Galaxies follow an evolutionary path that includes active growth and eventual decline. The term “dead galaxy” is the colloquial phrase astronomers use to describe a galaxy that has ceased forming new stars. This cessation of star birth, known as quenching, represents a significant transition in a galaxy’s life cycle, moving it from a vibrant, star-forming system to a more passive and stable one. Understanding how and why this happens provides insight into the physics that governs the evolution of the universe.
Defining Galactic Death
Astronomers define a dead galaxy, also called a quiescent galaxy, not by a complete absence of new stars, but by a dramatically low rate of production. This definition is based on the Star Formation Rate (SFR), which measures how quickly a galaxy is converting its gas into stars. Star-forming galaxies typically fall along the “star-forming main sequence,” where their SFR is directly proportional to their stellar mass. A galaxy is classified as quiescent when its star formation rate drops significantly below this main sequence. The distinction is between a gas-rich galaxy, like the Milky Way, which is actively forming stars, and a gas-poor, passive system that has effectively exhausted or lost its fuel supply.
Observable Characteristics of Dead Galaxies
The most immediate characteristic of a dead galaxy is its appearance, often described by astronomers as “red and dead.” This color is a direct result of its aging stellar population; without the continuous birth of new, hot, massive, and blue stars, the galaxy’s light becomes dominated by older, cooler, red stars. Quiescent galaxies frequently possess a distinct morphology, typically exhibiting an elliptical or spheroidal shape. This contrasts with the flat, disky, and spiral structures common in star-forming galaxies. A measurable lack of cold molecular hydrogen gas, the raw material for star formation, is a primary symptom of galactic death.
Mechanisms That Stop Star Formation
The physical processes responsible for shutting down star formation are collectively known as “quenching” and can be divided into internal and external mechanisms. Internal quenching is often linked to the supermassive black hole at the galaxy’s center, a phenomenon called Active Galactic Nuclei (AGN) feedback. This process involves the black hole ejecting powerful jets or winds, which heat the surrounding gas. This energy prevents the hot gas from cooling sufficiently to collapse and form new stars, acting like a thermostat that keeps the star-forming fuel too warm.
External mechanisms are driven by a galaxy’s environment, such as its membership in a dense galaxy cluster. One key environmental process is “ram pressure stripping,” where the high-speed passage of a galaxy through the cluster’s hot plasma exerts a drag force. This force effectively sweeps the galaxy’s cold star-forming gas out into space, a fast-acting mechanism that can strip a galaxy of its gas over tens of millions of years.
A slower environmental process is “strangulation” or “starvation,” which occurs when the supply of new, cold gas from the galaxy’s surrounding halo is cut off. The galaxy continues to form stars using its existing internal gas supply until it is exhausted. Tidal stripping and galaxy harassment, which are gravitational interactions with other galaxies in a dense cluster, can also physically distort a galaxy and remove its outer layers of gas.
The Long-Term Fate of Dead Galaxies
Once a galaxy is quenched, its evolutionary path is set toward becoming a massive, dark relic over cosmic time. Since star formation has effectively ceased, the galaxy’s existing stars simply age, cool, and dim. The bright, short-lived blue stars are the first to disappear, leaving behind the long-lived, low-mass red stars that dominate the galaxy’s light. These dead galaxies are expected to evolve into the cores of the oldest and most massive elliptical galaxies observed in the present-day universe. Mergers may increase the galaxy’s overall mass and further alter its spheroidal shape but do not reignite star formation due to the persistent lack of cold gas. The galaxy continues to fade as its remaining stars slowly burn out, eventually leaving behind a massive, gravitationally bound collection of stellar remnants, brown dwarfs, and dark matter.