Galaxy Hunting: Level 1

I was uncharacteristically calm before starting this internship. Clearly my subconscious knew that I would be going into a fun, supportive and (relative to the preceding stress of third year exams) relaxed environment. The work so far has been interesting and enjoyable, if not a little frustrating at times. This frustration has mainly been due to my lack of proficiency with computers and so I really feel like my computer skills are going to improve drastically over the next few weeks. Outside of the lab all the interns have been getting on well, and we’ve been warmly welcomed into the wider research group and department having been invited to tea and coffee breaks, the weekly journal club and a talk from a visiting academic from the University of Tokyo.

Our group’s first week has been focussed on getting to grips with the datacube. This is a 2.9GB file, containing a 2D image of a very small section of the sky, ‘taken to 3D’ by recording the incoming photon flux at a range of wavelengths. The data comes from The Multi Unit Spectroscopic Explorer (MUSE), an instrument of the Very Large Telescope (VLT) at the European Southern Observatory (ESO). (Astrophysicists like acronyms it would seem). We originally scanned through the wavelengths manually, identifying sources (i.e. galaxies) at random. We saw that there were both sources that appeared continuously throughout the range of wavelengths and ones which only appeared at very specific wavelengths. These specific wavelengths correspond to specific emission lines and eventually by extracting the spectra of each source and comparing with common galaxy emission lines, we hope to be able to determine the redshift of all the galaxies, as well as other interesting properties.

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The spectrum extracted from a merger of three of our galaxies, showing a very strong emission line at aroung 6700 Angstrom

However, after each obtaining a random selection of galaxies, very few of which overlapped, we soon realised we needed a more systematic approach. We wrote (ok, selectively copied) a programme to compress the image in the wavelength axis, simultaneously removing noise by only recording counts above the three sigma level. This three sigma noise level was itself determined from another programme we wrote. All of this allowed us to see all the sources at once. We split this image into five ‘streets’ (literally named after the streets each of us live on because why not) and each went through our streets recording every source that appeared. It was quite difficult to objectively determine sources from noise, even with the removal of most of said noise by the three sigma limit, so we used an image from the Hubble telescope to compare against our MUSE data. We have ended this week with a (hopefully) comprehensive catalogue of around 150 galaxies. This number is both exciting and intimidating and has really reminded me how incredibly massive and exciting our universe is.

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Just some of the galaxies in just one of our streets

My favourite part of the week was working out a naming convention for all our newly discovered galaxies. After Google determined that there is no completely standardised naming convention, we just decided to have fun with it. Somehow, as a group, we have become extremely fixated on pigeons (it’s best not to ask), so we originally tried to work an acronym around this. However PIGEON (PickInG lettErs frOm aNywhere) was decided to be a bit long for a catalogue name, so we shortened it to PIG (Potentially Interesting Galaxy). We then uniquely identified each galaxy using the first four digits after the decimal point of the RA and Dec angles – eg. PIG-7286-1237.

Finally, here are three of the most important lessons I feel I have learnt this week:

  1. If you know what you want to do with your code but just can’t get it to work, then ask for help. Preferably before you’ve wasted a whole extremely frustrating morning on it.
  2. Macs are actually quite good and I probably shouldn’t have avoided them for the first 3 years of my degree.
  3. Everything can be related to pigeons.

~Katie

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