Second Black Hole at the Center of the Milky Way

Tsalg pastes "A second black hole lurks at the centre of our Galaxy, according to astronomers who have watched a cluster of stars spinning around it. Just three years ago, astronomers confirmed that the Milky Way revolves around a supermassive black hole, called Sagittarius A*, which is about 2.6 million times more massive than the Sun. But now a much smaller black hole, just 1,300 times our Sun's mass, has been found orbiting about three light years away from its supermassive cousin. placing it intermediate between the relatively small (stellar mass) black holes in the Milky way Galaxy and the supermassive black holes found in the nuclei of galaxies."

Comments

Contradictory?

[dshaw858]

I thought that the goin theory was that at the center of each galaxy lay a black hole, which created the "spiral" effect (such as the one that we see in the Milky Way's "arms"). Does this contradict current knowledge, or is our galaxy just a fluke?

- dshaw

Re:Contradictory?

[Profane MuthaFucka]

That would be a movie effect, not reality. The spiral arms of the galaxy are a density wave propagating through the stars and dust of the galaxy's disk. Think of how sound can be described as a density wave propagating through the air. Same thing.

Re:Contradictory?

[Ayaress]

What the other poster said. But even if things where like you said, this is a 1600 solar mass black hole orbiting a 2.6 million solar mass black hole at a distance of 3 light years. At galactic distances, they can be approximated as a single 2.6016 million solar mass object. It's just not big enough to matter in that respect.

Re:Contradictory?

[CheshireCatCO]

There is (according to current understanding) definately a density wave there. Read Binney and Tremaine's "Galactic Dynamics", for example. While it's certainly true that stars virtually never collide, they don't have to to propogate a wave. Their mutual gravity binds them together quite nicely. (We see the same sorts of behaviors in Saturn's rings, incidentally. The rings are also collisional, but self-gravity is what lets most of the waves propogate.)

If the arms rotated because of the stellar orbits, you can easily see that the arms would be wound up beyond recognition by now. So that clearly doesn't work. (It's referred to as the "winding problem" in astrophysics.)

By the way, I'm pretty sure that sound waves aren't consider "density waves". The latter are driven by gravity, sound waves are pressure waves.

Re:Contradictory?

[Profane MuthaFucka]

Density waves don't need to be limited to just inelastic collisions. Look at traffic on a highway, in a traffic jam. You've got density waves there too, and no collisions, hopefully.

In a galaxy, gravity is the binding force. Imagine every star in the galaxy as a physical ball. And for simplicity's sake, imagine that it's connected to all it's neighbors by a spring. That spring is gravity. Now, move the sheet of interconnected stars that you've just made. Bingo, you have density waves.

black hole collision

[i_should_be_working]

any astronomers know what to expect to see when two black holes collide? we have pictures of stars colliding or ripping each other apart. we have ones of whole galaxies colliding. but what about black holes?

Re:black hole collision

[benhocking]

And some very interesting gravity waves will be generated!

Re:black hole collision

[chenzhen]

Actually this is a pretty tough problem to solve. The computation was attempted by several leading numerical relativity institutions some years ago, but met with no success. The professor I work for is currently building up a code that will hopefully someday be able to handle the binary collision problem.

One of the major problems is that programs crash pretty quickly when the evolution develops a singularity. A good method for avoiding this is called excision, where the singularity is removed from the grid and replaced with boundary conditions. This was recently implemented in my advisor's group and applied to the binary neutron star problem. At the end of the evolution, a black hole forms, so it doesn't seem like there are too many steps before a full black hole collision is possible.

Re:black hole collision

[chenzhen]

On second thought I shouldn't say no success. There have been successes in computing special, less physical cases, for example in treating the stars as frictionless dustballs not possessing magnetic fields. But these features are very important in determining the rotation structure of stellar fluids, and are probably essential in modeling the physically correct binary merger. The general problem remains to be solved, and the goal is to figure out what physical processes produce gravitational waves, so that we know what to be looking for experimentally.

Here are some visualizations of previous merger simulations:
1
2

Re:black hole collision

[Tablizer]

Actually this is a pretty tough problem to solve. The computation was attempted by several leading numerical relativity institutions some years ago, but met with no success.

Just divide by zero......twice :-)

Re:black hole collision

[scoser]

And here's a distributed computing project to detect such gravity waves: http://www.physics2005.org/events/einsteinathome/

Tithing

[AtariAmarok]

"black holes are a popular myth like god"

So, if you give money to a church, it goes to God; and if you pay taxes to government, it goes down a black hole?

Re:Is this a big surprise?

[Tsalg]

That this is a surprise depends on whom you ask. The real issue here is to understand how those huge f**off multi-billion solar mass black holes form. And so far there had not been such high-quality evidence for anything in between a stellar-mass black hole formed by a single massive star collapse, and those monsters in the middle of galaxies.

So those who think that they come from mergers of solar-mass BHs are comforted. There's also those who say that in no way those monsters had enough time to form by such a slow process. Read for instance Spin, Accretion and the Cosmological Growth of Supermassive Black Holes. Formation of supermassive black holes in turn is likely to have an impact on star formation rate in galaxies, another highly speculative area.

The other original thing here is that evidence for intermediate BHs in other galaxies comes from 1) luminosity measurements, which is a much more biased method than speed measurements of stars gravitating around it (to measure star velocities you have to be able to resolve them, which is only possible in relatively nearby objects) and 2) objects that were not in small clusters like here.

Death and Taxes

[RobertB-DC]

[T]hey calculated from the movement of the seven stars that they must be orbiting an intermediate-mass black hole, called IRS 13E, which spirals around Sagittarius A* at about 280 kilometres per second.

Is it just serendipity that this object, into which everything goes and never comes back, is named after an Earthly agency to which similar attributes are often ascribed?