In a paper recently published in the Monthly Notices of the Royal Astronomical Society, scientists led by astronomer and PhD candidate Toby Brown — of the Swinburne University of Technology and the International Centre for Radio Astronomy Research (ICRAR) — shared their findings on a phenomenon that has been happening all around the universe. It’s basically a rampant killing spree. And the victims are entire galaxies. Who’s doing the killing? Dark matter. Galaxies are being decimated by dark matter. It’s kind of poetic, isn’t it? But not in a good way.
Dark matter is one of those mysterious forces that belong to the ‘just because we can’t see it, it doesn’t mean it’s not there’ type. By observing roughly 11,000 galaxies, then combining data from Sloan Digital Sky Survey (SDSS) and Arecibo Legacy Fast ALFA survey, Brown’s team was able to confirm just how powerful dark matter is, and what it has been doing to the unsuspecting galaxies.
Galaxies are believed to be enveloped by clouds of dark matter called dark matter halos. And galaxies can’t stay still, so they move from one halo to another. When a galaxy falls into one of the larger halos, the ‘superheated intergalactic plasma’ between them sucks all the gas out in a process known as ‘ram-pressure stripping’. Without gas, a galaxy cannot form new stars. And old stars can’t last forever, they eventually cool down and die. So without new stars to replace the old stars, the galaxy eventually dies too.
Scientists have long been aware that ram-pressure stripping affects galaxies in large clusters, where dark matter exists in ginormous amounts. What’s new is finding out that the process also happens in smaller clusters where dark matter exists in lesser amounts.
As team leader Toby Brown explained in a statement they issued: “You can think of it like a giant cosmic broom that comes through and physically sweeps the gas from the galaxies… If you remove the fuel for star formation then you effectively kill the galaxy and turn it into a dead object.”
It’s not that it’s so uncommon for galaxies to die. In fact, scientists are aware of another galaxy-killing process. It’s called strangulation and it happens when the gas used to form stars is consumed faster than it can be replenished. So gas ultimately runs out, and the galaxy starves until dies. But this is a slow process, which makes it kind of natural and not-so alarming.
In contrast, ram-pressure stripping happens much quickly. Specifically, it takes around tens of millions of years to kill galaxies this way. It’s a bit hard to imagine but when it comes to astronomy, that time frame is extremely fast.
The idea that dark matter makes up approximately 80% of our universe is unsettling enough because we can’t help but be afraid of the unknown, right? If it’s just there to balance whatever needs balancing, then it’s fine. But knowing that it can exterminate galaxies in a hurry is an entirely different story. But then again, we’re talking about millions of years, so it would be an overreaction to think too much or in a panicky way about it. Besides, we’ve got enough problems on our plate as it is.