Wednesday, 8 July 2009

Active Galaxies

Active Galaxies:
Most galaxies are `normal' in that most of their light is generated by stars or heated
gas. This energy is primarily radiated away as optical and infrared energy.
However, there exists a subclass of galaxies, known as active galaxies, which
radiate tremendous amounts of energy in the radio and x-ray regions of the
spectrum. These objects often emit hundreds to thousands of times the energy
emitted by our Galaxy and, because of this high luminosity, are visible to the edges
of the Universe.
Active galaxies usually fall in three types; Seyfert galaxies, radio galaxies and
quasars. Radio galaxies often have a double-lobe appearance, and the type of radio
emission suggests that the origin is synchrotron radiation.

Active galaxies emit large amounts of x-rays and gamma-rays, extremely high
energy forms of electromagnetic radiation. Strong magnetic fields (synchrotron
radiation in the radiio) plus gamma-rays implies very violent events in the cores of
active galaxies.
Although active galaxies different in their appearance, they are related in the
mechanism that produces their huge amounts of energy, a Galactic mass black hole
at the galaxy's center. The gas flowing towards the center of the galaxy forms a
thick circle of orbiting material called an accretion disk many hundreds of
light-years across.
Since the infalling gas retains the direction of orbital motion of the companion, the
stream of material forms a rotating disk.

Friction between the gas in neighboring orbits causes the material to spiral inward
until it hits the event horizon of the central black hole. As the spiraling gas moves
inward, gravitational energy is released as heat into the accretion disk. The release
of energy is greatest at the inner edge of the accretion disk where temperatures can
reach millions of degrees. It is from this region that the magnetic fields are
produced for the synchrotron radiation and the collision between atoms to emit
x-rays and gamma-rays.
Our own Galaxy core may harbor a small active nuclei similar to those found in
quasars. In fact, all galaxies may have dormant black holes, invisible because there
is no accretion. Seyfert, radio galaxies and quasars may simply be normal galaxies
in an active phase.
This hypothesis has been confirmed by HST imaging of distant QSO hosts which
show the bright quasar core is in the center of fairly normal looking galaxies.
Lookback Time:
The large size of the Universe, combined with the finite speed for light, produces
the phenomenon known as lookback time. Lookback time means that the farther
away an object is from the Earth, the longer it takes for its light to reach us. Thus,
we are looking back in time as we look farther away.

The galaxies we see at large distances are younger than the galaxies we see nearby.
This allows us to study galaxies as they evolve. Note that we don't see the
individuals evolve, but we can compare spirals nearby with spirals far away to see
how the typical spiral has changed with time.

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