Neutral Hydrogen intensity mapping with MeerKAT

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During the last two decades we have entered a "golden era" of cosmology. Using satellites and ground based telescopes we have gathered high quality data from the very early Universe, essentially from light emitted right after the Big Bang explosion, as well as from the late Universe, through the light emitted from stars and galaxies. However, a big part of our Universe's history and volume remains unexplored. A way to attack this challenge is by observing the light emitted from the neutral hydrogen (HI) that filled the Universe for a long time after the Big Bang and before the first galaxies were formed. After that time HI resides within galaxies, so we can also use it as a novel way to study the late Universe. This is my main area of research; it is exciting because it opens a new observational window into the Universe and can push the boundaries of our understanding of astrophysics and cosmology. In the next few years, HI surveys of exquisite sensitivity will be performed using radio telescopes, and part of the proposed research is working out new observational methods and techniques in order to maximise their science output. I have pioneered a new observational method that does not require the -difficult and expensive- detection of individual galaxies but maps the entire HI flux coming from many galaxies together in large 3D pixels (across the sky and along time). I aim to use this method to provide a 3D map of the Universe using HI intensity mapping data from the MeerKAT array. MeerKAT is a radio telescope located in Karoo, South Africa, and it is a pathfinder for the Square Kilometre Array, which is going to be the largest radio telescope in the world. By using these data I want to answer fundamental questions about the Universe, for example what is the nature of dark energy - the mysterious substance responsible for the accelerated expansion of our Universe - and how galaxies evolve. But before doing that, much work is needed to improve the quality of the data and figuring out how to solve the many challenges that intensity mapping faces. One of these challenges has to do with the huge emitted radiation (called "galactic synchrotron") from our own Galaxy, that severely contaminates our HI maps. In other words, when MeerKAT observes the night sky, it maps the HI cosmological signal, but it also maps the emitted synchrotron radiation, which is a thousand times larger than the signal! The cosmological signal we want to measure is therefore "buried" under a much larger astrophysical signal. We can attack this problem by exploiting the fact that foregrounds are smooth functions of frequency, while the HI signal has a spiky structure. So, in principle, foregrounds can be perfectly removed without losing any signal information. However in practice this is very challenging. I will use the available MeerKAT data sets to test the performance of various foreground cleaning and calibration techniques. After the data has been processed and cleaned, I will move on to build a pipeline for the cosmological analysis of the HI intensity mapping signal. This pipeline will also account for the possibility of powerful synergies between HI and traditional optical galaxy surveys, by including cross-correlations data analysis tools. This is useful in order to obtain measurements that are free of systematic contaminations that often plague individual surveys (but drop out when combining them), and therefore more robust. Our developed tools will be used to analyse Science Verification data taken with MeerKAT, i.e. data that cover a larger part of the sky and observe for longer time, so they can be used to make precise cosmological measurements. We then hope to perform the first ever measurements of HI and cosmological parameters in the radio wavelength using the intensity mapping technique, exploit multi-wavelength synergies, and revolutionarise our understanding of galaxy evolution and dark energy.