Galaxies, with their billions of stars, are subject to a slow, inevitable process of transformation that astronomers refer to as “death.” This galactic demise is a cessation of the fundamental activity that defines a galaxy: the birth of new stars. The fate of these structures is tied to the depletion or removal of the cold gas that serves as their star-making fuel.
Defining Galactic Quenching
The scientific term for a galaxy’s death is galactic quenching, which is the process where a galaxy’s star formation activity is suppressed to a negligible level. Astronomers define a quenched galaxy as one whose star formation rate is less than about one-tenth that of a typical star-forming galaxy of the same mass. This transition takes a significant amount of time, often extending over billions of years, but is still considered rapid compared to the overall age of the universe.
The physical change is visibly dramatic, shifting a galaxy’s appearance from the “blue cloud” to the “red sequence” of galaxies. Star-forming galaxies, like our own Milky Way, appear blue due to the presence of hot, short-lived, newly formed blue stars. Once star formation ceases, these bright stars die off, leaving behind a population of older, cooler, redder stars.
Quenched galaxies are often smooth, gas-poor elliptical or lenticular shapes, sometimes referred to as “red and dead” because of their color and lack of star birth. This shift in color and morphology confirms that a galaxy’s life cycle has concluded. The cessation of star formation requires an explanation of how a galaxy’s reservoir of cold, dense gas—the raw material for stars—is removed or prevented from cooling.
Internal Mechanism: Exhausting Star-Making Fuel
One major mechanism for quenching originates from processes within the galaxy itself, particularly the activity of the central supermassive black hole. Nearly all massive galaxies host an Active Galactic Nucleus (AGN) at their core, formed by gas spiraling into the black hole. This accretion process releases enormous energy, often far exceeding the total light output of all the galaxy’s stars combined.
This intense energy output, known as AGN feedback, acts to heat the surrounding gas, preventing it from cooling enough to collapse and form stars. Outflows and jets driven by the AGN can directly expel vast quantities of gas from the galaxy and its surrounding halo into intergalactic space. This removes the fuel supply for future star formation, a scenario often referred to as the “maintenance mode” of quenching.
The heating effect is particularly effective in massive galaxies, where the black hole mass is proportional to the galaxy’s stellar velocity dispersion, allowing the AGN to self-regulate the galaxy’s growth. If a galaxy is above a certain mass threshold, roughly \(10^{12}\) solar masses for the surrounding dark matter halo, the heating from the AGN is the dominant process that keeps the gas hot and star formation suppressed. This internal mechanism explains the large population of quiescent, massive elliptical galaxies observed throughout the universe.
External Mechanism: Environmental Stripping
External forces are the primary drivers of quenching for galaxies residing in dense regions, such as galaxy clusters. As a galaxy plunges through the cluster, it encounters the Intracluster Medium (ICM), a vast reservoir of diffuse, superheated plasma. The fast-moving galaxy collides with this plasma, creating a powerful drag force that physically strips away its cold, star-forming gas.
This process is called ram pressure stripping. Ram pressure can sweep gas out of a galaxy quickly, sometimes in only tens of millions of years. The observational signature is a long, luminous tail of stripped gas trailing the galaxy, confirming the removal of its star-making fuel.
A related external process is “strangulation,” or “starvation,” which is a slower mechanism that affects a galaxy’s outer gas halo. As the galaxy moves through the cluster, the hot ICM prevents new, cold gas from accreting onto the galaxy from its surroundings. The galaxy then slowly consumes its remaining internal gas supply by forming stars, but with no resupply, it eventually starves to death over a few billion years.
The Role of Mergers in Galactic Transformation
Galaxy mergers, which involve the collision and eventual combination of two or more galaxies, act as accelerators of the quenching process. When two spiral galaxies collide, the gravitational shock waves compress the gas clouds within them, which initially triggers an intense, but short-lived, burst of star formation known as a starburst. This sudden, massive consumption of gas rapidly depletes the galaxy’s fuel supply.
The merger event also funnels gas toward the central supermassive black hole, fueling the AGN. The resulting AGN feedback then takes over, ejecting or heating the remaining cold gas in the galaxy and its halo, preventing any further star formation. This combined effect of rapid consumption and violent expulsion leads to a swift and permanent quenching of the galaxy.
Furthermore, these major mergers fundamentally transform the galaxy’s structure, often destroying the original spiral disk and resulting in a smooth, gas-poor elliptical galaxy. The newly formed elliptical galaxy is left with an older stellar population and a morphology incapable of sustaining a continuous supply of new stars.