Aging Galaxies in a Young Universe: Testing ΛCDM with JWST Observations of Quiescent Galaxies

Recent JWST observations have unveiled a surprising population of massive, quiescent galaxies at redshifts around 3 to 5, whose ages and formation times challenge the conventional ΛCDM cosmological model. This tension, often referred to as the "impossibly early galaxy" problem, raises fundamental questions about how and when the first massive galaxies assembled in the Universe. This thesis investigates the impact of the full-spectral fitting modeling assumptions, especially the role of age priors, on the inferred physical properties of these distant galaxies. By relaxing cosmological age constraints, the analysis explores whether observed star formation histories (SFHs) remain compatible with ΛCDM predictions or if they suggest alternative cosmological scenarios. The results reveal a complex picture: while some galaxies show consistent ages and SFHs across different parameterizations, others display a variety of SFHs that produce indistinguishable spectra yet imply widely different stellar ages. When age priors are removed, some of these ages exceed the age of the Universe at the observed redshift. These results raise potential concerns about interpreting some inferred properties as being in tension with the ΛCDM framework. Nevertheless, it is noteworthy that the median stellar ages remain remarkably stable across different models, reinforcing their reliability for cosmological studies. To clarify these findings, simplified theoretical models are developed to probe the degeneracies between old and young stellar populations in spectral fits. This work highlights the critical importance of modeling choices in interpreting early galaxy observations and contributes to the ongoing debate about massive galaxy formation in the young Universe. Moreover, it shows that detecting massive, passive galaxies at high redshift is promising, as younger stars reduce ambiguities and yield robust constraints, making them valuable probes of the ΛCDM model.