Chapter 19: Dark and Violent Universe

Energy from a Black Hole

- Gravitational potential energy of matter falling into a black hole turns into kinetic energy - Friction in the accretion disk turns kinetic energy into thermal energy (heat) - Heat produces thermal radiation (photons) - This process can convert 10-40% of E=mc^2 into radiation. Compare to fusion

Characteristics of Active Galaxies

- Luminosity can be enormous (>10^12Lsun) - Luminosity can rapidly vary (comes from a space smaller than solar system) - They emit energy over a wide range of wavelengths (contain matter with wide temperature range) - Some drive jets of plasma at near light speed - Galaxies around quasars sometimes appear disturbed by collisions

Black Holes in Galaxies

- Many nearby galaxies-perhaps all of them-have supermassive black holes at their centers - These black holes seem to be dormant active galactic nuclei - All galaxies may have passed through a quasar-like stage earlier in time

Fifteen years ago, a quasar was observed that was found to be located 8 billion light years away. If our universe is approximately 14 billion years old, when did the quasar emit the light that we observe?

8 billion years ago

Matter falling into supermassive black holes in the centers of galaxies is thought to be the source of:

1. Jets that shoot far into space 2. Active galactic nuclei 3. Strong radio emissions 4. Quasars ALL OF THE ABOVE

What can you conclude from the fact that quasars usually have very large redshifts?

1. They are generally very distant 2. They were more common early in time 3. Galaxy collisions might turn them on 4. Nearby galaxies might hold dead quasars ALL OF THE ABOVE

What is the luminosity of the most powerful quasars?

1000 times the luminosity of the Milky Way Galaxy

How big are active galactic nuclei?

A few light-hours across

Do supermassive black holes really exist?

Astronomers believe that supermassive black holes lie at the center of virtually all large galaxies, even our own Milky Way. Astronomers can detect them by watching for their effects on nearby stars and gas. - Orbits of stars at center of Milky Way stars indicate a black hole with mass of 4 million Msun - Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion Msun

Active galactic nucleus (continued)

Can shoot out blobs of plasma moving at nearly the speed of light Speed of ejection suggests that a black hole is present

Radio galaxies

Contain active nuclei shooting out vast jets of plasma that emit radio waves coming from electrons moving at near light speed - Don't appear as quasars because dusty glas clouds blick our view of their accretion disks

Active galactic nucleus

If the center of a galaxy is unusually bright Quasars are the most luminous examples - Highly redshifted spectra of quasars indicate large distances - Variability shows all this energy comes from region smaller than the solar system

What powers active galactic nuclei?

Matter faling into a supermassive black hole

Where in the universe are most quasars found?

More than halfway to the boundary of the observable universe

Which of the following is NOT true of supermassive black holes in the centers of galaxies?

The most massive detected so far is about 50 million solar masses

Imagine that you simultaneously receive the satellite transmission of 2 pictures of 2 people living on planets orbiting 2 different stars. Each image shows the people at their 21st birthday parties. Consider interpretations that could be made from your observations. Which do you think is the most plausible interpretation?

The person that is farther from you is actually the older of the two people

Galaxies and Black Holes

The mass of a galaxy's central black hole is closely related to the mass of its bulge - Galaxies with large bulges have large black holes and vice versa The development of a central black hole must somehow be related to galaxy evolution

Why should we not be surprised that galaxy collisions were rather common in the past?

The universe was much denser in the past, so its galaxies were much closer together, making collisions much more frequent

When we look at a very distant galaxy, billions of light years away, we see it...

When it was younger than now When the whole universe was younger than today

The black hole at the center of our own galaxy may once have powered an active galactic nucleus

Yes, active nuclei in other galaxies appeared to be powered by accretion into similar sized black holes

What is the power source for quasars and other active galactic nuclei?

Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies.

The Central Engine of an Active Galaxy

- A supermassive black hole, surrounded by an accretion disk - The strong magnetic field lines around the black hole channel particles into jets perpendicular to the magnetic axis - The black hole may be billions of solar masses