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High Energy Astrophysics > 7: ACTIVE GALACTIC NUCLEI (AGN) AND 8: GALAXY CLUSTERS > Flashcards

7: ACTIVE GALACTIC NUCLEI (AGN) AND 8: GALAXY CLUSTERS Flashcards

1
Q

What is an active galaxy?

A

An active galaxy is one in which a significant fraction of its emission is non-thermal; i.e. not originating from the stars or ISM. They make up ∼ 3% of galaxies.

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2
Q

What is an Active Galactic Nucleus (AGN)?

A

The central few parsecs of an active galaxy

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3
Q

What is the AGN region characterised by?

A

Emission of enormous amounts of energy and often jets of relativistic material.

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4
Q

Which two observed properties are AGN categorised into? Define these two properties.

A
  1. Quasars - Bright compact centres outshining the rest of their galaxy by 10 - 10^5. They emit a nearly featureless spectrum from radio to X-rays, but with broad emission lines in the optical.
  2. Radio Galaxies - Look like normal elliptical galaxies in the optical but very luminous in radio (10 orders of magnitudes brighter than radio emission from typical galaxies). The radio emission comes from the nucleus and/or pair of roughly symmetric lobes extending up to ~100kpc on each side of the nucleus.
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5
Q

What are radio galaxies classified into?

A

Fanaroff-Riley (FR) types.
1. FR I galaxies are brightest in the centre but less luminous than FR IIs. Jets are usually present but less collimated than FG IIs.

  1. FR II galaxies have powerful collimated jets, often terminating in bright hot spots and are brightest in the lobes.
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6
Q

What causes the differences between the two FR galaxies?

A

Might be due to velocity. With FR I having subsonic jets and FR II being supersonic.

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7
Q

What are Seyfert galaxies? (5)

A
  1. Spiral galaxies with very bright unresolved cores.
  2. Show strong optical emission
    lines from excitation and ionisation states too high to be produced by stars.
  3. Weak or no radio emission.
  4. The brightest Seyferts are as bright as faint quasars.
  5. Seyfert galaxies are more often involved in mergers with other galaxies compared to normal galaxies.
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8
Q

What are blazars? (3)

A
  1. Extremely luminous and variable sources dominated by synchrotron emission.
  2. Weak emission lines, swamped by the synchrotron continuum.
  3. As bright as quasars.
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9
Q

Active galaxies are further categorised into what and why?

A
  1. Radio loud
  2. Radio quiet

It depends on the ratio of their radio to optical luminosity.

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10
Q

Where are radio-loud AGN most likely to occur?

A

Elliptical galaxies.

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11
Q

There is evidence that all galaxies host a supermassive black hole. The gravitational potential of a SMBH can provide the energy to fuel an AGN. What are indicators that there is a SMBH there? (3)

A
  1. Kinematics of disk-like structures
  2. Kinematics of water masers
  3. Gravitational redshifts in X-ray FE lines
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12
Q

X-rays from AGN show variability with a similar pattern to that in Cyg X-1, but scaled up to longer
time scales. Why?

A

This is due to the causality limit on the minimum variability timescale ∆tmin (the minimum timescale is the time for light to cross the emitting region).

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13
Q

What is the mass of central black hole correlated to?

A

The mass of the host galaxy.

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14
Q

How to generate the enormous power emitted by AGN?

A

We would need the mass of the SMBH but the accretion rate need not be large.

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15
Q

What is the spectrum for the accretion disk?

A

Continuum emission with most emission in the UV.

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16
Q

Why are x-rays from the AGN thought to arise?

A

From a hot corona around the accretion disk, generated by inverse-Compton scattering disk photons.

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17
Q

Where does the radio emission in an AGN come from?

A

From the synchrotron emission from jets and lobes.

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18
Q

When jets are angled close to our line of sight, only one side may be visible. Why?

A

Doppler boosting.

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19
Q

For a jet axis close to the line of sight, with β ∼ 0.98, what is the specific intensity of the jet travelling toward the observed boosted by a factor of?

A

A factor of 1000. The receding jet is reduced by the same factor.

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20
Q

What is synchrotron emission observed from?

A

The jets and lobes that they create.

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21
Q

What is the physics of the jets?

A

They consist of relativistic electrons (and probably positrons or protons) and terminate in hot-spots at the end of the lobes.

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22
Q

What is referred to as “ageing” in the B-field?

A

When the electrons are no longer being accelerated they will lose energy to synchrotron emission as E^2B^2.

23
Q

Describe the features of the electron and synchrotron spectrum.

A

The highest-energy electrons age the fastest, so we expect the slope of the electron spectrum p to
steepen with time.

This is seen as the slope α of the synchrotron spectrum increasing from the hotspots back towards the galaxy (assuming that no energy is added to the electrons after they are
accelerated at the base of the jet).

24
Q

What is the role of the rotation axis of the accretion disk?

A

It provides a natural direction for the jets.

25
Q

What does synchrotron luminosity depend on?

A

B and number of electrons, N_e.

26
Q

What does the broad line region indicate?

A

They indicate the velocities of emitting regions.

27
Q

What are the consequences are of the broad line region line widths being so large?

A

Thermal broadening is impossible. It would require such large temperatures that the temperature of the gas would be fully ionised, so there would be no emission lines.

28
Q

What do the broad line region line width indicate?

A

They imply bulk velocities of ~ 10^5 km s^-1. Gas clouds moving with these velocities must be very close to a very massive object if their motion is caused by a gravitational field.

