29:50 Modern Astronomy
Fall 1999
Lecture 26 ...October 27, 1999
Radio Galaxies and Quasars
Watch the skies! Watch the skies!
On Sunday, Venus will be at
its maximum angular distance from the Sun, 
. Look at it and think about
the geometry of the solar system.
Over the next month, check out position of Moon relative to Jupiter and
Saturn. It will clearly illustrate the 
inclination of the Moon's orbit
to the plane of the ecliptic.
Contour map of 3C79, typical luminous radio galaxy. Radio galaxies
have up to 
times the radio power output of the Milky Way.
The distance to 3C79 is 1097 Megaparsecs! This is to be compared with the distance to
the Andromeda galaxy of 0.63 Megaparsecs.
Think of distance modulus and how faint even the brightest stars must appear in 3C79. The whole galaxy is a fuzzy little smear on long exposure photos with big telescopes. Its apparent magnitude (the whole galaxy which is substantially more luminous than the Milky Way) is 18.6.
How do we know this? The answer is crucial to appreciating the nature of the Quasars.
Return with us now to the thrilling days of yesteryear....
In the 1920's Edwin Hubble made two major discoveries: (1) the ``Nebulae'' were galaxies,
(2) Galaxy spectra showed an odd feature.
Diagram showing stylized spectrum of nearby galaxy, with rest wavelengths.
Question for audience: What does this mean?
Conclusion from Hubble's investigations: galaxies in all directions are moving
away from us. Even more odd, the further away they are, the faster they are moving.
Diagram showing Hubble's Law in graphical form.
This discovery is given in the form of an equation as
where 
is the speed at which the galaxy is receding from us (kilometers/sec)
d is the distance (in Megaparsecs), and 
is the Hubble Constant in units of
kilometers/sec/Megaparsec. Determination of the precise value of the Hubble constant has
been and continues to be one of the primary goals of observational astronomy. The current
best estimate is 
kilometers/sec/Megaparsec.
The implications of Hubble's Law are philosophically staggering, but we will wait to discuss them when we talk about Cosmology. In the meanwhile we will use it as a distance determination method.
Let's work out an example. 3C79 has a measured recession speed of 
km/sec.
Therefore 
.
, so
Megaparsecs.
This is the way we determine the distances to essentially all extragalactic objects more distance than the Virgo Cluster.
Quasars
Return with us now to those thrilling days of yesteryear... By around 1960
astronomers were realizing many of the brightest radio sources could not be
definitely identified with elliptical galaxies.
Picture of Virgo cluster
Examples were 3C48 and 3C273.
With improvements in radio astronomy, they were able to make precise measurements of
the position on the sky of these two quasars, and saw that they coincided with starlike
objects. Thus was born the class of astronomical objects called Quasi-Stellar Radio
Sources.
Look at Figure 24.2 from book showing spectrum of 3C273.
3C273 has a redshift of 0.16. Smaller than 3C79, but 3C273 is a 12th magnitude object ... you could image it for your lab project.
If we use Hubble's Law to calculate its distance, then use its apparent magnitude
to calculate its absolute magnitude, we end up with an incredibly brilliant object,
brighter than any galaxy. And it doesn't look like a galaxy.
As time went on, thousands of these objects were discovered, and they represented one of the more enigmatic objects in astronomy. They were featured in an episode of Outer Limits in which scientists were driven nuts by ``Quasar Rays''.
For these Quasars, the redshifts ranged up to z=4.4 and beyond. For such larger redshifts you need to use a more general expression for the Doppler effect. Look at Figure 24.3 of your book for a more general distance-recession velocity relationship.
Think about the concept of ``Look-Back Time'' to these objects.
For about 30 years there was a huge controversy about the nature of Quasars. The orthodox viewpoint was that they were extragalactic objects, at the distances indicated by Hubble's Law, and that they were due to enormously brilliant phenomena in the centers of galaxies.
The heterodox viewpoint was that they were an entirely different class of astronomical object.
We now know that the orthodox viewpoint is correct. The real proof was given by Hubble
Space Telescope images which showed the galaxy ``underneath'' the brilliant Quasars.
Blackboard drawing of Quasar model.
HST images of nearby Quasars
Other Topics
Quasars as cosmic ghosts.
The turn-on time for Quasars.
What is going on in Quasars and Radio Galaxies.
