Download Lecture 32 Quasars

Lecture 32
Active Galaxies and Quasars
Quasars
Blazars
Black Hole/ Jet Model
Apr 12, 2006
Astro 100 Lecture 32
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Quasars
• When radio telescopes were first invented
astronomers discovered many distant radio
sources, and tried to identify them with optical
counterparts.
• In the optical these radio sources looked just like
stars, hence they were given the name quasistellar objects, now shortened to quasars.
• Quasars have very unusual spectra that display:
– Very blue continuous emission
– Very high redshift emission lines
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Quasar Redshifts
Example: Quasar 3C273
• From the spectrum we can measure the redshift:
z = ∆λ/λ = 104 / 656 = 0.1583
for z < 0.5 or so, v/c = z, v = 47,000 km/s
line width of ~20 nm means internal velocities of ~ 8000 km/s
• If the mean velocity is due to the expansion of the Universe, Using
Hubble's Law (v = Hd) we can calculate the distance to the quasar:
– d = 47,000 km/s / 65 km/sec/Mpc = 720 Mpc
= 2.4 billion light years !
• This sort of calculation OK for z < 0.5, d < 2.5 Gpc, 8 b ly
Example: redshift 5 quasar
z = 608 / 122 = 5.0
for z > 0.5, use formula from relativity, v/c = 0.95, v = 285,000 km/s
• Naive application of Hubble's Law (really need cosmology model):
d = 4.4 Gpc = 14.2 billion light-years
look-back time ~ age of Universe
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Quasar Facts
• Quasars are being seen when Universe was younger, since
they are typically billions of light years away. (First
example of “look-back time” being important: a good
fraction of the age of the Universe)
– Most quasars are high-redshift objects. Most are found in the
range z = 2 - 4. They are not seen at the current time (i.e. at low
redshift), so they are evidence for evolution of the Universe.
• Quasars are extremely luminous. From distance and
brightness get luminosity
L = 4π d2 B
– A typical quasar has a luminosity 100 times as large as the Milky
Way's luminosity.
– Because of luminosity and distance, observing quasars can tell us
about the intergalactic medium (IGM) between us and the
source.
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Blazars and Radio Galaxies
• BL Lac
– a variable point-like object, changing its brightness markedly over
months. (Initially it was mistaken for a variable star)
– a smooth spectrum, with no absorption or emission lines. It is now
the prototype for a class of object known as blazars.
• Blazars- distance and size
– many blazars are now seen to have faint, distant elliptical galaxies
around them- they are in the centers of galaxies and far outshine
them
– Some blazars have variability time of days. Using the sizevariability argument, size < 1 light–day ~ size of solar system
• Radio galaxies are elliptical galaxies with strong radio
emission. They are often accompanied by jets and lobes,
with motions approaching the velocity of light
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A Unified Model of Quasars and Active
Galaxies
• A model trying to explain these phenomena must account
for the following properties:
–
–
–
–
High luminosity released in a very small space
High variability
High velocity motions
Production of jets and lobes
• The current model: energy is released by accretion of
matter onto a supermassive black hole at the center of a
galaxy. These are all Active Galactic Nuclei ("AGN's")
• Accretion onto compact object is a great way of producing
energy, since the efficiency is so high.
– hydrogen fusion in the Sun: E(out) = 0.007 Mc2
– In accretion disks: E(out) = 0.25 Mc2
– 25% of the mass is converted to light energy! To generate 100
L(MW): A typical QSO needs to eat 1 solar mass of material per
year. So in 1 million years it will swallow 1 million stars.
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Verifying the AGN Model
• Jets and high velocities formed by magnetic field squirting gas
out the poles before it is swallowed (like star formation jets)
• Can account for different types of AGN's by
– how much matter is being ingested
– accretion disk is surrounded by obscuring disk, type depends on the
viewing angle (eg: Blazars: down the pole)
• How can we check to see if supermassive black holes are
really there? High velocity motions in the center of
Milky Way
Andromeda Galaxy (M31)
Sombrero Galaxy (M104).
– Kepler's 3rd Law. For M31 there is 10 billion solar masses of material
in a volume 3 light years across.
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Formation
• How would a supermassive black hole form?
– Maybe low angular momentum (i.e. slowly spinning)
gas settled down into the center of a galaxy.
– Compact star clusters might have collapsed together
and merged.
– Once formed, the black hole swallows nearby gas and
stars.
• But why do we not see quasars in nearby galaxies?
– Current thought: central black holes formed and fed
only when galaxy is first formed
– Now (low redshift) most galaxies have already formed,
black holes are not being fed.
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Quasar 3C273
Obs: 760 nm
Lab: λ=656 nm
∆λ=104 nm
width 20 nm
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z = 5.0 Quasar
Lab: λ=122 nm
Apr 12, 2006
∆λ=608 nm
Astro 100 Lecture 32
Obs: 730 nm
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brightness
Blazar Variability
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Radio Galaxies and Jets
M87
Apr 12, 2006
Figure 10.20, p319, Arny
Astro 100 Lecture 32
Centaurus A
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AGN Black Hole Model
Figure 10.25, p323, Arny
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