Why Galaxies Stop Forming Stars?

Why Galaxies Stop Forming Stars?

The Quenching process of galaxies may vary from one to another.

A Galaxy Cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 1014 to 1015 solar masses. Keeping the definition in mind, Galaxy clusters are those recherché areas in the universe, constituting of trillions of stars along with hot gases and dark matter, where the mechanism of star formation is hindered in a process called Quenching. Dr. David Sobral, a Co-author of a study conducted in 2015, expressed his views in this regard by saying,

“Just like humans, galaxies are affected by both the environment in which they form and evolve, but also by their ‘nature’ and internal processes; both can have dramatic effects. Surprisingly, we find that the external processes are only really relevant in shutting down the production of stars in galaxies over the last eight billion years. At earlier times in the universe, internal processes are the main mechanism for shutting down star formation. In other words: back then, it was all about nature, not nurture, but later on, the environment starts to play a major role.”

The reasons for the possible occurrence of Quenching are still unknown. However, there are some factors which may explain the enigmatic process. Ryan Foltz led a new study at the University of California where they made the measurement of the Quenching timescale. The researchers observed how it differs in more than 70% of the universe which aids in the explanation of impediment of star formation in clusters. It is a known fact that an entering galaxy in cluster comes with cold gas that has not taken part in star formation. According to the latest study, the cold gas is ‘stripped’ away from the galaxy by the hot dense gas, which comes as part and parcel of the cluster, before it transforms the cold gas into stars. This causes the hampering in the process of star formation.

Another possibility is that the galaxies are ‘strangled’ which means that they stop forming stars because their reservoirs cease getting replenished with additional cold gas once they fall inside the cluster. This is predicted to be a slower process than stripping. Similarly, a valid possibility is that the energy from star formation repels much of the cold gas away from the galaxy and results in the cumbering of star formation. This ‘outflow’ scenario is predicted to occur on a faster timescale than stripping because the gas is lost forever to the galaxy and is unavailable to form new stars. These three possibilities justify why galaxies on different relative timescales in the history of the universe have a different rate of Quenching.

This deduction led to a conclusion that if astronomers could compare the number of quenched galaxies observed over a long time-baseline, the dominant process causing stars to quench would more readily become apparent. Having said that, until recently, it was very difficult to find distant clusters, and even harder to measure the properties of their galaxies. The International Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) conducted research on the measurement of more than 70% of the history of the universe which was attained by new cluster detection techniques. This helped the discovery of hundreds of new clusters in the distant universe.

Using some of the newly discovered SpARCS clusters, the new study discovered that it takes a galaxy longer to stop forming stars as the universe gets older. ‘Ram Pressure Stripping’ may also play an important role in quenching, where gas is stripped from a galaxy by plunging through the hot plasma. But this process should only work in clusters of galaxies, and many of our galaxies aren’t in galaxy clusters. Perhaps gas cannot cool to produce new stars because of the heating caused by Active Galactic Nuclei, which are powered by the in-fall of matter towards enormous black holes. Talking about that, Foltz said,

“Comparing observations of the quenching timescale in galaxies in clusters in the distant universe to those in the nearby universe revealed that a dynamical process such as gas stripping is a better fit to the predictions than strangulation or outflows.”

Gillian Wilson, a Professor of Physics and Astronomy at the University of California who led the SpARCS research, praised the role of different observatories for the investment they provided and thanked them for their help. He also discussed the future goals of his team in the following words:

“There are good reasons, however, to believe that lower-mass galaxies may quench by a different process. That is one of the questions our team is working on answering next.”

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