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Active Galactic Nuclei
Types of Active Galaxies
• 4 main types
– Seyfert galaxies
– Radio galaxies
– Quasars
– Blazars
What are they?
In general,
• They are small astronomical objects with a
large energy output.
• They are powered by the energy released
when matter is dropped into a massive
black hole in the center of the galaxy.
Seyferts
NGC 1068 and the X-Ray Flashlight
Credit: X-ray: P. Ogle (UCSB) et al.;
Optical: A.Capetti (INAF) et al.; CXO, STScI, NASA
A 10 minute exposure taken with an SBIG ST-9E CCD camera
thru our 20-inch telescope working at F/5.5. The field of view is
about 15x15 arc minutes.
The brightest Seyfert known is NGC 1068
Seyferts
• Has properties that lie in between Normal
Galaxies and Quasars
• Physical Appearance: Spiral galaxies
• Most interesting feature: its Galactic Nuclei
• Galactic Nuclei is compact ~ 1 light year
across
• Galactic Nuclei is powerful ~ output
energy more than several million
supernovae
M77 is a type Sb spiral galaxy in the constellation Cetus. NGC1068 is
the nearest and brightest example of a type II Seyfert galaxy,
showing broad and strong emission lines due to high velocity gas in
the galaxy's inner regions. A strong radio source known as Cetus A
sits in the nucleus.
Seyferts
• Comparison between Seyfect and the
NGC 1232
Impressive
``True-color'' stellar continuum image of the Seyfert 2 Galaxy
NGC 1068 (Messier 77), composed of UV, green, and red
narrowband filters chosen to reject bright emission lines and
only admit starlight.
Prime Focus Camera + Loral 512x512 CCD, 3.6-meter
Canada-France-Hawaii Telescope, Mauna Kea, Hawaii
Not so
impressive
The impressive spiral galaxy NGC 1232, which ressembles
the Milky Way galaxy in which we live. The distance is about
100 million light-years. The photo is a composite of three
exposures in difference wavebands (colours). (ESO VLT
FORS2 multi-mode instrument; 1998).
Seyferts
• Sub-classes of Seyferts:
• Seyfert 1:
• Broad and
Narrow emission
lines
• Seyferts 2:
• Only observe
narrow emission
lines
Seyferts
 Chaisson and McMillan
 Chaisson and McMillan
• Emit radiation of all wave lengths, but
emission lines mostly observed in the infrared
region of the spectrum
• Fluctuations are apparent in Seyferts
because.. HERMAN FILL THIS IN!
Seyferts
• Unification of Seyferts with other type
AGN types
Seyferts
Wind from Accretion Disk around a Black Hole
This illustration depicts a massive black hole at the center of a galaxy. Around it is a
swirling disk of gas, which gradually pours down into the black hole. As the gas falls
inward, it heats up and glows brightly, getting hotter and hotter the closer it is to the event
horizon. Some of the gas is blown away from the disk like steam from a kettle. As this gas
streams off the disk, the intense radiation generated by the very hot gas near the event
horizon forces the escaping gas into a cone and accelerates it to speeds as high as a
tenth the speed of light.
(Illustration: NASA/CXC/M.Weiss)
Seyferts
Illustration of Black Hole with Accretion Disk and Torus
An artist's conception shows a black hole surrounded by a disk of hot gas, and a large
doughnut or torus of cooler gas and dust. The light blue ring on the back of the torus is due
to the fluorescence of iron atoms excited by X-rays from the hot gas disk.
(Illustration: NASA/CXC/M.Weiss)
Radio Galaxies
• Has a very strong radio output ~ 1034W or
108Lsol
– Compared to our Milky Way which has radio output of
~ 1030W or 2500Lsol
• Radio output not seen in the visible spectrum
– When viewed in the radio spectrum, can see one or
two jets emerging
• Typically elliptical in shaped when viewed in
visible spectrum
A few Radio Galaxies…
Centaurus A, radio image is superimposed on the
visible image
Cygnus A in the
visible spectrum
And in the
Radio spectrum
Virgo A, radio image (left) and optical image (right)
Structure of Radio Galaxies
When viewed in the radio spectrum, one can notice
the following:
• The nucleus – the centre of the galaxy
• Jets - bright lines where strong radio emission
streams out from the nucleus
• Lobes – region around the jets
• Plumes – similar to lobes, yet they have a much
more elongated structure, replaces lobes
• Hotspots – bright spots typically at the end of a
jet
Quasars
• Brightest objects in the known universe
• Very far away with high redshift
– We see them as they were when the Universe
was only 1/6 of its present age.
• > 1012 Lsol
• Most emit in infrared
• Have jets and lobes like radio galaxies
Example of quasars
This is a classic double-lobed radio source with radio
emission from relativistic streams of high energy
particles generated by the quasar
Quasar 3C 273
This is the most luminous quasar known with an
absolute magnitude of -26.7 and approximately 2.2
billion years away.
Interesting fact
• Quasars show evidence of elements
heavier than helium
– This is taken to mean that galaxies underwent
a massive phase of star formation creating
hypothetical population III stars between the
time of the Big Bang and the first observed
quasars.
– Necessary in explaining the beginning of the
Universe
Blazars
• BL Lac Objects
– Named for their prototype BL Lacertae(1926)
• Was thought to have been an extragalactic
variable star, until they realized the redshift
– Weak emission lines
– Strong luminosities
– High rate of fluctuation
• Periods of fluctuation in days, even hours
– ‘Loud’ in radio spectrum (like all quasars)
• Optically Violently Variable (OVV)
Similar to BL Lacs w/ strong emission lines
BL Lac Object 0735+178, imaged in the microwave range by VLBA
Blazars
An artist’s
conception of
a quasar.
Counter jets
are not visible
in observation
due to the
luminosity of
the forward jet
and accretion
disk
M87
Nearby quasar
3C 273
Optical image by
Hubble.
This can be
considered a blazar
because it appears
that the jet is
pointed at us.
Summary of Active Galactic
Nuclei Classification
Orientation
Luminosity
Low
Seyfert 1
High
Quasar
“Face – on”
---
Blazar
“Edge – on”
Seyfert 2
Radio
“on – Angle”