Galaxy clusters, located at the node of the cosmic web, are the largest gravitationally bound structures in the Universe. Their abundance and spatial distribution are very sensitive to cosmological parameters, such the matter density in the Universe. Galaxy clusters thus constitute a powerful cosmological probe. They have proven to be an efficient probe in the last years (Planck, South Pole Telescope, XXL, etc.) and they are expected to make great progress in the coming years (Euclid, Vera Rubin Observatory, Simons Observatory, CMB- S4, etc.).
The cosmological power of galaxy clusters increases with the size of the redshift range covered by the catalogue. The attached figure shows the redshift ranges covered by the catalogues of galaxy clusters extracted from experiments observing the cosmic microwave background (first light emitted in the Universe 380,000 years after the Big Bag). One can see that Planck detected the most massive clusters in the Universe in the redshift range 0<z<1. SPT and ACT are more sensitive but covered less sky: they detected tens of clusters between z=1 and z=1.5, and a few clusters between z=1.5 and z=2. The next generation of instruments (Simons Observatory starting in 2024 and CMB- S4 starting in 2032) will routinely detect clusters in 1<z<2 and will observe the first clusters formed in the Universe in 2<z<3.
Only the experiments studying the cosmic microwave background will be able to observe the hot gas in these first clusters at 2<z<3, thanks to the SZ effect, named after its discoverers Sunyaev and Zel’dovich. This effect is due to high energetic electrons of the gas, which distorts the frequency spectrum of the cosmic microwave background, and is detectable in current experiments. But the gas is not the only component emitting in galaxy clusters: galaxies inside the clusters can also emit in radio or in infrared, contaminating the SZ signal. This contamination is weak at z<1 but increases drastically with redshift. One expects that the emission from radio and infrared galaxies in clusters are of the same order of magnitude as the SZ signal in 2<z<3.
One thus needs to understand and model the emission of the gas as a function of redshift, but also the emission of radio and infrared galaxies inside the clusters to be ready to detect the first clusters in the Universe. Irfu/DPhP developed the first tools for detecting clusters of galaxies in cosmic microwave background data in the 2000s. These tools have been used successfully on Planck data and on ground-based data, such as the data from the SPT experiment. They are efficient at detecting clusters of galaxies whose emission is dominated by the gas, but their performance is unknown when the emission from radio and infrared galaxies is significant.
This thesis will first study and model the radio and infrared emission from galaxies in the clusters detected in the cosmic microwave background data (Planck, SPT and ACT) as a function of redshift.
Secondly, one will quantify the impact of these emissions on existing cluster detection tools, in the redshift range currently being probed (0<z<2) and then in the future redshift range (2<z<3).
Finally, based on our knowledge of these radio and infrared emissions from galaxies in clusters, we will develop a new cluster extraction tool for high redshift clusters (2<z<3) to maximize the detection efficiency and control selection effects, that is the number of detected clusters compared to the total number of clusters.