The Galactic Death: Why Galaxies Stop Forming Stars Quiz

Quenching refers to the transition of a galaxy from an active, star-forming state to a "passive" or "red and dead" state. This occurs when the galaxy no longer has access to cold, dense gas, which is the primary fuel for creating new stars, leading to a shift in the galaxy's overall color and evolution.

Stars only form from cold, dense molecular gas. If a mechanism, such as radiation from a black hole or a supernova, heats this gas to millions of degrees, it becomes too energetic to collapse under gravity. Without the ability to collapse, the gas cannot form new stars, effectively shutting down the galaxy's growth.

An AGN is powered by a supermassive black hole at a galaxy's center. As it consumes matter, it releases immense energy in the form of radiation and relativistic jets. This energy provides "feedback" that can push the interstellar medium entirely out of the galaxy, leaving it without the materials needed for star birth.

When a galaxy falls into a massive galaxy cluster, it moves through a hot, thin gas known as the intracluster medium. The pressure from this motion acts like a wind, physically pushing the cold gas out of the galaxy. This "ram pressure stripping" can leave a galaxy as a gas-poor disk that can no longer produce stars.

Internal feedback comes from processes within the galaxy itself. Supernovae and stellar winds inject kinetic energy and heat into the surrounding gas, while AGN feedback from the central black hole can provide massive energy shocks. Ram pressure stripping is an external environmental factor caused by the galaxy's movement through a cluster.

When star formation stops, short-lived blue stars die off quickly. Only the older, cooler, and longer-lived red stars remain. Over time, the entire galaxy's light shifts toward the red end of the spectrum, placing these galaxies on what astronomers call the "red sequence" of the galactic population.

While quenching is often permanent, a galaxy can sometimes "rejuvenate." This happens if it later acquires a fresh supply of cold gas, perhaps through a merger with a gas-rich dwarf galaxy or by cooling gas from its surrounding halo. This influx of new fuel can re-ignite star-forming regions and change the galaxy's appearance.

The gas in massive galaxy clusters is often heated to tens of millions of degrees by gravitational energy and AGN activity. Gas at these extreme temperatures cannot cool down fast enough to settle into a galaxy and form stars. This "strangulation" prevents the galaxy from receiving the fresh supplies of gas it needs to continue growing.

Quenching is closely tied to morphology. As star formation ceases, the bright blue spiral arms fade away. Over time, gravitational interactions and the lack of new star-forming disks often result in a smooth, featureless elliptical or lenticular shape, dominated by a random swarm of older stars rather than organized rotation.

In very massive dark matter halos, the gas falling inward undergoes a "virial shock" that heats it to extreme temperatures. If the halo is large enough, the gas cannot radiate this heat away efficiently. Consequently, the gas stays in the halo as a hot atmosphere rather than falling into the galaxy to form stars.

Dwarf galaxies have much weaker gravitational fields. This makes it easier for external forces, such as ram pressure from a larger cluster or radiation from a nearby large galaxy, to strip away their gas. Because they have less "holding power," small galaxies are often the first to lose their star-forming capabilities in dense environments.

Galaxies in the "green valley" are in a state of evolutionary flux. They are no longer forming stars rapidly enough to stay blue, but they haven't yet aged enough to become completely red. Studying these intermediate galaxies helps scientists understand the timescales and specific triggers that cause star formation to shut down across the universe.

Stellar feedback occurs as stars live and die. Young, massive stars emit intense ultraviolet radiation and strong winds that push against nearby gas. When these stars explode as supernovae, they release massive amounts of energy. This collective activity keeps the interstellar medium turbulent and hot, which regulates and eventually can help limit the rate of star formation.

A central bar structure can act as a funnel, driving gas toward the galactic center. This often leads to a massive burst of star formation (a starburst). Because the gas is consumed so rapidly and the resulting feedback is so intense, the galaxy may exhaust its fuel much sooner than it would have otherwise, leading to premature quenching.

Sometimes a galaxy has gas, but it still won't form stars. If a galaxy has a large, massive central bulge, the gravitational environment can become so stable that it prevents gas clouds from fragmenting and collapsing. In this case, the galaxy is quenched not because it lacks gas, but because its internal physics prevents that gas from becoming stars.

Galaxies experiencing ram pressure stripping often look like "jellyfish." As they plow through the cluster gas, their own gas is pushed out behind them in long streamers. These streamers can even host "blobs" of new star formation, creating a striking visual trail that points back to the galaxy's path.

"Cosmic noon" represents the era when the universe was most active in forming stars. After this peak, the rate of star formation across the entire universe began to decline. This decline is driven by the increasing number of galaxies undergoing quenching as they grew larger, developed more active black holes, and merged into massive clusters.

Without quenching, gravity would continue to pull gas into massive galaxies indefinitely, making them much larger than what we observe. Feedback from supermassive black holes and hot halos provides a limit to this growth. By shutting off the fuel supply, quenching acts as a cosmic "thermostat" that regulates the maximum size of galaxies in the universe.

Since the peak of cosmic star formation 10 billion years ago, the rate at which the universe creates stars has dropped significantly. Quenching is the primary driver of this trend. By identifying the various ways galaxies die, astronomers can model how the universe will continue to change as more and more galaxies run out of cold gas.

Distant galaxies are redshifted, and their signatures of past star formation or hidden gas are best seen in the infrared. Instruments like the James Webb Space Telescope allow scientists to observe the chemical signatures and stellar populations of quenched galaxies in the early universe, revealing how long ago their star formation actually ceased.