XGAL Lancaster to the World, via Heather – Weeks 1 & 2

Welcome to my first internship blog post!

A big part of the internship that I have been working on is the educational videos for XGAL-DIY part of the XGAL Blog. The general idea of XGAL-DIY is to have a task that people of any age can carry out which involves finding high redshift, very distant galaxies, presented in an exciting way which is super easy to follow. This is all linked to the discovery of CR7, which was a real and impressive discovery by a group led by Dr David Sobral here at Lancaster University, hence the excitement of the XGAL team to share it with the public. 

So here’s the theory video! Feel free to like the video and subscribe to the XGAL channel.

This is the initial video of the series, which is the theory video, explaining a lot of the background concepts. I was working from an initial draft made by Amaia, which definitely made my job easier. In order to build on it though I had to refresh my knowledge of redshift and telescope filters but entirely learn from scratch about the Lyman break. This is the feature seen in the spectra of high redshift galaxies where, below a wavelength of 1216Å  (Å = Angstrom = 10^-10m), no light is collected by the telescope for the galaxy. The reason that wavelengths shorter than 1216Å are not seen by the telescope is because they are absorbed by the clouds of neutral hydrogen within the galaxy itself and along the line of sight (in between the galaxy and the telescope).  The value of 1216Å refers to the rest-frame wavelength (the wavelength actually emitted by the galaxy), but the observed wavelength (the wavelength collected by the telescope) where the Lyman break occurs will depend on the redshift of the galaxy, described by the following equation:

Here, λ is the wavelength and z is the redshift. In order to fully explain the concepts of both the Lyman break and redshift, I made this video:

Figure 3: A visualisation of how redshift changes the wavelength at which the Lyman break is observed.

This is the spectrum of a test galaxy and it shows that increasing the redshift also increases the wavelength that the Lyman break is seen. It also involves the filter profiles of the telescope used to collect this galaxy’s data. These were added to illustrate the idea that, depending on the redshift of the galaxy, it will be visible in some filters but not visible in others, due to the Lyman break.

The graphs used in this clip were made using a Python script which implements one of the fancy new LancAstro.py methods to plot the spectrum. For more information of the LancAstro.py project, check out the blog: https://xgalweb.wordpress.com/lancastro-py/.

The Plot2D.py script from LancAstro.py made it very simple for me to plot graphs which looked incredible without having to spend hours adjusting all of the axes and ticks etc; this had already been done for me! In order to have the redshift increase by 0.1 repeatedly, I needed a ‘for’ loop. This essentially meant that the equation shown above was calculated for values of redshift from 0 to 10, in steps of 0.1. Hence, 100 graphs were made, each one with the spectrum shifted slightly to the right compared to the previous one. This was converted into a smooth video using iMovie, which was really simple to use. 

So this is just the theory part of the task, meaning the next steps are to produce the instructions video, so people can find their own galaxies from real data. This is a super exciting thing to be involved with and I really hope it gets a lot of engagement and teaches people some astrophysics whilst they also have fun!

Another part of the project that I have been working on is the 3D visualisation of a ‘fly through’ of the COSMOS field data. This initially felt like an overwhelming task that would never be completed! However, when broken into simple steps is seemed much more achievable. First of all, I had to learn how to use DS9, reacquaint myself with Python (it has been a while since I finished my MPhys project) and more specifically learn how to use .fits files in Python. Oh, and learn how to use a Mac! DS9 is a super useful program which makes viewing and editting images taken by telescopes.

After lots of hard work and confusion with scale parameters (see Figure 2 in Amaia’s blog: https://xgalweb.wordpress.com/category/xgal-from-lancaster-to-the-world/) we finally had some videos which were a little on the way towards what we wanted:

Figure 1: The most basic fly-by, just positioning galaxies and zooming in on them.
Figure 2: A dramatically improved fly-by with many more galaxies, but still in need of a lot of improvements.

The first little clip, Figure 1, is a little example which just took a few images that had been created by hand in DS9 and, using a Python script, they were zoomed in on in a way that kind of represented us flying, but in an extremely basic way which didn’t include the redshift or true positions of the galaxies – this was just one of the many small steps necessary to complete on the way to the final video!

The next, Figure 2, is slightly more impressive. This actually involves all of the galaxy images in the COSMOS field which had been created fully in Python rather than DS9. However some adjustments to these images need to be made, because, as you can see, some are just blurs or grainy images that don’t even slightly look like galaxies. The first 20 seconds actually looks like we are flying towards all these galaxies, as we zoom in on them gradually. However, after 20 seconds, things get a bit…. Weird. The zooming just has a mind of its own as we zoom outwards again and then everything gets a little warped and stretched, but fear not – improvements are on the way! 

So this explains the first two weeks of my internship, and I’m super excited for the next four! I’d like to thank everyone else in the AstroLab – Emma, Charlie, Amaia, Cass, Josh and Harry – for all the help with coding and ideas and, more importantly, all the fun we have in between working and making me feel really welcome here. Amaia also gets a special shout-out for being my internship buddy, producing the first draft of the theory video and helping out whenever I don’t know how to use the Macs. 

Keep your eyes out for more blog posts, updates on the visualisation and more uploads to the XGAL-DIY section!

Heather Wade

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