New Class of Black Hole Space Objects?Jul 10th, 2009 | By Rebecca | Category: News, Science
Merging galaxies that forcefully eject supermassive black holes have theoretically created a whole new class of astronomical object—and now scientists think they know how to find them.
Black holes that get kicked out should carry with them clusters of nearby stars, a new study says.
These stars can act as signposts and can reveal details about the now galaxy-less black hole’s past life.
In theory, hundreds of massive black holes left over from the age of galaxy formation could be lurking in the nearby universe.
“Every such black hole that’s ever been kicked out is still potentially observable, and that’s very encouraging,” said lead study author David Merritt of the Rochester Institute of Technology.
“It’s not quite what anybody has seen so far,” he said. “We’re just talking about what they would look like if you were to find them.”
Observations made with the Hubble Space Telescope and re-examined with ground telescopes have come close, Merritt said.
“People are now just starting to make the kind of observations to see these kinds of things, if they are there.”
Most galaxies are thought to harbor black holes at their centers that are millions to billions of times the mass of our sun.
When galaxies merge, their respective supermassive black holes start to coalesce in a process that creates a spurt of gravitational waves.
If the waves are strong enough, the kick they provide should drive the newly merged black hole outside the host galaxy, simulations from the past few years suggest.
That kick would also leave the “rogue” black hole surrounded by any orbiting stars that got taken along for the ride, Merritt and colleagues report in the July 10 issue of the Astrophysical Journal.
(Related: “New Class of Black Hole Found?”)
And knowing the speed of the stars that accompanied the black hole can reveal the speed of ejection, Merritt said.
That’s because stars that were orbiting slower than the force of the kick would stay in the host galaxy.
“[It's] like the sun in our solar system: If you gave it a kick, it would take along some of the inner planets, like Mercury, but it would leave behind the outer planets because they’re just too weakly bound to the sun to move with it,” Merritt said.
Milky Way’s Mergers
Scientists earlier this year came to similar conclusions in a study that focused on our home galaxy, the Milky Way.
“The Milky Way is thought to have been a collection of small galaxies early on that later merged together,” said Avi Loeb of Harvard University, a co-author of the paper, published in the April issue of the Monthly Notices of the Royal Astronomical Society.
“If this [merger] process took place at each stage of evolution, then black holes would have escaped from the merger remnant but would have still been [surrounded by nearby stars],” he said.
Both new theories are reasonable, based on estimates made in the last few years of how strong a merging black hole’s kick might be, said Christopher Reynolds, of the University of Maryland in College Park.
“The question is if that actually does happen in nature,” said Reynolds, who was not involved in either study.
Merritt’s work is “good science in the sense that it lays out a very clear path by which we can actually use observations to address these questions,” Reynolds added.
“If we don’t find [the black holes with star clusters], that could tell us that the estimates of how many of these black holes get ejected with very high velocity are off.”