In our universe, there are two
types of galaxies: live ones and dead ones. For two decades, scientists didn't
understand what could possibly kill a previously thriving galaxy, but in 2017,
research may be pointing to whodunit. As recently as 2015, scientists had two theories for how galaxies die—that is, how they lose the hot, X-ray-spewing gas
known as the intra-cluster medium, thereby halting the formation of new stars.
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One involved the gas being
slowly choked off over time, in a process aptly dubbed strangulation. The other
was a quick death called ram-pressure stripping, in which the gas is violently
torn away like dandelion seeds in a strong wind.
Research published in the
journal Nature in 2015 found that the vast majority of galaxies—those up to 100
billion times heavier than the sun, or roughly 95 percent of the galaxies out
there—died via strangulation.
But in 2017, new research published in the
Monthly Notices of the Royal Astronomical Society showed that ram-pressure
stripping was more common than previously thought, affecting galaxy clusters
both large and small.
According to Toby Brown, lead
researcher on the ram-stripping study, strangulation is more of a natural
death. "Strangulation occurs when the gas is consumed to make stars faster
than it's being replenished, so the galaxy starves to death," he said in a
press release. "It's a slow-acting process." Ram-pressure stripping,
on the other hand, is a crime carried out by none other than dark matter.
Galaxies are embedded in
invisible halos of dark matter. As galaxies fall through these halos, the
"superheated intergalactic plasma" between them quickly strips them
of their star-forming gas, leaving them cold and lifeless. We find the
defendant guilty as charged.
