Monday March 15th 1999
Announcements:

Lecture notes:

Class 25

At 8 PM at night find North Star then turn around 180 degrees to see
Orion - Rigel Betelgeuse
Canis Major - with very bright star Sirius
Canis Minor with Procyon

Gemini - twins Castor and Pollux - in plane of ecliptic 2 planets discovered while in Gemini - Uranus (1781) Pluto (1930)

Auriga - charioteer - named after mythical inventor of chariot.  supposed to be his face - Capella is his eye.

Quasars and Active Galaxies.
This is a subject that Dr. Brandt spends much research time on.  He is an X-ray astronomer and studies the giant black holes that we think powers quasars and active galaxies.

Start by telling you what a quasar is and then we'll talk about the history of them and how they were discovered and our
theories for how they work.  Then we'll talk about active galaxies in general.

Quasar - most luminous type of object in the Universe.  Light comes from a very small point and looks like a star.  Originally
meant "quasi stellar radio source" - will explain this in a minute.  We think it is the very center of a galaxy that has a big black
hole in it - as the black hole swallows gas makes a great deal of radiation - very hot millions to billions of degrees.

Video shown of an accreting black hole.
Zoom in to the center of the galaxy.  See a black hole.  Disk of gas around the black hole is being sucked in.  Then you will also see jets that are being fired out from the quasar.

Basic properties of quasars.
- Extremely high luminosites - more than 1000 times Milky Way.
- Lots of energy comes out at almost all wavelengths - radio - IR - optical - UV - X-ray.  Unlike stars which are mostly
optical.
- The apparent brightness of quasars can vary quite quickly in time.
For example in optical some quasars can double in a week.  In X-rays even faster - we just discovered one that doubled in
about 3000 seconds.

The Variability is important - tells us the main power source is quite small.
Explain light travel time argument.

Lets say you had an emitting region 100 light years in radius. Let's say it suddenly switches on.  Light from the far side takes
100 years more time to reach you than light from the near side thus variability would take roughly 100 years.

We think most of energy in quasars generated in a region about the size of our solar system.

Quasars show strong and broad emission lines in their optical spectra.
- made by gas that is irradiated by radiation from central
source.  Causes atomic transitions in the atoms in the gas and this makes the lines - just like we learned about before.  Broad
due to Doppler effect.

How Quasars were discovered.
Shortly after WWII people started building high quality radio telescopes.  Found many galaxies with quite strong radio
emission.

Also found a few pointlike radio sources that appeared to be associated with stars.  One of these was the source 3C273.

In 1962 Maarten Schmidt at Caltech took an optical spectrum of 3C273 and found strong spectral lines but could not figure out what the lines were - they did not resemble the lines from any star or galaxy he knew.

Then in 1963 he actually realized they were standard lines from hydrogen.  He had not understood them before because they were shifted to the red by an extremely large amount - 16%.

This meant 3C273 was flying away from us at 16% the speed of light - was a huge speed!

V=H₀D Hubble law
D=V/H₀ = .16 * 3x10⁵/70 = 685 Mpc -- was 685 Mpc away!
huge distance - most distant object known meant the universe was very big.

Because quasars are so luminous we can see them very far away.  Today we see them many thousands of megaparsecs away. (remember distance to M31 is about 1 Mpc for comparison)

In fact the most distant objects known are quasars.  Some of the most distant ones were discovered by PSU Professor Don Schneider.

The light from the most distant quasars takes an extremely long time to reach us.  In fact light from most distant quasars known
takes over 10 billion years to reach us.  The photons left those quasars before the Earth even existed.

Quasar demography.
- There were many more quasars during part of the early Universe than there are now.
- This means that many quasars must have "shut off" over time.
Black hole stopped swallowing matter so much became faint.

Where are all the quasar black holes?  Many are in "normal" galaxies we think.  Our Galaxy has a black hole for example.
Our Galaxy was probably never a powerful quasar though since the central black hole mass is too low.

How are quasar black holes made?
- not from collapse of 1 star and that's all.
- thought to have started with a little black hole that grew and grew.

Active Galaxies.
- objects that are like quasars but less luminous
- intermediate between normal galaxies and quasars.  10 - 100 times as luminous as Milky Way.

There are many different types but we think they are basically all the same thing - black holes eating gas.

Slides were shown of:
physical model of AGN
Cyg A in radio and optical
M87 radio jet
M87 jet in detail
3C279 variability.