Transcript solar.gmu.edu

ASTR 113 – 003
Lecture 11 April 12, 2006
Spring 2006
Introduction To Modern Astronomy II
Review (Ch4-5): the Foundation
1.
2.
3.
4.
5.
6.
7.
Sun, Our star (Ch18)
Nature of Stars (Ch19)
Birth of Stars (Ch20)
After Main Sequence (Ch21)
Death of Stars (Ch22)
Neutron Stars (Ch23)
Black Holes (Ch24)
Star (Ch18-24)
Galaxy (Ch 25-27)
Cosmology (Ch28-29)
Extraterrestrial Life (Ch30)
1. Our Galaxy (Ch25)
2. Galaxies (Ch26)
3. Active Galaxies (Ch27)
1.
2.
Evolution of Universe (Ch28)
Early Universe (Ch29)
ASTR 113 – 003
Lecture 11 April 12, 2006
Spring 2006
Quasars, Active Galaxies,
and Gamma-Ray Bursters
Chapter Twenty-Seven
Guiding Questions
1. Why are quasars unusual? How did astronomers
discover that they are extraordinarily distant and
luminous?
2. What evidence showed a link between quasars and
galaxies?
3. How are Seyfert galaxies and radio galaxies related to
quasars?
4. How can material ejected from quasars appear to travel
faster than light?
5. What could power the incredible energy output from
active galaxies?
6. Why do many active galaxies emit ultrafast jets of
material?
7. What are gamma-ray bursters? How did astronomers
discover how far away they are?
Quasars: Discovery
•Quasars, or quasi-stellar radio sources, look like stars but
have huge redshifts.
•They were first discovered in radio wavelength; they were
strong radio sources in the sky, e.g., Cygnus A
Quasars: Distance
•The redshifts (>0.05 to > 5) indicate that quasars are at
least several hundred Mpc away, and often several
thousand Mpc away
3C 273
Z=0.158
d=682 Mpc
(or 2 billion ly)
PKS 2000-039
Z=3.773
d=3810 Mpc
(or 12.4 billion ly)
Quasars: Luminous Objects
•A quasar’s luminosity can be calculated from its apparent
brightness and the distance using the inverse-square law
•Even though small, the luminosity of a quasar (1038 to 1042
Watts) can be very larger, i.e., several thousand times more
than the entire Milly Way Galaxies (1037).
•A quasar has emission spectrum, not the absorption
spectrum of ordinary stars or galaxies.
•We now know that about 10% of all qauasars are strong
sources of radio emission and are therefore called “radioloud”
•The remaining 90% are “radio-quiet”, or quasi-stellar
objects, or QSOs
Quasars: Distribution
•Quasars are most populated in 1 to 4 billion years after the
Big Bang.
•There are no nearby quasars (>250 Mpc)
Quasars are centers of active galaxies
•A quasar is not a star
•A quasar is the ultra-luminous center of an active galaxy
Missing Links
•Quasar are extreme galaxies.
•What are the missing links between normal galaxies and
quasars:
– Seyfert Galaxies
– Radio Galaxies
Seyfert galaxies
•Seyfert galaxies are spiral galaxies with bright, compact
nuclei that show intense radiation and strong emission
lines in their spectra.
•The nucleus of Seyfert galaxies resembles a lowluminosity quasar nearby
Radio galaxies
•Radio galaxies resemble low-luminosity, radio-loud
quasars
•Radio galaxies are often elliptical galaxies with a nucleus
of intense activities. Including jets
Radio Galaxy Centaurus A
•In visible light, it is a elliptic galaxy; about 4 Mpc away
•In radio wavelength, it shows a central source and two lobes
•In x-ray, it shows a jet
•It looks similar to a quasar in the radio and X-ray
wavelengths
Jet
•Jets are from the synchrotron radiation of relativistic
particles that are ejected from the nucleus of a radio galaxy
along two oppositely directed beams
•Jets are collimated by the twisted magnetic field lines along
the rotational axis of the central object
Synchrotron Radiation
•Synchrotron radiation
•Produced by relativistic
electrons spiraling around
magnetic field lines
•is non-thermal radiation
•Is polarized radiation
•Blackbody radiation
•Produced by the random
thermal motion of the
atoms that make up the
emitting object
•Is thermal radiation
•Is un-polarized radiation
Super-luminous Motion of Jets
•Some jets appeared to move faster that the speed of light,
the super-luminous motion
•For example, the blob seems moving 10 times faster than
the speed of light
Super-luminous Motion of Jets
•Super-luminous motion is a
projection effect
•Because the blob is moving
toward us close to the speed of
light, the signals from the blob
always reach us earlier, which
makes any lateral motion
appear faster.
