PHYS 2600 Chapter Notes - Chapter 23: Accretion Disk, Radio Galaxy, Quasar
Unit 12: Cosmology
Quasi-Stellar objects (Quasars)
Quasars: Ultra luminous centers of distant galaxies.
Unusual properties of Quasars
1. These objects look like stars in an optical telescope.
2. They are almost always emitters of large amounts of radio energy unlike stars. The Sun is the
only star for which radio energy can be detected. Some galaxies, on the other hand, emit radio
energy in prodigious amounts.
3. There are no quasars with blue shifts, i.e., none are moving toward us.
4. Quasars have very large red-shifts, which means their recession velocity is very large, certainly
much larger than any "normal" galaxy. If the Hubble Law applies to them then they are much
further away than all other "normal" galaxies.
5. These objects must be radiating enormous amounts of energy - perhaps 10000 times the energy
output of a "normal" galaxy. If they were normal galaxies they would be beyond our ability to see
them.
6. There is strong evidence that this enormous amount of energy is produced, not in an object the
size of a galaxy, but in a region at least as small as our solar system and perhaps even smaller
Active Galactic Nuclei (AGN): Class of objects that are powered by hot gas accreting around a
supermassive black hole
Active Galaxy: Host galaxy that contains an AGN which can be easily observed
Radio galaxy: Active galaxy which emits strongly at radio wavelengths
Accretion: The gravitational accumulation of matter around a black hole (basic model of quasar
emission)
Eddington limit: There is a limit to the luminosity that can be radiated by accretion onto a compact
object like a black hole. This applies to any object held together by its own gravity and applies to stars as
well as quasars.
Represents the balance between radiation pushing gas-fuel outward and gravity pulling gas inward.
LEDD = 30,000 (M/Mo) Lo
LEDD = maximum luminosity that can be radiated by accretion around a black hole.
M = mass of the black hole
Mo = mass of the Sun
Lo = luminosity of the Sun
Super massive black holes: At the centers of nearby galaxies and thought to be the remnants of AGN. At
the centre of a quasar is a black hole, rapid fluctuations in the brightness of quasars indicate that the
region that emits radiation is small.
Accretion and jets around black holes: Hot gas accreting around the black hole is thought to give a
quasar its intense luminosity.
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Superluminal motion: Faster than the speed of light (some quasars seemed to show this type of motion
when looking at “blobs” of radio emitting particles near the very base of their radio jets however the blobs
in the jets move near the speed of light – relative jets – never faster)
Unified model: Explains how different AGN, at their core are fundamentally the same. A thick dusty
doughnut shaped torus surrounds a quasar that, when viewed edge-on, blocks a direct view of the
luminous accreting gas.
Dead quasar flare: As the star’s matter is accreted and is consumed by the black hole over a period of a
year or so, it can shine brightly.
Hypothesis
In the early stage of the formation of a galaxy it is formed with a black hole at the centre and a lot of
material (gas, dust, stars) nearby that can feed its accretion disk producing enormous amounts of energy.
As time goes on the material available for the disk begins to be used up. The black hole is still there but
there is increasingly less material to fall into it and so it generates less energy. Nearer our own epoch,
then, there is even less power developed in the galactic nucleus. Therefore the most powerful objects
would be seen at the greatest distance - furthest back in time.
The Cosmological Implications of Hubble’s Law
Hubble time: Estimate the age of the universe
t = 1/H0
The Hubble Law describes the continuing expansion of the universe.
Hubble Flow: Galaxies are not actually moving through space, but the space between them is increasing
as space expands.
Cosmological redshifts: The redshift seen from distant galaxies are caused by cosmic expansion. These
redshifts develop as photons travel through expanding space, getting stretched along their journey. The
redshift of a distant galaxy is a measure of the relative size and age of the universe at the time the galaxy
emitted its light.
The Cosmological Principle: Cosmological theories are based on the idea that on large scales, the
universe looks roughly the same at all locations and in every direction (universe is homogenous and
isotropic).
The Big Bang Hypothesis and the Cosmic Microwave Background
The Big Bang Theory: The universe began with nearly infinite density and began its expansion in the
event called the Big Bang, which can be described as the beginning of time and space.
One of the most powerful confirmations of the Big Bang is the existence of the Cosmic Microwave
Background Radiation. This remnant of the high temperature radiation of the Big Bang fills all of space.
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Document Summary
Unusual properties of quasars: these objects look like stars in an optical telescope, they are almost always emitters of large amounts of radio energy unlike stars. The sun is the only star for which radio energy can be detected. If the hubble law applies to them then they are much further away than all other normal galaxies: these objects must be radiating enormous amounts of energy - perhaps 10000 times the energy output of a normal galaxy. Active galactic nuclei (agn): class of objects that are powered by hot gas accreting around a supermassive black hole. Active galaxy: host galaxy that contains an agn which can be easily observed. Radio galaxy: active galaxy which emits strongly at radio wavelengths. Accretion: the gravitational accumulation of matter around a black hole (basic model of quasar emission) Eddington limit: there is a limit to the luminosity that can be radiated by accretion onto a compact object like a black hole.
