup$ – pela Jan 31 '20 at 20:56
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They would only be visible in IR if they emitted IR They would only be visible in IR if they emitted IR, Right? Even if that's not true, I'm pretty sure he was talking about the fact that those very old quasars are only visible in IR (I say this because quasars definitely emit more than just IR). Quasars emit energies of millions, billions, or even trillions of electron volts so it would make since for them to emit infrared, but according to NASA, "In addition to infrared light, quasars also emit ultraviolet rays, radio waves, visible light, X-rays, and gamma-rays."

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Far away quasars emit light at many frequencies. However, we can only see them in the infrared. This is because all of the light emitted with a shorter wavelength gets absorbed.

See this illustration enter image description here

The light from distance quasars is redshifted on its way towards us and absorbed by the intergalactic medium. Because of this, the "bluer" light emitted by distant quasars can't reach us. Therefore, we see these objects in the infrared, but not in the visible part of the spectrum.

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