Active Galaxies

1
Active Galaxies
2
Some external galaxies are extremely luminous
in some instances their luminosities may exceed
that of the Milky Way by factors of hundreds and
even thousands. Such active galaxies include
double-lobed radio sources, Seyfert galaxies, BL
Lac objects and, most luminous of all, the
enigmatic Quasars. Most of these objects are
strong radio sources many radiate strongly
across the entire electromagnetic spectrum.
3
Double-Lobed Radio Sources
The scale of Double-lobed radio sources can be
enormous the largest known is 5.8 Mpc across.
4
Double-lobed radio sources are among the
brightest radio sources in the sky. They
consist of

• A central compact active galaxy

• A pair of jets emanating from this galaxy

• Radio lobes and hot spots caused by the
  collision of the
• jets with the inter-galactic medium.

5
3C31 is a double-lobed radio galaxy that shows a
cork- screw in its jets and radio lobes. This
cork-screw strongly suggests that the engine in
the central galaxy is precessing, implying
the presence of rotation.
6
Seyfert Galaxies
About 2 of spiral galaxies have an extremely
luminous nucleus, some showing the presence of
jets. These galaxies are known as Seyfert
Galaxies.
7
(No Transcript)
8
The spectrum of a Seyfert galaxy is dominated by
strong, broad emission lines. The very broad
profiles suggest either that the gas in the
nucleus is very hot (100 million K) or that the
gas is rotating very rapidly around a
very massive object in the core of the galaxy.
9
Seyfert Galaxies as well can vary significantly
on time scales as short as a few months. This
suggests that the central engine in these
galaxies cannot be much larger than a light-year
in size surprising considering the enormous
luminosity of these galactic nuclei.
10
BL Lac Objects
BL Lac Objects are active galaxies similar to
Seyfert galaxies, except that they occur in
elliptical galaxies. However, their spectra do
not exhibit any emission lines. BL Lac objects
can vary significantly on time scales of a few
days in the X-rays, again suggesting that the
central engine is very small indeed.
11
Quasars Quasi-Stellar Objects QSOs
Quasars were first detected as extremely strong
radio sources. When astronomers obtained optical
images of them, they looked just like stars
hence the name QSO.
3C 273
12
Quasars
The optical spectra of these mysterious objects
were at first undecipherable, consisting of a few
broad unidentified emission lines.
In 1963, Maarten Schmidt discovered these
emission lines are actually highly
redshifted lines of hydrogen.
13
Quasars
Later on, examples of radio-quiet quasars were
discovered, but all quasars exhibited extremely
high redshifts. According to Hubbles Law,
these large redshifts suggest that these objects
are at immense distances, implying that they are
extremely luminous up to 1000 times more
luminous than normal galaxies.
14
Quasar Redshifts
Redshift z
The largest detected redshift so far is z 6.4,
which implies this quasar is receding from us at
more than 99 of the speed of light, and is near
the edge of the observable Universe!
15
Update In March 2004 the highest detected
redshift is z ? 10 a galaxy about 13.2 billion
light years away!
16
Quasars
Recent optical studies of Quasars show that these
luminous objects are extremely active nuclei of
galaxies.
17
(No Transcript)
18
Quasars also show variability on time scales of
years, months and even days, again implying that
the central engines of these luminous objects
are very small.
19
The Energy Source of Active Galaxies

• Clues, such as
• Variability on short time scales
• The presence of jets
• The precession of the jets
• SUGGEST
• The central engine
• is a massive
• Black Hole with
• an Accretion Disk.

20
Calculations suggest that 10 M? /year falling
into a central massive blackhole of 1 billion M?
could account for the entire luminosity of a
typical Quasar.
21
(No Transcript)