29
Q

What is the narrow line region and where is it thought to come from? (3)

A
  1. These lines still have measurable widths that correspond to bulk velocities of 100 − 1000 km s−1.
  2. The region emitting these narrow lines has been resolved in some Seyfert galaxies and sizes from 50 pc to 200 kpc have been found.
  3. This narrow line
    emission is thus thought to come from gas clouds further out than the broad line region, which move more slowly, but are still ionised by the disk emission.
30
Q

What is the molecular torus and where is it thought to originate?

A
  1. Quasar spectra show a bump of emission in the IR that is not from the disk or jets.
  2. The emission is consistent with BB emission from dust at ∼ 80 K, and is believed to originate from a dusty torus that absorbs the emission from the nucleus and re-emits it in the IR.
  3. The torus extends from 1 pc to about 100 pc (beyond the accretion disk, but in the same plane), and has been imaged for some nearby galaxies.
31
Q

What features does the AGN consist of? Is it radio-loud or radio-quiet?

A
  1. Central black hole
  2. Accretion disk
  3. X-ray emitting corona around disk
  4. Broad line region
  5. Narrow line region
  6. Molecular torus

It can be radio-loud (jets and lobes) or radio-quiet (no jets and lobes)

32
Q

What does the viewing angle relative to the jet axis affect?

A

The obscuration by the torus and beaming along the jet.

33
Q

For radio-loud AGN when theta is approaching 90 degrees, what is seen and what is obscured? What does this appear as then?

A
  1. Direct emission from the accretion disk and broad line region are partly obscured by the torus.
  2. Emission from the narrow line region, jets and lobes are visible.
  3. Appears as a radio galaxy with narrow but not broad emission lines.
34
Q

For radio-loud AGN with intermediate theta, what is seen and what is obscured? What does this appear as then?

A
  1. Inner regions are no longer obscured.
  2. It will be a radio galaxy with broad and narrow emission lines.
  3. As the angle approaches the jet axis, Doppler boosting effects become important and the jet will appear more symmetric.
35
Q

For radio-loud AGN when theta is approaching 0 degrees, what is seen and what is obscured? What does this appear as then?

A
  1. Beaming of jet emission becomes stronger and the source becomes more compact.
  2. Emission from disk and broad and narrow line regions is seen, plus synchrotron from the jet.
  3. Source appears as a radio-loud quasar.
36
Q

For radio-loud AGN when theta is equal to 0 degrees, what is seen and what is obscured? What does this appear as then?

A
  1. Same source viewed directly down the jet would appear as a blazar.
  2. Doppler-boosted jet emission drowns out the emission lines and small variations in jet speed or angle give large variations in boosting factor, explaining the variability
37
Q

For radio-quiet AGN, when theta approaches 90 degrees, what does it appear as?

A

a Seyfert galaxy with narrow but not broad emission lines (these are called Seyfert2 galaxies)

38
Q

For radio-quiet AGN, when theta is intermediate to θ = 0: what does it appear as?

A

A Seyfert galaxy with broad and narrow emission lines (a Seyfert1) if
low luminosity, or a radio-quiet quasar if high luminosity.

39
Q

What are galaxy clusters and how were they formed?

A

They are the largest gravitationally bound structures in the universe. They formed from the highest density peaks in the early universe. The primordial gas falls into the potential well of a galaxy cluster. Infalling gas gains bulk KE, converted to internal energy by shocks with the gas already in the cluster. This heats the gas to high temperatures.

40
Q

What is the gas in the galaxy cluster called and once in the cluster what state is it in?

A

Intra-cluster medium (ICM). The ICM will be in virial equilibrium (not expanding or contracting) so the virial theorem applies: 2KE = -PE

41
Q

For gas at temperature T, how much KE does each gas particle have?

A

KE = (3/2)kT

42
Q

For a cluster of mass M and radius R, each particle has how much PE?

A

PE = - fGMm/R with f ~ 1 (exact value depends on the ICM distribution).

43
Q

At the temperature reached in clusters, what can the ICM be treated as? What is the average mass of a particle then?

A

Fully ionised hydrogen plasma. m ~ m_p/2

44
Q

What equation does the virial theorem give?

A

T ~ GMm_p/6kR

45
Q

What are among the most luminous X-ray sources in the sky?

A

Clusters. They appear as centrally peaked X-ray sources.

46
Q

What is the primary emission mechanism for clusters?

A

Thermal bremsstrahlung.

47
Q

What is the equation expressing the bremsstrahlung spectrum exponential “cut-off”? And what can be determined from measuring its frequency?

A

ν ≈ kT/h
The temperature can be determined.

48
Q

How does measuring the temperature differ for cool galaxy clusters and hot clusters?

A

The cool cluster will have emission lines in the spectrum that can provide temperature measurement. For hot clusters, the plasma will be fully ionised, and the frequency of the bremsstrahlung cut-off will be the primary temperature diagnostic.

49
Q

How much dark matter is in clusters?

A

Clusters are 90% dark matter.

50
Q

What emission lines are seen in X-ray spectra? What does this point to?

A

It has strong emission lines notably from highly-ionised Fe. The ICM has been significantly enriched by metals produced in SN in the cluster galaxies which have been mixed in the ICM and accumulated over the life of the cluster.

51
Q

What do clusters record history of?

A

Star formation in member galaxies.

52
Q

The emissivity of the ICM to bremsstrahlung radiation is ∝ ρ^2. What does this have to do with cooling? Is this observed?

A

In the denser core regions the ICM cools most rapidly and so contracts increasing its density and hence cooling faster. This is a runaway process called cooling flow. This should lead to massive star formation in the central galaxy in the cluster as cold gas accumulates on it.

It isn’t observed. Something must be balancing the cooling.

53
Q

What is responsible for balancing the cooling? What is the evidence for this?

A

The energy output in the jets of AGN in the central galaxy.

AGN jets are seen inflating huge bubbles in the ICM.

High Energy Astrophysics (11 decks)

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