Blazar
•Similar to quasar, a blazar is an extraordinary luminous,
compact star-like object that is the core of distant galaxies
•But unlike quasar, the spectrum of a blazar is featureless,
without emission line or absorption line
•A blazar is dominated by synchrotron radiation
AGN: Active Galactic Nuclei
•Because the similar properties among quasars, blazars,
Seyfert Galaxies, and radio galaxies, they are now
collectively called active galaxies
•Active galaxies possess active galactic nuclei, which cause
intense radiations, fast variations, jets, lobes, et al.
AGN: Variation and Size
•One common property of all AGN is variability
•Variability place strict limit on the maximum size of a
light source
AGN: Variation and Size
•A principle: an object can not vary in brightness faster than light
can travel across the object
•E.g., flash from an object 1 ly across reaches us over I yr period
Super-massive black holes: the “central
engines” of AGN
• AGN is powered by the
accretion of galaxy
material onto a supermassive black hole at the
center
• The energy for AGN is
the gravitational energy
converted to radiation
• Material in an accretion
disk spirals inward toward
the black hole
Super-massive black holes: the “central
engines” of AGN
• The fast orbital motion of
stars at the core indicates
the presence of a central
object
• Calculations show the
object to be 3 X 107 solar
mass
• Super-black hole exists in
the nucleus of almost
every galaxy, including
Milky Way
Rotation Curve of Andromeda Galaxy (M31)
Super-massive black holes: the “central
engines” of AGN
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Estimate the mass of the central black hole for 3C273
The luminosity is 3 X 1013 Ls
Assuming the luminosity is at the Eddington limit
Eddington limit: radiation pressure, the pressure
produced by photons streaming outward from the infalling material, is equal to the gravitational force.
• The minimum mass of black hole in 3C273 is 109 Ms
– If BH mass were smaller than this number, the in-falling
material would be pushed away from the radiation pressure
Jets from a Super-Massive Black hole
•The rotation of the accretion disk surrounding a super-massive
black hole twists the disk’s magnetic field lines into a helix.
•Relativistic subatomic particles are channeled along the field
lines
A Unified Theory of Active Galaxies
• Blazars, quasars, and radio galaxies may be the same
type of object, viewed at different angles
• The same object is consisted of a super-massive black
hole, its accretion disk and its relativistic jets
A Unified Theory of Active Galaxies
•Why there are no nearby quasars
– Because of the strong accretion, over time, most of the available gas
and dust surrounding a quasar’s central engine is accreted onto the
black holes; the central engine becomes less active
•The collision of galaxies transfer gas and dust from one galaxy
to another, providing more fuel for the super-massive black
Gamma-ray Bursters
• Short (in seconds), intense bursts of gamma rays are
observed at random times coming from random parts of
the sky
Optical Counterparts of Gamma ray Burster
• Tracking the “Afterglow”, indicating Gamma X-ray bursters
are from distance galaxies
• E.g.,optical object z=3.418, 12 billion light years away
Origins of Gamma Ray Bursters
• Supernova explosion
• Collision between two neutron stars, or between a neutron
star and a black hole, or two black holes
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Key Words
accretion disk
active galactic nucleus (AGN)
active galaxy
blazar
collapsar
double radio source
Eddington limit
gamma-ray burster
head-tail source
nonthermal radiation
polarized radiation
quasar
radio galaxy
radio lobes
Seyfert galaxy
superluminal motion
supermassive black hole
thermal radiation