Download No Slide Title

Chapter 15: Normal & Active Galaxies
Normal and
Active
Galaxies
Chapter 15: Normal & Active Galaxies
Three different types of Galaxies
Spirals
Hubble devised a classification scheme to describe different
shapes. Spirals typically have a center of older stars, and spiral
arms which are sites of star formation.
Chapter 15: Normal & Active Galaxies
Ellipticals
Ellipticals have no spiral arms, no flattened disk, very little
dust or star formation in progress.
Chapter 15: Normal & Active Galaxies
Irregulars
Irregular galaxies don’t fit into nice categories. They
tend to be rich in dust, gas, and active star formation
regions. Believed to be the result of “collisions” of
normal spiral galaxies.
Chapter 15: Normal & Active Galaxies
We have two irregular galaxies in orbit around the Milky Way
called the Magellanic Clouds which were named after the
famous explorer.
Chapter 15: Normal & Active Galaxies
Hubble’s classification scheme
Chapter 15: Normal & Active Galaxies
Comparing properties of the different kinds of galaxies
Chapter 15: Normal & Active Galaxies
Since more massive galaxies are likely to be brighter, and
because more massive galaxies are likely to rotate faster. The
Tully-Fisher relation allows us to estimate the luminosity of the
galaxy by its rotation speed.
Chapter 15: Normal & Active Galaxies
We now have
measuring
techniques that
allow us to
determine
distances over 200
Mpc.
Chapter 15: Normal & Active Galaxies
Distribution of Galaxies in Space
Distribution is not random. Galaxies are concentrated in
some regions which are beside regions which are empty or
void.
Local
Group of
about 20
galaxies.
Chapter 15: Normal & Active Galaxies
Local Super Cluster
Chapter 15: Normal & Active Galaxies
When Hubble began to take
spectra of the galaxies, he
eventually noticed that the
spectral lines where extremely
red shifted. Using, primarily
Cepheid variables to
determine their distance, he
noticed a tight correlation
between velocity and distance
Chapter 15: Normal & Active Galaxies
This relationship is
knows as Hubble’s
Law
Chapter 15: Normal & Active Galaxies
Now we can
measure distances
all the way to the
seeable edge of
the universe.
Chapter 15: Normal & Active Galaxies
The spectrum from
normal galaxies is
dominated by black
body radiation from
stars.
The spectrum from
active galaxies have an
additional component
due to synchrotron
radiation.
Chapter 15: Normal & Active Galaxies
Galaxy Interactions
When galaxies “collide” they pass through each
other without stars actually colliding. We think this
is how irregular galaxies are made.
Chapter 15: Normal & Active Galaxies
Galaxy Interactions
When galaxies “collide” they pass through each
other without stars actually colliding. We think this
is how irregular galaxies are made.
Chapter 15: Normal & Active Galaxies
Synchrotron radiation is caused by high speed electrons
spiraling around strong magnetic field lines. The radiation is
stronger in the radio part of the spectrum and these are called
radio galaxies
Chapter 15: Normal & Active Galaxies
Chapter 15: Normal & Active Galaxies
Radio loud galaxies were found in
radio surveys but their visible image
looked faint and star like and were
called Quasi Stellar Radio Sources
or Quasars for short.
Chapter 15: Normal & Active Galaxies
Because active galaxies vary
in brightness over a period of
months, they must be at most
a few light months in size.
Chapter 15: Normal & Active Galaxies
Chapter 15: Normal & Active Galaxies
We can measure the Doppler velocities and combine this with
distances to use Kepler’s laws to determine the mass of the
“engine”. Answer (3 billion suns).
Chapter 15: Normal & Active Galaxies
The source of the radio emission
is called Synchrotron radiation
which is produced by
accelerating electrons. This
radiation has a fundamentally
different spectrum than black
body radiation.