What is the Biggest Ever Redshift?

A discussion on the Physics Teachers’ Network requested advice on “What is the biggest ever redshift detected?”

Research shows it was a redshift, z = 11.09 for galaxy GN-z11; and the measurements were  taken in the near infra red using Hubble’s Wide Field Camera.

This is a big question because effectively we are seeing the furthest galaxy back in time.  It is 32.2 billion years away and came into existence 400 million years after big bang.  So if the Universe is only 13.8 billion years old then how come we can see something so far away?

During this time the Universe was opaque and full of neutral atoms.

Professor Martin Hendry supplied an interesting reply.

In some cases we can determine the redshift of a galaxy by measuring the wavelength of a particular spectral line that corresponds to a particular transition of an electron in a hydrogen atom.  For example the Lyman alpha emission line is the result of an electron dropping down from the n=2 energy level to the n=1 energy level, and the presence of this spectral line is often seen as an indicator of a recent burst of new stars forming as one might expect to see in a very young, recently formed galaxy.  (This line was proposed as a tell-tale sign of a very young galaxy by Bruce Partridge and Jim Peebles – awarded the Nobel Prize for physics this week: see e.g. https://en.wikipedia.org/wiki/Lyman-alpha_emitter).  This line has a wavelength of 121.567 nm in the rest frame of the hydrogen atom.  If a galaxy is a strong Lyman alpha emitter, and the line is observed at wavelength lambda, then by comparing the observed wavelength with the 121nm at which it was emitted we can measure the redshift of the galaxy.

(Of course if this spectral line is redshifted then how do you know it’s a Lyman alpha line?  Likewise for any other spectral line.  Often it’s the combination of several spectral lines and their relative spacing that gives the game away – a bit like a bar code in the supermarket.  You could imagine enlarging the image of a bar code in a photocopied and, generally, it’d still be recognisable as the overall pattern would still be the giveaway).

In fact for this record-holding galaxy, the redshift was determined a slightly different way, from the Lyman series but not the Lyman alpha line and not from an emission line but an *absorption* line: specifically it was determined from the “Lyman break” – i.e. the limiting wavelength that corresponds to the amount of photon energy you need to absorb to allow an electron in the n=1 energy level to escape from its hydrogen atom altogether.   That is a higher energy (and so a higher frequency, and a shorter wavelength) than the Lyman alpha line, and in fact corresponds to about 91 nm in the rest frame of the hydrogen atom.   Any photons that have even higher energies (and thus even shorter wavelengths) than this get absorbed by the (lots of) neutral hydrogen that is around in the Universe at that time; these photons thus *ionise* that neutral hydrogen.  This is sometimes referred to as “re-ionisation” in the sense that the universe was fully ionised when it was much younger, because it was much hotter, then it cools enough for neutral hydrogen to form – i.e. when the CMBR was emitted – and now it’s being ionised again.  Where are the high-energy photons coming from to do this ionising (being absorbed in the process)?  They are believed to come from hot young stars – i.e. the newly formed stars in these young galaxies.  (Remember, the more massive the star the hotter their surface temperature, so massive blue stars emit lots more of these energetic photons than cooler red stars do).

So, in summary, the spectrum of light from a galaxy as a whole drops off at the Lyman break, like a “cliff edge” because at shorter wavelengths than the Lyman break these photons get absorbed, ionising the hydrogen gas in their environments.

You can then play the same game as with an emission line: look for where this “cliff edge” appears in the observed spectrum and then use that observed wavelength (which will be much longer than 91nm) to estimate the redshift.

The research paper on GN-z11 is at https://arxiv.org/pdf/1603.00461.pdf, and is actually pretty readable I think…

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