We investigate in detail the various evolutionary phases of galaxies — active star-forming, quiescent, and green valley. We specifically focus on post-starburst (PSB) systems, which are galaxies that, according to Maltby et al. [2018], ”have experienced a recent burst of star formation followed by a rapid quenching on the timescale of tens of Myrs”. We base our work on a sample with 248,307 sources at 0.5 < z < 3.5 from the COSMOS2020 dataset where we use UV J color-color diagrams (divided into 0.5 redshift bins and then into 1 Gyr bins) to analyze and classify the galaxies into star-forming, quiescent and green valley galaxies, observing the dichotomy between them. We find that the sample is predominantly composed of star-forming galaxies (∼ 92%). With CIGALE simulations, we reproduce the COSMOS data and analyze a sample representative of passive galaxies with 12, 084 sources with the simulator. We classify PSB as galaxies that have a quenching timescale lower than 0.7 Gyr. Our results indicate that while the absolute number of passive galaxies decreases with redshift, the percentage of PSB systems increases, reaching∼ 31% at z ∼ 2.5 and ∼ 2% at z ∼ 1.0, which aligns with previous studies. We conclude that models with just star formation history (without AGNs or mergers) can predict a small fraction of PSB galaxies, ∼ 3%. Lastly, we also find evidence that the speed of transition from star-forming to passive galaxies depends on redshift: a slow quenching at lower redshifts and a fast quenching at higher redshifts, increasing in stellar mass. After the peak of star formation and quiescence, quenching mechanisms tend to become more secular, leading to slower quenching timescales and a lower fraction of PSB galaxies. Before the peak, quenching mechanisms are dominated by faster processes, resulting in shorter timescales and a higher fraction of PSB systems.

Tese de mestrado, Física (Astrofísica e Cosmologia), 2025, Universidade de Lisboa, Faculdade de Ciências