Monster Galaxies Eat Younger Ones to Gain Weight After They Stop Forming Stars

It turns out that massive galaxies don't expand by making their own stars. Instead, they gain weight by chomping on nearby galaxies. Scientists have found that monster galaxies tend to absorb smaller ones in order to grow larger.

While small galaxies are very efficient at creating stars from gas, most massive ones are much less efficient at star formation. In fact, our own Milky Way is at a tipping point and is expected to now grow mainly by "eating" smaller galaxies, rather than by collecting gas.

"All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars," said Aaron Robotham, one of the researchers, in a news release. "Then every now and then they get completely cannibalized by some much larger galaxy."

In order to learn a bit more about galaxy growth, the researchers analyzed more than 22,000 galaxies. This showed them that the larger galaxies were munching on the smaller ones. This included our own Milky Way.

"The Milky Way hasn't merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we've cannibalized," said Robotham. "We're also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years."

What's interesting, though, is that the Milky Way isn't even the largest galaxy out there. The scientists found that because the Andromeda Galaxy is larger, in about 5 billion years, the Milky Way will eventually merge with Andromeda.

"Technically, Andromeda will eat us because it's the more massive one," said Robotham.

So why is there a slowdown in star growth in large galaxies? It's likely that star formation slowed in really massive galaxies due to the fact that there are extreme feedback events in a very bright region at the center of a galaxy known as an active galactic nucleus.

"The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars," said Robotham.

The findings reveal a bit more about our own Milky Way galaxy and how other galaxies form in the universe.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.