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ASTR 200 : Lecture 26
Supermassive black holes and AGNs
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Seyfert Galaxies
• Around 1940, Carl Seyfert noticed that some spiral
galaxies have very broad emission lines in their
spectra, compared to normal spiral galaxies
• We now call such objects Seyfert galaxies
• Why are the lines so broad?
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Normal galaxy Seyfert Galaxy
Doppler
(or Thermal)
Broadening
• If gas particles are moving
in many directions at great
speeds, some will be blue
shifted and some will be
red shifted
• This makes the emission
line very broad
• The derived velocities are
MUCH larger than the
~300 km/s one finds in
galaxy rotation curves
• Seyfert doppler widths
can be ~10,000 km/s (!)
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A clue : Bright nucleii
The galaxy Markarian 500
• In optical images,
Seyfert galaxies look
much like normal
spiral galaxies with
one exception
• They often have a very
bright unresolved
nucleus, far brighter
than the nucleus of a
normal galaxy
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A second phonomenon: Radio active galaxies
Two emission regions on either side of Cygnus A
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•
Astronomical objects can
sometimes be detected in radio
waves.
•
Perhaps surprisingly, some of
the sky's strongest radio
sources are related to distant
galaxies
•
Often the radio emission does
NOT come from the galaxy,
even though there is an
obvious connection with the
central galaxy (linear jets, in
this case)
•
The jets clearly transport a
large amount of energy out
from the galaxy and it is
deposited into interactions with
material beyond the galaxy
(producing radio emissions)
~ 50 kpc
Sometimes jets can be also be seen at optical wavelengths
<<< at left; an amateur astronomer's image of the cD giant elliptical M87 (in the Virgo cluster)
● A linear jet emerging form the galaxy ●
~1 kpc
Much higher resolution views from HST and a radio telescope
●
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A THIRD phonomenon: Quasi-stellar objects
•
By the late 1950s, radio telescopes
were detecting many extragalactic
sources, which were consistent with
unresolved points in the radio (beam is
large on the sky).
•
Many had no obvious optical source
association with them
•
In 1962, the moon passed in front one
of these and helped localize it.
•
An optical image yielded a very faint
source (with even fainter tiny jet), but
the main source looked like a point
(and thus nearly stellar), hence the
term `quasi-stellar object' (QSO),
which became quasar.
•
Spectra revealed very broad emission
lines, but at strange wavelengths
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Optical image of 3C273
Quasars at great distance?
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•
Astronomers realized that they were
looking at lines of hydrogen, but with
redshifts of about 0.15
•
This implied the source was moving
away at 15% of the speed of light!
•
Seemed impossible that stars could be
moving 45,000 km/second... and ALL
the quasars were moving AWAY, none
were blue shifted
•
The resolution (see next week) is that
the objects are at cosmological
distances and the motion is due to the
expansion of the universe
•
3C273 is about 700 Mpc away, but is
is as bright as a typical star viewed in
a moderate telescope (13th magnitude)
•
It's luminosity is thus 4 trillion solar
luminosities (a few hundred times the
total luminosity of the Milky Way).
HST Optical image of 3C273
Quasars at great distance?
HST Optical image of 3C273
• There was a great deal of debate for
many years about the nature of quasars
• Some astronomers argued there was
some strange process producing the
high red shifts.
• The question is now settled, as very
deep imaging with the central source
blocked out (like is done to image
exoplanets around stars) reveals the
very faint galaxies (due to distance)
that surround the incredibly bright
central quasar, which is at the nucleus
of the galaxy
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Deep image with central source blocked out by coronograph
So what the heck are these various objects?
• They appear to be related to the very center (nucleus) of the galaxies
• Can we get any clues by looking at the center of OUR galaxy?
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So let's try and study the nucleus of our galaxy
• It turns out the center of our galaxy is off in the direction of the
constellation Sagittarius, but as we learned earlier, it is incredibly obscured
by dust. In optical light no photons from the center reach us.
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Hard to study the center of our galaxy
• Below, an image in galactic coordinates. (central plane is that of the
galaxy and center of the image is at the galactic center).
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But in the infrared, dust absorbs much less light
• Below, an IR image in galactic coordinates towards galactic center
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Zooming in on
the galactic
center
• Using IR light to be
able to peer past the
dust
• At the very center of
the Milky way there
is a tight star cluster
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Stars near the galactic centre
• There are a lot of stars
here....
• There about 107 stars per
cubic pc at the galactic
centre (compare to ~0.1 in
the solar neighbourhood)
• Is there a way to actually
define where the `center'
is?
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Where is the Center of our Galaxy?
• In fact there IS a good way to anchor
this question, because there is a very
bright radio source that has been
known for decades
• It is called Sagittarius A* = Sgr A*
• It is bright and very compact
– Using radio interferometry the scale of
the source is ~0.3 au = 44 million km
• Via proper motion studies, this source
is right at the distance of the galactic
center and also centered in the dense
star cluster
• About 1000 solar luminosities
• Many astronomers believed the radio
emissions was coming from hot gas in
orbit around a supermassive black hole
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Motions
• Amazingly, it has been possible to watch (in the infrared) stars ORBIT in
the very center of this cluster very close to Sgr A*
• The periods vary, but some stars go around in 15-50 years and can be
watched...!
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Elliptical orbits!
• These stars follow beautifully
elliptical orbits around a
point which is centered
precisely on Sgr A*
• Semimajor axes are a few
thousand au
• This means that the central
mass must completely
dominate the dynamics
• Applying our friend
3
au
M S + M SgrA *=a /P
2
yr
to each of these stars yields
the same answer of
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M SgrA *≈4 ×10 M ⊙
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S.M. Black holes inhabit the nuclei of other galaxies
• The evidence for SM black
holes in the nuclei of other
galaxies is provided by high
spatial resolution
spectroscopy which reveal
the rotational dynamics
• At right, the very central core
of M31
• The velocity dispersion
shows that there are many
objects within the central
0.5” moving at ~300 km/s
• To rotation curve also shows
ordered fast rotation, with
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M M 31 bh≈4×10 M ⊙
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`Super massive' Black holes
• This is MUCH larger than the 5-100 solar mass black holes we think
may form after supernovae explosions of massive stars
• How could it form?
• Very likely started as a stellar-mass black hole and then grew via
acretion of other material (stars, gas, and other black holes!)
• Some objects just pass directly into the
event horizon and disappear
• Others (especially clouds of gas) will be
broken up by tidal gravitational forces
and because they are then dissipational,
will settle into an accretion disk around
the supermassive black hole
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`Super massive' Black holes
• In this model, nuclei of galactic bulges host SM BHs
– Chicken or egg argument...unclear which came first
• The material in the accretion disk is very hot, moving very fast, and
ionized, so it generates a very powerful magnetic field
• As material spirals into
the event horizon, it
approaches v~c and if
charged some material is
ejected along the jet of
the magnetic field
• The material in the jet is
moving relativistically
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Unified AGN model
• In this model, all these energetic Active Galactic Nuclei phenomena
(Seyferts, quasars, etc) are energy released from the region of a SM
BH, and our viewing angle determines what is seen
• The very brightest
objects are when looking
right down the jet
• Less energetic, but still
high-energy phonemena
occur when viewing
from the side
• IMPORTANT: The
galaxy will only be an
AGN if it is recently
accreting a large amount
of mass...
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