The galaxy next door to the Milky Way, the Large Magellanic Cloud (LMC), could be hiding a monstrous secret. This dwarf galaxy, a satellite of our galaxy, may have its own supermassive black hole.
Evidence for this hidden cosmic titan was delivered via hypervelocity stars at the edge of the Milky Way; these ‘runaway stars’ that appear to have been fired from the LMC by a hitherto undiscovered supermassive black hole.
The team behind this shocking discovery arrived at their findings when they investigated 21 hypervelocity stars traveling so fast they will soon break away from our galaxy.
Tracing the trajectory of these super-speed stars using the European Space Agency’s star-tracking Gaia satellite, the researchers discovered that around half of them were accelerated by the Milky Way’s own supermassive black hole, Sagittarius A* (Sgr A*).
The other half, the team believes, likely fled to the outskirts of the Milky Way after a gravitational encounter with a supermassive black hole at the heart of the LMC separated these stars from their stellar binary partners.
“It is astounding to realize that we have another supermassive black hole just down the block, cosmically speaking,” team leader Jesse Han of the Center for Astrophysics | Harvard & Smithsonian (CfA) said in a statement. “Black holes are so stealthy that this one has been practically under our noses this whole time.”
What has hypervelocity stars on the run?
Hypervelocity stars are thought to be created when a binary star system gets too close to a supermassive black hole.
While one of these stars gets captured in a tight orbit around the black hole, or even possibly devoured in a violent Tidal Disruption Event (TDE), its partner gets ejected at speeds exceeding many millions of miles per hour.
“We knew that these hypervelocity stars had existed for a while, but Gaia has given us the data we need to figure out where they actually come from,” team member and California Institute of Technology researcher Kareem El-Badry said. “By combining these data with our new theoretical models for how these stars travel, we made this remarkable discovery.”
A pre-existing theory had suggested that if a supermassive black hole exists in the LMC, it would create a cluster of hypervelocity stars at one the edge of the Milky Way as a result of how this satellite dwarf galaxy moves around our galaxy.
The researchers theorized that the properties of hypervelocity stars seen at the edge of the Milky Way cannot be explained by other possible acceleration mechanisms not involving a supermassive black hole, like a “kick” from a companion star undergoing a supernova explosion, for instance.
In addition to gathering evidence that supports the possibility of a supermassive black hole in the LMC, the scientists were able to use the speeds of those stars and their quantity relative to ones accelerated by Sgr A* to infer the mass of the LMC black hole.
All in all, the team determined that the mass of the LMC black hole is about 600,000 times the mass of the sun.
This actually makes it rather diminutive in terms of supermassive black holes. Sgr A* at the heart of the Milky Way has a mass 4.3 million times that of the sun, while the supermassive black hole in the galaxy Messier 87 (M87) has a mass around 5 billion times that of our star!
“The only explanation we can come up with for this data is the existence of a monster black hole in our galaxy next door,” team member and CfA researcher Scott Lucchini said. “So in our cosmic neighborhood it’s not just the Milky Way’s supermassive black hole evicting stars from its galaxy.”
The team’s research has been accepted for publication in The Astrophysical Journal with a preprint version available on the repository site arXiv.