#STFCastro: Lyman-alpha forest

A forest. Not a Lyman-alpha forest.

A forest. Not a Lyman-alpha forest.

A couple of weeks ago now I attended the STFC Astronomy Introductory Summer School for incoming PhD students. I have a general overview post of the whole week here.

One of the first lectures was by Ross McLure from Edinburgh University. It was about high redshift galaxies and his particular research into them. These galaxies are extremely far away and therefore the light has travelled a very long way (and for a very long time) to get to our telescopes, so we see them as they were near the beginning of the Universe.

The part that stood out for me was his explanation of the ‘Lyman-alpha forest’. This is a feature in spectra of distant galaxies and quasars and something that has come up a few times when I’ve been reading about spectra but that I’d never been able to find an understandable explanation for.

The only way we can find out about distant astronomical objects is to look at the light reaching us from them. One way of analysing the light is to split it into all the different wavelengths you see and look at the ‘features’ you can see – essentially which wavelengths of light are bright or dim and by how much compared to the rest. This is a technique called spectroscopy (which I’ll be using a lot over the next three years!). It is a great way of working out which elements are in certain objects, and how ionised they are (how many electrons they have lost – for example by photoionisation, when a photon collides with an atom and gives one of its electrons enough energy to escape the atom).

Each line in a spectrum has its own name, related to the element and electron movement that causes that line. The line causing the Lyman-alpha forest in spectra of distant quasars is (unsurprisingly) called Lyman-alpha – this is the line for an electron within Hydrogen moving between its ‘normal’ position closest to the atom (n=1) and the next level up (n=2). The direction of electron movement determines whether it is an emission or absorption line.

In fact, spectroscopy was the technique used to first work out what our own Sun is made of. If you split the visible light from the Sun into all its different wavelengths you get an image like this:

Solar Spectrum - visible light

Solar Spectrum – visible light

Each of the black lines in this spectrum corresponds to part of the Sun’s light that is absorbed before it reaches us. Each line relates directly to a particular element or ion which absorbs that wavelength of light. This absorption actually happens in the Sun’s own atmosphere, so by working out which elements these lines correspond to we can work out which elements are present in the Sun.

Part of the Solar spectrum shown as a colour image and as a graph

Part of the Solar spectrum shown as a colour image and as a graph

While the image above is beautiful, it is a little difficult to quantitively analyse for strengths and widths of lines, so instead the data is put into graphical form. You can see how the graph corresponds to the image to the right. This shows the Sun’s spectrum in much less detail than above, but you can see how each black line in the colour variation corresponds to a dip in the graph just above it (the Y axis is the intensity of light).

You can do exactly the same in all wavelengths of light, not just visible. (For more on multiwavelength astronomy see my previous post here).

So now onto the Lyman-alpha forest:

Some of the brightest objects in the Universe are AGN (active galactic nuclei) called quasars. As they are so bright, they can be seen at a huge distance from the Earth. In the same way as certain wavelengths of the Sun’s light is absorbed by its own atmosphere before it reaches us, light from these distant quasars has to pass through vast distances of space before reaching us, and this space isn’t necessarily empty. There are gas clouds of neutral hydrogen in many directions in space, and they leave their own absorption signature on the spectra of distant objects.

How the Lyman-alpha forest occurs

How the Lyman-alpha forest occurs. From Ross McLure’s powerpoint at #STFCastro.

The further away the quasar, the more space there is for neutral hydrogen to get between it and us. It is important to remember that each of these neutral hydrogen clouds will be a different distance away from us (see image on the left).

Due to the expansion of our Universe, the further away an object is from the Earth the faster it is moving away. The Doppler effect then causes light from all distant objects to be shifted towards the red end of the spectrum (stretched towards longer wavelengths). The shift is larger for objects further away because they are moving away from us faster.

The same spectral line, for example Lyman-alpha, is then seen at different wavelengths in spectra from objects closer to us (lower redshift) than in spectra from objects further away from us (higher redshift).

The Lyman-alpha forest is the result of these effects. If you’re looking at light from a distant quasar then the light has to travel a long way to reach us. Along the way it encounters lots of clouds of neutral hydrogen. These clouds all absorb some of that light. When the light finally reaches us it has had clouds at many different distances absorb part of it. The different distances of each cloud puts its absorption line at a slight different wavelength of the spectra. If enough clouds are in the way, eventually a huge section of the spectra will have been absorbed. This section is called the Lyman-alpha forest, and once it is totally absorbed it is known as the Gunn-Peterson trough.

The image below shows the effect of this on spectra from quasars at different redshifts (distances from us). The redshift is shown as z; the higher the z value the further away the quasar is. You can see that the further away the quasar, the more of this section of the spectra is absorbed.

Spectra from quasars at different redshifts, showing the increasing effect of the Lyman-alpha forest as distance to the object increases. From Ross McLure's #STFCastro powerpoint

Spectra from quasars at different redshifts, showing the increasing effect of the Lyman-alpha forest as distance to the object increases. From Ross McLure’s #STFCastro powerpoint

As I said above, this was the first explanation of the Lyman-alpha forest that really helped me understand what was happening. I found it really satisfying to look at this effect in the spectra and be able to think about the physical processes leading to it.

If anything I’ve said isn’t clear to you please let me know in the comments and I’ll try to help.

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