ASTR 113 – 003 Spring 2006Lecture 11 April 12, 2006

Review (Ch4-5): the Foundation

Galaxy (Ch 25-27)

Cosmology (Ch28-29)

Introduction To Modern Astronomy II

Star (Ch18-24)

1. Sun, Our star (Ch18)2. Nature of Stars (Ch19)3. Birth of Stars (Ch20)4. After Main Sequence (Ch21)5. Death of Stars (Ch22)6. Neutron Stars (Ch23)7. Black Holes (Ch24)

Extraterrestrial Life (Ch30)

1. Our Galaxy (Ch25)2. Galaxies (Ch26)3. Active Galaxies (Ch27)

1. Evolution of Universe (Ch28)2. Early Universe (Ch29)

Quasars, Active Galaxies,and Gamma-Ray Bursters

Chapter Twenty-Seven

ASTR 113 – 003 Spring 2006Lecture 11 April 12, 2006

Guiding Questions1. 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 273Z=0.158d=682 Mpc(or 2 billion ly)

PKS 2000-039Z=3.773d=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 “radio-loud”•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 low-luminosity 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 super-massive 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

• 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 in-falling 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

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