Have you ever wondered how a galaxy dies? Astronomers have found that their deaths are slow and painful. Cosmic events cut them off from the gases (hydrogen and helium) that they need to create new stars, cruelly strangling them to death over the course of four billion years.
Researchers at the University of Cambridge and the Royal Observatory Edinburgh banded together to conduct the new study. They surveyed thousands of nearby galaxies (over 26,000), both living and dead, to establish whether they die slowly or suddenly.
Living galaxies are galaxies that produce stars (the Milky Way), while dead galaxies are the ones that don’t produce stars. Living galaxies have a rich supply of cold gases, while dead galaxies have a very low supply of them.
The team of researchers detected high levels of metals in dead galaxies. These metals build up during a star’s formation and act as reliable “fingerprints” that point to a period of slow strangulation. Their findings were published today, in the journal Nature.
Dr Yingjie Peng, lead author from the University of Cambridge, explained that metals are a very powerful tracer of the history of star formation. The more starts that are formed by a galaxy, the more metal content you’ll be able to see.
There were two hypotheses that Peng and his colleagues worked with.
If a galaxy’s death was quick and violent, with the hydrogen and helium suddenly being yanked out of the galaxy by either internal or external forces, it would immediately stop forming starts, leaving its metal content exactly the same.
If a galaxy’s death was cut off from its supply of gas, but continued to use what it already had in storage, metal would continue to build up until the galaxy slowly but surely suffocated.
In order to figure out which hypothesis was the correct one, and to answer one of the biggest, most challenging questions astronomers have had in the past 20 years, the researchers used data from Sloan Digital Sky Survey.
Professor Roberto Maiolino, co-author of the study, explained that they “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”. This isn’t what they’d expect to see in a case of sudden gas removal, but it is consistent with the strangulation scenario.
The team concluded that most galaxies are being strangled to death and Dr. Peng said it was the first study to offer conclusive evidence on the matter.
The analogy is a simple one: during strangulation a victim uses up the oxygen in their lungs, but keeps producing carbon dioxide, which remains trapped in the body, not unlike how galaxies hold on to their own reserves of hydrogen and helium.
Computer simulations followed, and they revealed that the strangulation process usually last about four billion years. This is further proof that supports the conclusion as the star-forming galaxies researchers looked at were at least four billion years younger than dead ones.
Peng did note however that strangulation is only a sure thing when it comes to galaxies that are up to 100 billion times bigger than sun (about 95 percent of all galaxies). For larger galaxies, the evidence was inconclusive, with scientists not being able to prove either hypothesis.
The lead author also expressed interest in exploring what exactly could be causing the strangulation, as a clear and obvious suspect is yet to be found. A possible next step could involve looking at more distant galaxies, in order to get a sense of what the universe looked like when it was young.
Image Source: dailygalaxy.com