Why Astronomers Love Spectra

Jonathan McDowell, 2022 Jul

The public are excited by the newly released JWST images, but astronomers are as excited by the spectra, like this one. I want to talk about why.

The data are the white wiggly line which show the amout of light at each precise color. There are particular spikes where there's more light, for example the one marked 'hydrogen' at 4.61 or so.

The presence of four such spikes ("spectral lines") in a very precise pattern is a fingerprint of the element hydrogen. We know that light must have been produced by hydrogen atoms. The spacing of the four lines marked in blue is also distinctiive and is a fingerprint of oxygen

You can think of the pattern as a tune. Going from left to right in the image, the oxygen lines are like "dah...long pause .. dah.. long pause... DAH'DAH!" (small spike, gap, small spike, gap, two big spikes next to each other). It's oxygen's theme tune.

We can see the same 'theme tune' when we heat oxygen gas in the lab. Here we're seeing it (redshifted, but the same relative pattern) in a galaxy 13.1 billion light years away. This tells us two things:

• 1) There's oxygen in that there galaxy!
• 2) Oxygen atoms behave the same billions of years ago and billions of light years away as they do here and now - a profound proof of the unity of physics

The exact pattern of the oxygen lines, and their individual profiles, let us figure out the properties of the gas that shone to make them. So this spectrum tells us things like density, temperature, speed, distance... lots of stuff

I think it's helpful to see the SMACS J0723.3-7327 image (and other astronomy images) as a crime scene. Which galaxies are causing the bending in the light of the others? Which galaxy smashed into that other one to make the tidal tail? So, we are the galaxy police...

First on the scene, yes, we take a picture of the crime scene. But the next thing we do is send in the CSI unit to take DNA swabs of all the galaxies for forensic analysis.

The spectra are the astronomer's equivalent of the DNA swabs. What the galaxies were up to is ENCODED IN THEIR LIGHT. The spectrum is like a DNA fingerprint of their history, and we can go all CSI on it to figure out what is going on, in far more detail than for the image.

So yes, you can do science with the pretty pictures, but often it's the spectra that really let us tell the story. End rant!