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. 
January 9th, 2008 at 12:49 pm
About 6 or 7 years ago Sky & Telescope had a article about a disrupted globular cluster that had been smeared into a stellar streak apparently moving so rapidly that it was torn apart, leaving a trail of stars across the halo. Perhaps this was the effect of tidal forces in the Milky Way but could it also relate to massive black holes shooting out of its core?
January 9th, 2008 at 12:56 pm
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.
Tsk, tsk… you should know I can’t think of the forces that can do that if you don’t give me a timeframe! (f=ma) It could be a small force working over ‘billions and billions of years’ or a huge force working over a few seconds…
(Sorry, the physicist in me always gets nervous when someone equates high velocities with high forces. Granted, high masses and limited timeframes are involved, but still. It’s like getting people to understand the maximum velocity of ion propulsion vs chemical propulsion. While the force from a chemical rocket might be larger, the maximum velocity of ion propulsion can be greater due to the length of time the force can be applied.)
Now, if you want me to think about momentum, we can talk!
January 9th, 2008 at 1:27 pm
Tometheus: I would think, that with the cluster as our frame of reference, the black hole is accelerated to it’s high velocity in a very short time. This is based on the explosive nature of gamma-ray bursts, some of which may (stress may) be caused by black hole - black hole mergers. But I don’t know of any bh - bh merger obseravtions being confirmed.
And yeah, the momentum is awesomely awe-full
January 9th, 2008 at 1:30 pm
Amazing stuff.
By the way, these fragrance ads all over the page today are hilarious.
January 9th, 2008 at 3:05 pm
On the other hand, aren’t we on the edge of the Milky Way? And if one of these doo-hickies were, in fact, headed our way, we’d never see it coming , would we? In fact, we’d probably never even know what hit us. One moment we’re arguing about Hillary’s pantsuits, and the next we’re bug splatter on the windshield of some super-sized globular cluster black hole.
Well, sounds like a pretty good reason for me to ditch my New Year’s resolutions.
January 9th, 2008 at 4:10 pm
Are any other types of energy released besides gravitational when these intermediate mass black holes collide? Does their interaction release massive amounts of gamma and x-rays? What type of forces are we talking about here? Also, would the interaction be any different if the black holes themselves were active or not during the collision? For example, what if both were active, or one, but not the other, and if neither was active, how would the range of interactions be different here?
January 9th, 2008 at 5:50 pm
If black holes have such small radii, how do they collide, as opposed to just orbiting each other eccentrically? Also, are intergalactic black holes detectable by the lensing of distant objects?
January 9th, 2008 at 7:44 pm
Good grief, what a concept! I find the image of a black hole just sitting (relatively) still, drawing in the closest matter to it, eerie enough. But the prospect of these things haring through the galaxy at (relatively) high velocity is beautifully terrifying! Actually finding one of these “great black sharks” would be an amazing discovery.
January 10th, 2008 at 2:41 am
1 solar mass = 2×10^30kg
20 solar masses = 4×10^31kg
v = 4,000,000m/s
e=0.5mv^2
e=3.2×10^44 joules
e=3.2×10^51 ergs
As I recall, the total output of a supernova is of the order of 10^51 ergs. This, people, is one HELL of an energetic black hole cannon.
January 10th, 2008 at 5:01 am
Lugosi writes:
[[On the other hand, aren’t we on the edge of the Milky Way? And if one of these doo-hickies were, in fact, headed our way, we’d never see it coming , would we?]]
Actually, we’re well within the Milky Way Galaxy, in the Orion arm. We’re about 2/3 of the way to the edge of the main disc.
January 10th, 2008 at 5:02 am
Lab Lemming writes:
[[If black holes have such small radii, how do they collide, as opposed to just orbiting each other eccentrically?]]
Given enough time. Also, when two of them merge, the resulting black hole is twice the linear size (and thus one-fourth the density) of a single black hole.
January 10th, 2008 at 6:19 am
So the old sci-fi plot device of a black hole threatening Earth isn’t so far-fetched after all? Maybe such a black hole was the engine the drove Moonbase Alpha on it’s journey. Or would that be too neat and tidy?
And yes, the fragrance ads have been quite silly…
January 10th, 2008 at 7:37 am
Eh, Barton? A black hole doesn’t technically have size, if I have this figured right. Each black hole has an infinite density. The “size” I think you are referring to is the event horizon. That’s a boundary, not a physical structure of the black hole.
January 10th, 2008 at 8:38 am
Thanks for the astro articles! Great facts, great comments and great questions. Very brain-stretching this early in the AM.
January 10th, 2008 at 10:38 am
So, if a black hole is active, does that mean a life bearing planet could possibly orbit such, drawing its light from the disruption of material close in to the Black Hole? Interesting story possibility here,,,
GAry 7
Really boss photos, Phil.
January 10th, 2008 at 11:18 am
Earl — yes, I was referring to the Schwarzchild radius. The actual black hole is a point. But for collisional purposes, if the Schwarzchild radii intersect, you’ve got a merged black hole. Relativity and all that.
January 10th, 2008 at 3:15 pm
OK, but that still doesn’t explain how orbiting black holes can lose relative angular momentum.
January 11th, 2008 at 7:28 am
I thought there “is no escape” once past the event horizon, surely another hole’s singularity is no exception to this.
January 11th, 2008 at 11:30 am
“Usually, this radiation [from merging black holes] 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.”
So, it is the asymmetry of the “explosion”/energy release (from the merging black holes) that is propelling it/them out of the cluster. The black holes have still merged.
January 12th, 2008 at 8:29 am
Lab Lemming: There are 3 main mechanisms by which orbiting black holes can lose energy and angular momentum, as far as I know. The first is through dynamical friction (a global transfer of energy and angular momentum to stars), the second is by super-elastically scattering stars that come near the black hole binary, and the third is by emitting gravitational radiation (by far the most efficient, but only operates when the black holes are relatively close to each other). If there is a significant amount of gas present near the black holes, then gas accretion and torques can also help get rid of angular momentum.
January 21st, 2008 at 10:12 pm
If a supermassive black hole was rocketed out of our galaxy, what are the chances of it ripping through the lining of our universe? Is that possible? (guessing no)
Another possibility would be that if it orbited the universe leaving a trail of destruction, it would either have to end up in the center of the universe after a very long tim or else it would have to rip through the lining of the universe.
Wouldn’t it?