Chapter 6 Flashcards
(33 cards)
What is Doppler Boosting?
If a material is moving at relativistic speeds, radiation from a source within the jet will be beamed in the forward direction of the jet with an observed increase in intensity
With which images are knotty jets seen the clearest?
Under radio and infrared images
Radio monitoring of the “knots” in the jets show they are moving
What is magnetic field in a galactic plane parallel and perpendicular to?
Parallel to jet close to nucleus
Perpendicular to jet further away
What does a boost in intensity of radio emission depend on?
Spectral index
Why do some components of a source appear to separate at velocity?
The particle beam is aimed almost directly at the Earth
How can large fluxes of relativistic plasma
and magnetic fields survive so far from the
galactic nucleus?
The radio spectrum is much flatter in the “head” of a lobe than in the “tail” suggesting the head contains particles energised more recently
The plasma is being replenished
What is the nucleus a source of?
Double beam
What are radio galaxies?
Giant elliptical galaxies with strong radio emission
Where does the emission from a radio galaxy come from and stretch to?
From nucleus and often from pair of symmetric lobes that extend for several kpc on both sides of nucleus
What features do quasars have?
Host galaxy barely detectable
Nearly featureless spectrum from radio to hard X-ray
Variable emission across whole spectrum
What are the two types of quasars?
Radio-loud quasars (strong radio emission)
Radio-quiet quasars (weak radio emission)
What do Active Galaxies have?
Nuclei exhibiting quasar-like phenomena: point like sources of non-thermal radiation varying in intensity on timescales from years to days
What does the correlation between the mass of a black hole and the optical luminosity of the bulge of its host galaxy?
It suggests a link between galaxy formation and black hole formation
What is the sphere of influence?
The region where gravitational potential of the black hole is larger than kinetic energy of surrounding stars
(but angular momentum conservation means BH growth takes long time and gas falls in instead)
How can a black hole’s mass be determined?
From motion of surrounding matter (the dynamics of stars governed by gravity or of gas (influenced by pressure too)
What is relaxation time?
Time required for gravitational interactions to alter orbits
What relaxation time do most galaxies have?
Greater than the age of the universe (this time must elapse before a group of stars collapses into the BH
What causes in falling gas that powers AGNs?
Galaxy mergers
What does more accretion lead to?
A higher accretion luminosity which in turn leads to a larger radiation pressure acting on the in falling material
What happens when a luminosity exceeds a certain value (Eddington Luminosity) ?
The outward radiation force becomes larger than the force of gravity and the accretion is halted
Eddington luminosity is the upper limit beyond which radiation pressure would blow away infalling matter
What is the rate of increase of BH mass limited by>
The radiation pressure
What is Big Blue Bump?
Thermal emission from the disc peaks in the U.V.
What has to happen fro matter to be accreted onto the BH?
Angular momentum has to be transported out through the disc
What is the big blue bump evidence for?
For the existence of a hot disc, bright in the U.V., superimposed on a power law
spectrum from the central object.
