Galaxy Death by Strangulation?

It has been well established that Galaxies have formed during the last 13.7 Billion years of cosmic evolution.  They didn’t just pop into existence, but developed in a long and arduous process that spans immense time.  Many of them will continue to flourish for many Billions of years.  If Galaxies do indeed have a birth, as has been seen, it stands to reason that they should someday ‘die’ as well.  But have we ever seen the death of a galaxy? Have we ever seen the end for a massive collective structure of stars?

This is an artist’s impression of one of the possible galaxy strangulation mechanisms: star-forming galaxies (fed by gas inflows) are accreted into a massive hot halo, which ‘strangles’ them and leads to their death. Credit: Re-active

We have seen galaxies collide and merge to form new, larger galaxies.  But this isn’t so much galaxy death as it is merger.  Could a galaxy simply cease to exist? We have seen dwarf galaxies absorbed into larger galaxies many times, to the point where the original galaxy no longer exists.  But despite all of this, we have never seen a Galaxy simply die.  It may be because the smallest red dwarf stars within can survive many times the age of the Universe, so perhaps a Galaxy never truly dies.

When we speak of life and death in galactic terms, ‘life’ refers to the formation of stars, while ‘death’ refers to a lack of star formation.  By these terms our Milky Way is alive, while giant ellipticals with no gas are considered ‘dead.’  We know what makes galaxies come alive, but until now we have never had a supported explanation of what causes them to go dead and end their star formation.

There are two schools of thought on this: Either the supply of cold gas required for star formation is suddenly removed from the galaxy by internal or external forces, or there is a mechanism that prevents the galaxy from obtaining any new cold gas at all.

Astronomers used the Sloan Digital Sky Survey (SDSS) Data to look at the metallicity of a large population of galaxies.  The fraction of elements heavier than Helium by mass, known as the metallicity, is a tracer of galactic evolution. The more stars that have formed and lived out their lives, the higher the resulting metallicity.

If galaxies are killed by an immediate loss of cold gas, the metallicity should be the same as when it died, since star formation would halt.  Whereas in the case of death by strangulation, the metallicity would keep rising as the galaxy continued to form stars and use up the gas it had left. By looking at a large population of galaxies and statistically analysing the metallicity, they found that most galaxies do in fact die by strangulation.

“We found that for a given stellar mass, the metal content of a dead galaxy is significantly higher than a star-forming galaxy of similar mass,” said Professor Roberto Maiolino, co-author of the new study. “This isn’t what we’d expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario.”

In the strangulation scenario, Galaxies move into a cluster’s central region, where any reservoir of cold gas is blocked off by the galaxies at the cooler edge, essentially strangling them of the new gas they need to continue their state of star formation.

“This is the first conclusive evidence that galaxies are being strangled to death,” said Dr Yingjie Peng of Cambridge’s Cavendish Laboratory and Kavli Institute of Cosmology, and the paper’s lead author. “What’s next though, is figuring out what’s causing it. In essence, we know the cause of death, but we don’t yet know who the murderer is, although there are a few suspects.”


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