A review of galaxy quenching — Part I: Defining the problem and observational results

The goal of this review article series is to provide a comprehensive overview of galactic star formation and quenching from both an observational and theoretical perspective. Drawing on a vast quantity of literature, we attempt to answer a deceptively simple question: why do galaxies cease forming stars? In Part I, we concentrate on observational results, especially from the past two decades, where there have been enormous advances made toward answering this fundamental question. In Part II, we focus on theory and simulations, including discussion of direct observational tests thereof. Observationally, the advent of wide-field spectroscopic galaxy surveys, spatially resolved spectroscopy, high-resolution sub-mm and radio observations, X-ray observations, the Hubble Space Telescope (HST), and most recently the James Webb Space Telescope (JWST) have all contributed significantly to our empirical knowledge of star formation and its demise in galaxy quenching. We discuss the fundamental problems which quenching aims to solve, various routes to identifying quiescent galaxies in observations, observational constraints on quenching from studies of galaxy populations, and the spatially resolved view of quenching. We end this part of the review with a detailed discussion of the latest results on galaxy quenching at the high-redshift frontier from JWST observations.