New research from the University of Western Australia node of the International Center for Radio Astronomy Research (ICRAR) suggests that large "monster" galaxies eventually stop producing stars for growth, but instead, dine on their smaller nearby galaxy neighbors.
The seven-year-long research project looked at over 20,000 galaxies before making this determination. Over 90 scientists participated. Their observations showed that smaller galaxies grew by creating stars from gas, an easy process for them. However, once galaxies become larger, it's harder to produce stars this way, so the only way they can grow is by merging and absorbing other galaxies.
"All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars," says Dr Aaron Robotham of the ICRAR. "Then every now and then they get completely cannibalized by some much larger galaxy."
Our own Milky Way galaxy is also subject to this phenomenon and is now the size where making stars is more difficult for it, so it will start eating smaller galaxies. There is already evidence of previous galaxies it has absorbed, but eventually, it will also pull in two small nearby galaxies, the Large and Small Magellanic Clouds, in about 4 billion years. Eventually, though, in about 5 billion years, the Andromeda galaxy will eat the Milky Way when the two galaxies collide. Basically, it's the Universe's survival of the fittest, with the larger galaxies coming out ahead.
In a simulation, scientists show how the collision between the Milky Way and Andromeda will happen, including the merging of the two into one massive galaxy.
Astronomers explain that this theory makes sense because larger galaxies have more gravity than smaller ones. This gravity pulls in the smaller galaxy, allowing for its absorption.
But what causes star formation to slow down once a galaxy grows larger? Larger galaxies start experiencing "feedback events" in the part of their centers called the active galactic nucleus. These events keeps the gas hot, preventing the cooldown necessary for forming stars.
In the very far future, gravity will eventually bring all of the galaxies together, leaving only a few massive supergalaxies.
"If you waited a really, really, really long time that would eventually happen but by really long I mean many times the age of the Universe so far," says Dr. Robotham.