Time travelling through the Universe to discover and study primeval galaxies – Week 1

During my first week, I started to gather the knowledge I will need to complete my project. My internship revolves around the study of SC4K, a catalogue that contains much information about 3908  galaxies located in the COSMOS field, a region of the sky that has been much studied by astrophysicists. 

This project relies on the use of Python, a programming language I am familiar with. However, I had never used Astropy before starting this internship. Astropy is a collection of software packages written in Python and designed to be used in astrophysics projects. It allows us, among many other things, to manipulate astronomical quantities that have values and units, to use astronomical time, dates and coordinate systems, to handle data tables, to create models, and to visualize data. I spent time during this first week learning about the basics of Astropy, using mostly different sources: the official astropy documentation (https://docs.astropy.org/en/stable/), https://python4astronomers.github.io/, learn.astropy.org, and https://astropy4cambridge.readthedocs.io/en/latest/

I was given the complete SC4K catalogue, as well as the two Python scripts that were used to generate it. I did my best to analyse them and to try to understand how all the modules and functions work, what they do and how they are used. The scripts make use of an algorithm called SExtractor, which prompted my interest, and I managed to find the original paper in which it was published, Bertin and Arnouts, 1996 (https://ui.adsabs.harvard.edu/abs/1996A&AS..117..393B/abstract), to learn more about the mechanisms behind it. 

The SC4K catalogue is available as a FITS file, a format mostly used to store scientific data. This file can be opened with Astropy, or with astronomy softwares such as TOPCAT (Tool for OPerations on Catalogues), which is described as “an interactive graphical viewer and editor for tabular data” by its creator Mark Taylor. Here is a screenshot of the SC4K catalogue as seen in TOPCAT:

In order to have a better understanding of the theory behind the creation of the SC4K catalogue, I started carefully reading the main paper that explains its conception, Sobral et al., 2018 (https://arxiv.org/abs/1712.04451). Reading scientific papers is an arduous art that I just started learning, but I found some very interesting guidance in a paper intended to explain how to efficiently read astronomy papers for people who are not used to it yet: Cooke et al., 2020 (https://arxiv.org/abs/2006.12566). As with many other activities, I am convinced that practice is key, and I will probably have to read plenty of papers before I become more comfortable with them. It’s actually something I’m looking forward to!

But for now let’s go back to Sobral et al., 2018: it presents works performed with data obtained from the Subaru and Isaac Newton telescopes, located respectively at Mauna Kea  in Hawaiʻi and on the island of La Palma in the Canary Islands. The said data was used to identify nearly 4000 galaxies in the COSMOS field using 16 narrow- and medium-band filters, thus slicing the Universe into 16 cosmic periods, from redshift z ~ 2 to z ~ 6. These galaxies are all Ly-α emitters (LAEs), selected among thousands of possible candidates according to strict criteria. Ly-α transitions occur in Hydrogen atoms, when an electron goes from the level n = 2 to to the level n = 1. Because Ly-α emission lines are very strong, they are excellent tools to detect distant galaxies. David Sobral and his team were even able to create a 3D map of the galaxies they identified:

Source: Sobral et al., 2018

I also carefully read parts of the thesis of Dr Jorryt Matthee of and the thesis of (soon Dr) Sérgio da Graça Santos. They gave me important information about cosmology and extragalactic astrophysics that helped me understand better the research ongoing in the XGAL group. Going through scientific works of hundreds of pages discussing cutting-edge research is both intimidating and fascinating, but I mostly focused on the introductions of these two thesis, as they are the parts that will be the most relevant for my project. 

This first week was also an opportunity to discover the strange world of remote working. Socializing behind a screen is not an easy task, and it requires skills that I will need to acquire along the way. This summer, the weekly astro lunch and astro tea sessions are held on Zoom, the famous conference app that became viral all across the world in the past few months as more and more countries imposed a lockdown and urged their citizens to work from home when possible. I was very happy to see again students and professors I had met last year during my first internship at Lancaster University, and to meet new people. In addition to these informal meetings, each Tuesday members of the XGAL group discuss their more recent work together. Finally, on Thursday evenings, the journal club takes place, during which several members from the Observational Astrophysics group present and explain astronomy papers recently published by other researchers.

This first week was full of discoveries for me, from having my first Zoom meetings to learning about the detection of galaxies in the early universe, and I am very excited about the next steps of the project!

Louis Marinho Fernandes

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