DEPARTMENT OF PHYSICS DISSERTATION DEFENSE: Bernardita Ried Guachalla

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Title: 

Tracing Cosmic Baryons with the Sunyaev-Zel'dovich Effects

Abstract: 

The cosmic microwave background (CMB) constitutes one of the most powerful probes of the expanding Universe. Originating approximately 380,000 years after the Big Bang, this relic blackbody radiation is remarkably isotropic, with small temperature fluctuations, known as primordial anisotropies, that encode fundamental information about the physics and composition of the early Universe. Beyond its primary anisotropies from the early Universe, the CMB also carries imprints of the late-time Universe: as CMB photons traverse the evolving cosmic web, they acquire secondary anisotropies through interactions with intervening structures formed during cosmic evolution. In particular, inverse Compton scattering of CMB photons off energetic electrons in ionized gas gives rise to the Sunyaev–Zel'dovich (SZ) effects. The thermal component (tSZ) traces the integrated pressure of hot gas, while the kinematic component (kSZ), arising from Doppler shifts due to bulk motions of electrons, probes the momentum field of baryons. These effects provide complementary observables, enabling the CMB to act as a cosmic backlight for studying ionized gas across a wide range of halo masses and redshifts. Such measurements offer a unique avenue to address outstanding problems in astrophysics and cosmology, including: the distribution of the ``missing baryons'' in circumgalactic media; the efficiency and spatial extent of feedback processes in galaxy formation; and the characterization of baryonic physics for precision weak gravitational lensing analyses. Recent advances in survey capabilities are enabling increasingly precise measurements of these signals across cosmic time. The combination of data from the Atacama Cosmology Telescope (ACT) with spectroscopic samples from the Dark Energy Spectroscopic Instrument (DESI) provides unprecedented statistical power, with further improvements anticipated from the Simons Observatory and the Vera C. Rubin Observatory. This dissertation presents a comprehensive study of the Sunyaev–Zel'dovich effects as probes of gas thermodynamics and large-scale velocity fields. It includes: (i) precision measurements of extended gas profiles via kSZ stacking, achieving signal-to-noise ratios exceeding 10$\sigma$ and 18$\sigma$ with ACT×DESI Data Releases 1 and 3 respectively; (ii) the development and validation of velocity reconstruction techniques for spectroscopic and photometric surveys, with performance benchmarks for DESI and Rubin Observatory; (iii) systematic analyses of gas thermodynamics using state-of-the-art hydrodynamical simulations (e.g., IllustrisTNG) and N-body simulations (e.g., AbacusSummit). Our measurements provide strong evidence that gaseous halos around galaxy groups extend significantly beyond their dark matter distributions, suggesting that feedback processes redistribute baryons more efficiently than predicted by current hydrodynamical simulations. These conclusions rest on high-significance kSZ detections and are validated through systematic treatment of halo occupation distribution uncertainties, extensive tests against simulations, and demonstration across multiple galaxy populations including luminous red galaxies, bright galaxies, and emission-line galaxies. Overall, this work establishes the Sunyaev–Zel’dovich effects as precision tools for probing baryonic physics and large-scale structure, and lays the groundwork for future analyses with next-generation cosmological surveys.