By Joshua Winn, Princeton University
A galaxy called M87 is an elliptical galaxy centering around a black hole. It seems kind of dull, but looking closely, we see a tiny blue streak in its middle. It might look like a glitch in the image, but it’s not. A closer look shows that there’s something extraordinary about this galaxy. The center is glowing brightly, and it’s emitting a beam of light and accreting gas.
Black Holes Actively Accreting Gas
The M87 is an example of an active galaxy. There’s a supermassive black hole at the center, just as there is in all big galaxies, but what’s different here is that the black hole is actively accreting gas. Gas is funneling toward the black hole. The gas slowly loses energy and angular momentum, causing it to spiral inward, speed up, and heat up. By the time it’s within a fraction of a parsec of the black hole, it’s glowing brightly.
After it crosses the event horizon, we never see it again and the black hole gets slightly more massive. But just before that, at the innermost edge of the accretion disk, two powerful jets of plasma are being launched up and down, perpendicular to the disk. Why this happens is only partly understood. One thing that’s clear is the force propelling the jets is electromagnetic. By this point, the gas is a hot, ionized, plasma, and like many plasmas, it’s prone to developing a tangled internal magnetic field. As the plasma and magnetic field approach the black hole, they whirl around in a frenzy, inducing electric fields that can accelerate charged particles vertically.
Black Holes Can Rotate
The electromagnetic field becomes so strong that electrons and positrons emerge spontaneously, out of pure energy, a phenomenon called pair production. Positrons are the antimatter equivalent of electrons; same mass, but opposite charge. All those newborn charged particles fly away from the disk at nearly the speed of light. And the underlying energy supply for all these fields and particles might be the black hole itself. Black holes can rotate.
Accreting black holes absorb so much angular momentum from the spiraling material that they probably rotate close to the speed of light. This leads to a relativistic effect in which the surrounding space starts spinning, too, and if there’s plasma there, the rotational energy can be converted into magnetic energy. So, the jets of an active galaxy might be rotation-powered, in the same way that we can see the luminosity of the Crab Nebula coming from the rotation of the neutron star at its center.
Now, none of this is obvious. This so-called Blandford-Znajek mechanism relies on concepts from plasma physics and by itself, it doesn’t explain why the ejected particles end up forming such a narrow beam. To explain that, theorists figure that the accretion disk is not like a thin, flat plate surrounding the black hole; it’s more like a fat inner-tube, or a torus, which restricts the escaping charged particles to narrow cones surrounding the black hole.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
Different Types of Active Galaxies
There are many different types of active galaxies that go by different names, depending on how luminous they are, and from what angle we’re viewing them. For example, an optical image of Cygnus A shows a galaxy with an irregular shape. When we look with radio and X-ray telescopes, we see that the galaxy is surrounded by a haze of X-rays, and at radio wavelengths we see 2 jets that puff out into galactic-scale fireballs. This type of active galaxy is called a radio galaxy, or to be persnickety, a Fanaroff-Riley Class II radio galaxy.
Other active galaxies look more prosaic. A Hubble image shows two stars, along with some galaxies in the background. But, in fact, the star in the middle of that image isn’t a star. It’s an active galaxy, millions of times farther away than the star. The giveaway is the spectrum, which shows emission lines from the hot gas in the accretion disk. The accretion disk in this case is 100,000 times more luminous than all the stars of the galaxy put together. With that bright light shining in our faces, we can’t even see the galaxy. This kind of active galaxy is called a quasi-stellar object, or quasar.
Why Do Accretion Disks Glow?
How can the light from an accretion disk overwhelm an entire galaxy-worth of stars? How long could such a beacon possibly shine, before it runs out of energy? Let’s figure it out. First we need to remember why accretion disks glow. Gravity pulls the gas inward, speeding it up, converting gravitational potential energy into kinetic energy. Then, within the vortex of material, a lot of that kinetic energy is dissipated as heat.
When a small mass, dm, falls from far away, the energy released is GM times dm over 2R, where in this case, M is the black hole mass, and R is the inner edge of the accretion disk, which is about three times the Schwarzschild radius. That’s the location of the innermost stable circular orbit, the ISCO.
Calculating the Resulting Luminosity
To calculate the resulting luminosity, energy per unit time, we divide by the time interval dt over which the mass is falling, and simplify. The answer is 1/12 dm/dt times c-squared. We can also solve dm/dt. That’s the rate at which the black hole must be fed, in order to produce a given luminosity.
An entire galaxy has a luminosity of order 10 billion suns, and if the accretion disk is 100,000 times brighter, then L is 10 to the 15 solar luminosities. What value of dm/dt is required? The answer is 5.1 times 10 to the 25 kilograms per second. That sounds like a lot. But is it? Let’s convert to more relevant units, say, solar masses per year. Numerically it comes out to be 810. Thus, it’s not so much, after all.
Common Questions about Accreting Black Holes
M87 is an example of an active galaxy. There’s a supermassive black hole at the center, just as there is in all big galaxies, but what’s different here is that the black hole is actively accreting gas.
Accreting black holes absorb so much angular momentum from the spiraling material that they probably rotate close to the speed of light.
There are many different types of active galaxies that go by different names, depending on how luminous they are, and from what angle we’re viewing them.
