Quenching (astronomy)

In astronomy, quenching refers to the shutting-down of star formation within a galaxy. A galaxy where star formation has quenched is known as a quenched or quiescent galaxy. Quenching is an important phenomenon in the study of galaxy evolution, as all galaxies can be divided into two fundamental types: actively star-forming or quenched. Compared to a star-forming counterpart, a quenched galaxy tends to be redder in the visible spectrum and contain older stellar populations, a direct consequence of its star formation being shut off. Most elliptical and lenticular galaxies known to date have these features, which, along with their weak star formation, qualify them as quiescent. Additionally, quenched galaxies also exist in more massive dark matter halos and can be found in denser environments, such as clusters or groups. Until recently, most quenched galaxies have been found in the local Universe. Since the late 2010s, deep-field surveys in near-infrared bands, including some by the James Webb Space Telescope, have found a number of quenched galaxies in the early Universe. Various mechanisms have been proposed as drivers of quenching, but their relevance depends on the age, mass, and environmental conditions of each quenched galaxy. These mechanisms can be divided into two classes based on their origins: internal (coming from within the galaxy being quenched) and environmental (coming from surrounding galaxies). Internal mechanisms, most notably active galactic nucleus (AGN) feedback, are responsible for most of the quenching seen in high-mass galaxies, while environmental mechanisms contribute to the quenching of low-mass galaxies, especially if said galaxies are satellites around a more massive central galaxy.