Are there massive black holes screaming around just outside our Galaxy?
A new study says: Maybe.
Globular clusters are roughly spherical clusters of stars, some with populations numbering in the millions. They're old, as old as the Galaxy, which means they've been around billions of years. There are no young stars in globular clusters, which means there are no massive stars, either: massive stars blow up after only a few million years.
But when these stars die, they can form black holes, which are abundant in globulars... and stars are so packed together in these clusters that they can interact gravitationally. These interactions can happen over great distances; enough that the star doesn't get eaten, but enough that the path of the star can be affected by the hole; and vice-versa too.
When that happens, the more massive of the objects sinks to the center of the cluster, while the less massive object moves outwards. Over billions of years, this means the black holes tend to hang out in the center of the cluster, while normal stars tend to populate the suburbs. Eventually, the black holes crowded into the center will interact with each other. Most of these black holes will start out with masses close to that of the Sun (called stellar mass black holes) but as they merge they can grow in size to hundreds of times the Sun's mass (called intermediate mass black hole).
But, it so happens, when they collide and eat each other, bad things can happen.
When black holes merge, they can actually distort the fabric of space/time, like a rock thrown into a pond ripples the surface. They emit a huge amount of energy in the form of these ripples (called gravitational radiation). Usually, this radiation is emitted in all directions, but sometimes it can be sent out more to one side than the other. When that happens, it acts like a rocket, pushing the black hole in the other direction. It is possible, if conditions are right, to accelerate a black hole to the incredible speed of 4000 kilometers per second (2500 miles per second)!
Stop for a moment, and think on the forces that can move an object that outweighs the Sun to a speed of well over ten million kilometers an hour. It makes the hairs on the back of my neck stand up.
Anything moving at that kind of speed will blow right out of the cluster; there isn't nearly enough gravity in the cluster to hold on to such a stellar bullet.
This new study on how globular cluster black holes merge and what happens when they do shows that of the 150 known globulars surrounding or Galaxy, well over 100 may have blasted black holes from their cores, shooting them out into the Universe at large. There is a thin halo of stars (and dark matter, but that's neither here nor there for this work) surrounding the Milky Way, called its halo. And now we might be able to add another citizen to its census: a few dozen or even as many as one hundred black holes, moving at phenomenal speed, silently cruising the intergalactic depths. We're in no real danger from these Great Black Sharks, since they are so few in number and space is so mind-bogglingly vast. But to me, there's something appealing in knowing they're out there... especially since it makes it easier to write the black hole chapter of my book. :-)