The closest star to Earth is the Sun. The difference between our Sun and other stars is that the dynamics are observable with telescopes. A complex and strange part of our Sun is the chromosphere which is a part of the solar atmosphere. The chromosphere is located between the corona and photosphere where the temperature increases very rapidly within a short distance. A wavelength suited for observing this region is millimetre wavelength. A millimetre observing radio telescope suited to be pointed at the Sun is Atacama Large millimetres/submillimeter Array (ALMA). ALMA is using smaller telescopes to synthesis a larger aperture by correlating the difference between antenna pairs. This technique is called interferometry. When sampling a larger telescope, the projected distance between the smaller telescopes determines the sampling points in Fourier space. When the distance between the antennas increases, they will experience different noise due to Earth's atmosphere. This difference in noise is because the signal travels a different path through the Earth's atmosphere. Different Precipitable Water Vapour (PWV) levels in this path play a major role in this disturbance[1]. To acquire further knowledge of how different seeing effects affect ALMA it is important to enhance the understanding of what could be expected from actual observation. Realistic simulated observations can be a useful tool to extend this knowledge and is investigated in this report. This is done with the CASA (Common Astronomy Software Applications) package and simalma and the simulator tool. The simulator tool gives the possibility to include phase noise from Earth's atmosphere. This noise is calculated with atmospheric transmission at microwaves model and the aatm library [2]. This phase noise is simulated as a fluctuating PWV screen over that array that blows at specific wind speed[3].  Traditionally only thermal noise has been implemented when simulating an observation with the CASA task simalma and simobserve. Initial results indicate a big difference between traditional thermal noise and phase noise. Phase noises generate a greatly increased error as a function of radius compared to a noise free simulated observation. This effect is enhanced for higher PWV levels. This behaviour is due to that the antennas are more sensitive in the centre. This tool shows great potential since more realistic simulations give the possibility to investigate different phenomena in a controlled environment. One could optimize the reconstruction algorithm for noisy observations and investigate how physical phenomena could be affected by different seeing effects. There are a large variety of cases where this type of simulation could be used. Optimization of PWV fluctuating for specific cadences should be done first. This is important because the atmosphere is expected to behave differently at a different cadence because of different movement and averaging. However, optimization and comparison for 1.1 s cadence is doable with data generated from cosmological observations with ALMA[1].

The Cosmic Microwave Background (CMB) serves as a powerful backlight, illuminating thestructures throughout the universe. As the CMB photons travel to our telescopes from the surface of last scattering, their interactions with matter imprint detectable signatures in the CMB spectrum, known as the CMB secondary anisotropies. Among these late-time phenomena, the Sunyaev–Zel’dovich (SZ) effect—caused by scattering of the CMB photons off free electrons—is one of the most powerful, providing a unique window into the pressure and density of the electron gas. As the ionized gas and its feedback on the underlying dark matter distribution via high-energy processes present a significant obstacle to obtain precise cosmological constraints from the matter power spectrum, the SZ effect serves as an invaluable tool to address these challenges. This thesis uses the measurements of the CMB secondary anisotropies, particularly the SZ effect, from the state-of-the-art experiments, the Planck satellite and the ground-based Atacama Cosmology Telescope (ACT), in combination with Large-Scale Structure data to probe the ionized gas in distant galaxies. Chapter 2 presents the second measurement of the kinetic SZ effect in the unWISE galaxies with Planck using the projected-fields estimator. This work concludes that the ionized gas abundance in these galaxies matches the primordial-CMB predictions. Chapter 3 describes the work done to model the galaxy-halo connection of the unWISE catalog with Planck CMB lensing data using the halo model framework. It constrained the halo masses of these samples to ≈ 2 ×10¹³ _⊙/ℎ, and found that they are dominated by central galaxies, rather than satellites. These constraints can be directly used in other cross-correlations of unWISE with, e.g., the tSZ or the kSZ effect in the halo model with the upcoming CMB experiments. Chapter 5 discusses the ongoing work of cross-correlating the Dark Energy Survey Maglim galaxies with the thermal SZ maps from ACT. It measures very extended pressure profiles around Maglim, which suggest strong feedback activity in low mass objects, pushing the ionized gas far outside of the halo. This thesis also presents novel techniques to tackle the key systematics in cosmological cross-correlations. The analysis of Maglim galaxies employs the new Cosmic Infrared Background (CIB) cleaning technique, the moment-deprojection method, which ensures that the measurement is robust to this foreground. Chapter 4 discusses three new methods to remove the CIB and tSZ contamination, using the external Large-Scale Structure data which show a large correlation with both fields (e.g., the unWISE catalog). With the new methods presented, it is possible to remove those contaminants to enhance the measurements of the blackbody component of a CMB map. The results presented in this thesis offer a unique window into the baryons residing in distant galaxies through the SZ effect, confirming there is no missing baryons, and indicating that the feedback is stronger than predicted in simulations. These analyses lay the groundwork for cross-correlations of the upcoming high-resolution, low-noise CMB experiments such as the Simons Observatory, and high density galaxy surveys, including DESI, Euclid, or LSST. The upcoming measurements will yield precise constraints on gas physics, transforming our understanding of galaxy formation, and enabling cosmological constraints from the matter power spectrum, where baryons currently represent the primary uncertainty.