The compact centers of galaxies can emit huge amounts of energy:
the brightest are over 1012 LSun.
We know this energy does not come from stars. Some active nuclei changes its luminosity rapidly, within weeks or even days. This means that the bright region is only light-days across -- size of the solar system!
You can't put 1012 stars in a volume the size of the solar system; they would collide with each other and the arrangement would not last long.
We think that an active nucleus is powered by material falling onto a gas disk
orbiting a black hole of 108 to 109 MSun.
These are just like the black holes in binary stars from Chapter 16, except that the gas in the disk comes from the rest of the galaxy, not from a companion star.
A black hole of 109 MSun has a Schwarzschild radius of 3000 million km or 20AU. The brightest part of the disk is within a few times the Schwarzschild radius, so it is small enough to vary fast.
Power from gravity is much more efficient than from stars. Coverting a mass M of hydrogen into helium releases just less than 1% of Mc2 as energy. But if it falls into a black hole, up to 42% of Mc2 can be given out by the hot gas as it spirals inwards in the disk.
The spinning disk has a strong magnetic field, which builds up because
the gas of the disk is ionized (so conducts electricity) and spinning.
As in pulsars, twin jets of fast electrons come out from the
poles: look at Figure 19.24.
We see synchrotron radiation (Box 16.1) when the electrons moving at near-light speeds spiral around magnetic fields. Mostly this is radio waves, but can be visible light or even X-rays.
The elliptical galaxy M87 has an active nucleus:
Visible: a bright jet about 5000 light years long emerges from the center (Fig 19.4)
Radio: we see that energetic material comes out in two directions. The "2cm" map shows the same region as the visible-light map
Stars shine mostly in the infrared, visible and ultraviolet regions;
the dust around them shines in the far infrared.
But in active nuclei, the gas disk is hot enough to shine in the ultraviolet and at X-ray energies.
Electrons spiralling in magnetic fields produce radio waves and also gamma rays.
Radio galaxies are bright at radio wavelengths but otherwise look
like normal elliptical galaxies.
Quasars have very bright centers in visible light and in X-rays; some are also bright at radio wavelengths, others are not.
Seyfert galaxies are low-power versions of quasars; they are never very bright at radio wavelengths.
Having an active nucleus is much more common in 'young' galaxies; when the
Universe was a third of its present age, powerful active nuclei
were 100 to 1000 times more commmon than they are now.
What happens when the nucleus stops being active? You still have the black hole.
All fairly big galaxies seem to contain black holes at their centers.
Perhaps these used to have active nuclei.
The Milky Way's black hole is about 4 x 106 MSun.
It is close enough that we can watch the stars move across the sky, and also measure their speeds with the Doppler effect.
Animation of observed star orbits near the Galactic center: watch how the stars speed up at closest approach (MPE Garching)
The Milky Way's black hole is almost totally inactive: it gives out only about 104 LSun.
We don't know why; there is plenty of gas for it to eat.
The black hole in the Andromeda galaxy (M31) is also inactive.
We don't know. But we think it is related to the growth of the rest of the galaxy, because the black hole 'knows' which galaxy it is in.
The black hole is more massive in a more luminous galaxy, where the stars also move more rapidly (&sigmar is larger).