The strangler of galaxies

Astronomers have solved a riddle that is even partially behind the death of galaxies. In fact, according to a study published in Nature, the leading case concerning the “death” of a galaxy is by “strangulation” due to lack of supply of the raw material that is used to give rise to the formation of new stars.
What determines the order of the galaxies to the point that they are no longer able to produce new stars? This is one of the great cosmic mysteries that astronomers today, as real investigators, revealed, albeit partially. In fact, a new study published in Nature shows that the main cause of the “death” of a galaxy is to “choke” when they are cut out of the running to supply the raw material that is used to generate new stars.


Credit: Peng et al. 2015

Researchers at the University of Cambridge and the Royal Observatory of Edinburgh found that the proportion of metals present in the galaxies now deceased provides a kind of “fingerprint” which frees up those leads that are used to determine the cause of death of galaxies. Generally, galaxies can be divided into two broad categories: about half are galaxies still “alive”, i.e. they are in full stellar fertility, while the other half is made up of those “dead” where the processes of star formation have already become extinct. The living galaxies as our own Milky Way are full of cold gas, mainly hydrogen, which is needed to form stars unlike the now inactive galaxies where the gas reserves are very small.
So, to solve the mystery that causes the death of galaxies, the researchers introduced two major hypotheses, as shown: 1) the gas needed to form stars is consumed quickly by the presence of any internal or external forces to the Galaxy (panel higher); 2) the supply of gas is stopped in some way, “choking” slowly Galaxy up to bring it to a sort of “asphyxiation galactic” over a long period of time (bottom panel). To test their ideas, the team used data from the Sloan Digital Sky Survey (SDSS) in order to analyze the percentage of metals present in more than 26,000 galaxies from the intermediate size and that are distributed in our cosmic neighborhood.
“The metals are a powerful indicator of the history of star formation more stars form a galaxy, the greater its metal content,” says Peng Yingjie of Cambridge’s Cavendish Laboratory, Kavli Institute of Cosmology and lead author of the study. “Therefore, the analysis of the percentage of metals present in galaxies dead we should say how they died.” If the galaxies remain inactive due to the lack of gas, then the metal content of a galaxy deceased should be the same as it was before dying given that star formation should stop quickly. Instead, in the case of “death by strangulation” the content of metals present in the galaxy should increase and then stop responding, given that star formation could go on until the gas is completely consumed.


Credit: Peng et al. 2015

Now, while it is not possible to analyze galaxies individually because of the enormous time scale involved, investigators-astronomers were able to discover the cause that leads to the death of those galaxies intermediate size going to examine statistically the difference in the percentage of metals present in the two types of galaxies. “We found that for a given value of the stellar mass, the metal content of a galaxy deceased is significantly higher than that of a galaxy in full fertility and similar stellar mass,” says Roberto Maiolino co-author of the study, Astronomical Observatory of Rome and now a professor at the University of Cambridge. “This is not what we would expect to see in the case of a rapid consumption of gas, but it is consistent with the scenario of the so-called strangulation.”
The scientists then tested their findings by examining the age of the stars in the two types of galaxies, regardless of the content of metals, finding a difference in mean age of 4 billion years to undergo this process of “strangulation galactic”, in agreement with time required from a galaxy still alive that is where there is star formation activity. “This is the first conclusive evidence that galaxies undergo asphyxia to death. The next step now will be to understand what causes their death. In other words, we begin to learn more about the mechanism behind their death but we do not yet know what really kills them, although we have some suspicion, “said Peng.

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