This episode is sponsored by The Ridge.

Go to ridge.com/scishow and use promo code “scishow” to get 10% off your next order. {♫Intro♫}. When astronomers observe distant galaxies, some of them are contemplating a murder mystery.

Our Milky Way is alive and well, producing new stars all the time. But there's another group of galaxies out there, populated only by venerable red dwarf stars -- the young stars are nowhere to be seen. These are called quiescent galaxies because at some point in the galaxy's life, its star formation stopped.

In effect, these galaxies are... dead. But, how? There are no witnesses to these murders -- the distances and time scales are simply too vast for us to observe.

So scientists have proposed a few different ideas for how it could happen. The first scenario has an appropriately whodunit-y name: strangulation. We know a galaxy can live for many billions of years, as new stars are constantly being created from cold hydrogen gas that's hanging around in interstellar space.

And normally, new gas flows into the galaxy from the intergalactic medium to refresh that supply. But sometimes that inflow of gas stops -- and we're not sure why. It might be because of interactions with other, neighboring galaxies, which compete for intergalactic gas supplies.

Or perhaps the nearest clouds of gas just... run out. Either way, when the inflow stops, all that's left is the galaxy's own stars, and the gas they expel at the end of their lives. And since stars fuse hydrogen into heavier elements, the overall proportion of hydrogen in the galaxy decreases over time, replaced by heavy metals.

This continues until there's not enough hydrogen for new stars to form. Without external input, the galaxy is slowly strangled to death, suffocated by its own waste products. To test the strangulation hypothesis, in 2015 researchers from the University of Cambridge and the Royal Observatory of Edinburgh surveyed the light spectra of local galaxies.

Their autopsy revealed the characteristic signs of strangulation: the quiescent galaxies had much higher levels of heavy metals than the living, star-forming galaxies. They also showed that the dead galaxies were on average 4 billion years older than the living ones -- which makes sense, because it takes time for a galaxy to die by strangulation. Based on their results, the researchers concluded that this was the primary cause of death for galaxies up to about twice the size of the Milky Way.

But strangulation doesn't explain all galaxy deaths. Some galaxies still have plenty of hydrogen-rich gas, but for some reason they just stop forming stars. This could happen if the gas is too hot and energetic for it to condense into new stars -- a mechanism called suppression.

Scientists think that black holes could be the perps in this case. In 2016, researchers made observations of a galaxy nicknamed Akira, which has a supermassive black hole at its center. As supermassive black holes like these grow, they're thought to occasionally emit powerful winds.

In the 2016 paper, researchers found evidence for those blasts of gas. They believe this wind may be powerful enough to heat up the gas in the galaxy -- enough to prevent it from condensing into stars. And the heat blasts keep coming, which keeps the gas hot and suppresses the formation of new stars long term.

Which supports the idea that galaxies like Akira could die by suppression -- and that black holes are the killers. Finally, some galaxies don't fit into either of these categories. They don't have much hydrogen gas to go around, but they don't have the heavy metal signature of strangled galaxies either.

They seem to have died before their time, by having their life-giving gas physically stripped out in a process called ram pressure stripping. Although galaxies might appear still to us here on Earth, they're actually moving through space at millions of kilometers per hour. In part, they're being dragged by the gravity of invisible dark matter.

And as they move, scientists think that they pass through clouds of intergalactic charged plasma. This plasma acts like an atmosphere in space, applying drag to the galaxy, and stripping it of its lightweight gas. Amazingly, observations of a galaxy falling into the Virgo cluster actually seem to show this process in action.

The galaxy is curved back at its edges from the pressure of the intergalactic wind. The light from the disk reveals streamers of gas and dust trailing behind, pushed out by the force of the intergalactic plasma. A bit like a dog sticking its head out of a car window.

Ram pressure stripping has been thought to affect galaxies inside clusters since the 1970s. We now know that's because these clusters have huge amounts of dark matter in them -- enough to put some serious hurt on the galaxies within. But a study in 2016, which surveyed more than ten thousand relatively isolated galaxies, found that this could also be a cause of death for those loners.

They saw that the dark matter that surrounds a single galaxy could be enough to tug on neighboring galaxies, pulling them along and stripping out their gas. It seems that no single mechanism can explain this er mystery. Strangulation, suppression, or stripping are all capable of putting a stop to star formation.

But the cause of death will depend on what's going on inside and around an individual galaxy. So we'll have to wait and see what lies in store for our own Milky Way. The good news is, we've probably got a couple billion years before the killer strikes, so we'll all be dead anyway.

Thanks for watching this optimistic episode of SciShow Space. And thanks to The Ridge for sponsoring this episode. The Ridge makes wallets that are light, sleek, and industrial, and designed to hold everything you need -- and only what you need.

And they're so confident that you'll like it, that they'll let you test drive it for 45 days -- and you can send it back for a full refund if you're not totally starstruck. You can get 10% off today — with free worldwide shipping — by going to ridge.com/SCISHOW and using the code “SCISHOW”. If you're interested, there's a link in the description. {♫Outro♫}.