Date: January 22, 2010
Title: Black Holes and the Formation of Galaxies
Podcaster: Robert Knop
Description: A monster lurks in the center of our galaxy!
Bio: Dr. Rob Knop is associated with MICA, the Meta-Institute of Computational Astronomy. You can find us on the web at www.mica-vw.org.
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Transcript:
I am Dr. Rob Knop; I will be joining the faculty of Quest University in Squamish, British Columbia in the middle of this year, where I will teach Physics. I’m also associated with the Meta-Institute of Computational Astronomy; you can find us on the web at www.mica-vw.org.
So what about this monster lurking at the core of our Galaxy? Astronomers have used infrared light– so as to peer through the thick dust that is uibiquitous in the plane of our Galaxy– to look at the motions of stars right at the center of our Galaxy. What they see is that those stars are orbiting a very massive dark body. This black hole is about 4 million times the mass of the Sun.
That sounds like a lot, but it’s not much compared to the mass of the Galaxy as a whole. The mass of stars in our Galaxy is about 10 billion times the mass of the Sun. That’s ten thousand times more than the mass
of the black hole at the Galaxy’s core. Most of the total mass of the Galaxy is in dark matter, and the mass of the Galaxy’s dark matter is about 100 billion times the mass of the Sun. So, while the supermassive
black hole at the core of the Galaxy is 4 million times the mass of the Sun, its gravity is insignificant compared to the gravity of the stars and the gravity of the dark matter in our Galaxy.
We think of black holes as being very massive things that pull in anything around them. However, it is not the black hole that holds our Galaxy together. The Sun orbits the Galaxy once about once every 250 million years. But the Sun is in fact not orbiting the black hole. It is the mass of the stars and the dark matter that holds the Sun, and all the other stars, in their orbits arond the Galaxy. The black hole’s mass is irrelevant to the dynamics of the Galaxy– except for the stars closest to it right at the core– because even though it’s large compared to a star, it’s small compared to a Galaxy.
Our Galaxy is not alone. Other galaxies also have supermassive black holes at their cores. Every time astronomers have looked carefully enough at the core of a big galaxy, they’ve found a supermassive black hole there. Indeed, it turns out that even though there’s a monster lurking at the core of our own Galaxy, it’s pretty wimpy as these things go. For example, the black hole at the core of the Andromeda Galaxy is
about 30 million times the mass of the Sun, an order of magnitude larger than our own black hole. The elliptical galaxy M87 in the Virgo cluster harbors a black hole that is 3 billion times the mass of the Sun, three
orders of magnitude larger than our own black hole.
It turns out that the masses of central black holes are related to the masses of bulges of galaxies. Our galaxy is composed of a large dark matter halo, a tenuous halo of stars and globular clusters embedded in
this dark matter halo, a disk where most of the stars, including our Sun, are, and a spheroidal bulge at the center. The Andromeda Galaxy has a similar construction. Elliptical galaxies like M87 are “all bulge”. If you measure the mass of the bulge of a galaxy, and compare it to the mass of the supermassive black hole at the center of the galaxy, you find that they’re linearly related. Less massive bulges have less massive black holes, and more massive bulges have more massive black holes. However, because, as in our Falaxy, the mass of the black hole is small compared to the mass of stars and dark matter in the whole galaxy, this is rather surprising. The black hole is not important to the gravitational dynamics of the galaxy as whole, so why should the masses be so tightly correlated? Clearly, there is some other physical process besides simple gravity linking the black holes to the stellar bulge.
Astronomers’ best understanding today is that this process is something we call “AGN feedback”. AGN stands for “Active Galactic Nucleus”. A quasar is an example of an AGN. You get an AGN, or a quasar, when you
feed the black hole. If you allow a large amount of gas to build up around the supermassive black hole at the core of a galaxy, things happen. Right around the black hole, you get what’s called an accretion disk. This is a disk of gas swirling around the black hole at a tremendous rate. It can be extremely hot, so hot that it emits X-rays and gamma rays. What’s more, as a result of this gas near the black hole, you can get narrow jets of gas that shoot out from the poles of the black hole at extremely close to the speed of light.
Galaxies first formed probably a few hundred million years after the Big Bang. As the dark matter, originally spread nearly uniformly through the Universe, started to clump together, it made gravitational potential wells that would draw in the gas. Through processes astronomers still don’t fully understand, there were seed black holes at the center of these potential wells. The gas falling into the potential wells would do two things. First, stars would form out of the gas. Second, the gas would feed the AGN. Even after the very first formation of galaxies, galaxies collided with each other and grew, making the larger galaxies we see today. Each time they collided with each other, you would get a burst of star formation as the gas clouds ran into each other, but you’d also funnel some of that gas down to the core to feed the supermassive
black hole.
As the black hole was fed, eventually the accretion disk around the black hole would heat up enough to turn on a powerful quasar. The energy released from that quasar would blow out into the surrounding galaxy. This energy wouldn’t do very much to the stars; stars themselves are gravitationally bound well enough that they wouldn’t be too bothered by the quasar’s energy. But the gas in between the stars would be blown out of the galaxy by energy ultimately generated in the quasar. As the gas got blown out of the galaxy, that would cut off further star formation, as there was no more gas left to form stars.
This is the physical process that couples the central supermassive black holes to the masses of the galactic bulges. Even though the gravity of the black hole isn’t directly relevant to the dynamics of the galaxies, the fact that a supermassive black hole can generate a huge amount of energy in the quasar phenomenon allows it to clear out the galaxy of gas, thereby coupling the central black hole with the stars, and the formation of stars, in that galaxy.
End of podcast:
365 Days of Astronomy
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