29:50 Modern Astronomy
Fall 1999
Lecture 25 ...October 25, 1999
Hyperactive Galaxies
In the Sky... Full moon last night, Mercury at greatest angular
distance from the Sun in the evening.
Begin with a summary of characteristics, main results of our understanding
of galaxies from last time.
90 % of the mass of the
Milky Way and other similar galaxies is composed of Dark Matter. There are no
good ideas of what comprises Dark Matter.
1000 galaxies in a region a few
Megaparsecs in diameter. The biggest of the Virgo clusters are giant ellipticals like
M84 and M87, which you can see with binoculars. 
National Geographic poster with Virgo cluster. Also look at Figure 25.11
on p587 of your book. Galactic Oddballs. The biggest and brightest elliptical galaxies like M87 are several times brighter than the Milky Way. However, this doesn't fully convey how varied they are.
If one observes these galaxies at radio wavelengths or x-ray wavelengths, they are much more remarkable.
Look at the sky at wavelengths from 3 meters (frequency of 100 MHz) to 2 cm
(frequency of about 15000 MHz). The Milky Way is a weak emitter of radio waves.
NRAO poster of sky at 1400 MHz (wavelength of 22 centimeters).
Sky full of point sources, like stars. Stars are objects typically 50 - 500 parsecs away.
The point sources in the radio are radio galaxies at typical distances of
50 - 500 MEGAPARSECS!!! .
The most luminous radio galaxies are more than 
times the radio luminosity of the
Milky Way. What is going on to make this so?
Radio galaxies, including the Milky Way radiate (shine) by a radiation mechanism called synchrotron radiation. Moving charged particles such as electrons and protons are bent by a magnetic field. When they change direction, they feel an acceleration. According to physics, accelerated charges radiate, or emit electromagnetic waves.
Demonstration of bent electron beam.
When the electrons are traveling very, very close to the speed of light, the radiation emitted is called synchrotron radiation.
Cosmic Ray Electrons 
Magnetic Field Synchrotron Radiation
The fact that some of these radio galaxies are so much more luminous than the Milky Way means they have more cosmic ray electrons, more energetic ones, and stronger magnetic fields than the MW.
Pictures of Radio Galaxies. You might expect that radio galaxies would be
nebulous glowing clouds of radiation. In actuality , the strongest ones show a striking
double structure, in which the radiation originates in two large lobes, or
clouds. Each of these lobes is typically 50 to 100 kiloparsecs (sometimes more) from the
nucleus of the galaxy. Pictures of examples are shown on p562 and 563 of your textbook.
Radio Galaxy Cygnus A
One of the amazing things to think about is how a galaxy creates these big blobs of electrons traveling at close to the speed of light, hundreds of thousands of light years from the center of the galaxy.
Observations answer this question by showing the existence of beams or jets that connect the galactic center with these lobes. The jets apparently serve as conduits for the flow of energy from the galactic center to the lobes.
In addition to radio galaxies, there are even more intriguing objects called Quasars. To really appreciate Quasars, we need to backtrack a bit and talk about how we know the distances to these objects.
