Galaxy Hunting: Level 2

Now that we had all the galaxies, it was time to start exploring them.

The focus of this week has been analysing the spectra of our sources. With a spectrum, you can deduce all sorts of properties – whether a galaxy is close or distant, metal-rich or metal poor, star-forming or dead.

In a lab on Earth, you find that each element emits light at specific wavelengths. Energised electrons in atoms can jump to a lower energy, releasing their energy as light. On a spectrum, these appear as bright ’emission lines’ against a dark background.

You find how far away a source is by finding it’s ‘redshift’. As the universe is expanding, light from distant galaxies is stretched out as it travels towards us. This stretching increases the wavelength – in other words the light appears redder (hence ‘redshift’). When looking at their spectra, all the emission lines appear shifted towards the red. By identifying the correct line and comparing where it is to where it should be, you can find the redshift. Light from more distant sources is stretched out more, so by finding the shift gives you can also calculate its distance.


A nice example emission spectrum, from source PIG-7354-1408. The clearest spectral lines are labelled.

It turns out there is no easy way to code this, so we have been identifying peaks by eye. Many of our spectra are currently full of noise, or only have one line, which could be almost anything. Despite this, we have now identified over 30 sources with two or more identifiable spectral lines, allowing us to calculate the redshift (and therefore distance) with confidence!

At the moment most of our redshifts are quite small (less than 1), which is quite close in astronomical terms, but we expect to see more distant sources once we reduce the noise further.

Lines kept appearing at the same points from sources at completely different locations. These could only be actual lines if each source was at exactly the same distance, which seems suspicious.  We tried inventing a new element (pigeonium) to explain them, we realised the truth was much less exciting and sadly not worthy of a Nobel Prize. Sky-lines are emissions from our atmosphere that appear exactly as emission lines would when you look at the spectra (see Katie’s post for more details).

By finding the area under a spectral line, we found its ‘flux’ (roughly speaking, how bright the source appears on Earth). Since we know the distance, we can find the actual luminosity of the source, and not just how bright it appears to us.

Since then we have started finding more properties about the sources, such as  star formation rate (SFR) (mass of stars formed in a galaxy per unit time) and metallicity (the percentage of mass in a galaxy that isn’t hydrogen or helium – yes, any element other than those two counts as a ‘metal’ in astrophysics). There are fairly simple mathematical relations between the fluxes of particular spectral lines and these properties.

In our first attempt at finding the SFR of a galaxy we found a value of 3.9×10^20 solar masses per year. This would mean that a single galaxy produces a billion Milky-Way’s worth of stars each year. A bit much, perhaps. 

Again, it turns out we had not made a major discovery. Instead, we had overlooked a factor of 10^20 in our units! Correcting for this, we got the much more reasonable value of 3.9 solar masses per year. I have made plenty of these sort of mistakes, but it’s all valuable learning!

Discovering these galaxies is very exciting for me. It is brilliant to learn from the academics and PhD students here, who have far better knowledge of all this stuff than I do! I have only just begun to appreciate how much you can find out from a simple spectrum. Think of it this way – that picture above holds almost all of the information that we can ever hope to find about these galaxies! I am looking forward to next week, as we find delve deeper into the properties of galaxies that we ourselves have found.


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