Lecture 6 Active Galactic nuclei

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X-ray Astronomy
Lecture 6 - Active Galactic nuclei
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X-ray Astronomy
Lecture 6 - Active galactic nuclei (AGN)

• Last time
• Cosmology with galaxy clusters
• Constraining Om with x-ray cluster gas masses
• Constraining Om with x-ray cluster luminosity
  function
• Constraining H0 with the SZ effect

• This time
• History of the discovery of active galactic
  nuclei (AGN)
• Evidence for supermassive black holes as AGN
  power source
• The Eddington luminosity limit
• Where do x-rays come from in AGN?
• Broad emission lines in x-ray spectra of AGN
• X-ray background

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Active galactic nuclei
Definition galaxy containing supermassive black
hole (gt 106 M?) which is actively accreting
material and converting gravitational potential
energy in EM radiation
AGN standard model

• Why is this the standard model?
• Very high luminosity
• Very rapid variability (small volume)
• Copious X-ray emission

E 0.1mc2 (gravity) E 0.007mc2
(fusion)
Jargon Quasar luminous AGN
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Active galactic nuclei
A bit of history The discovery of quasars
Quasars first identified as the optical
counterparts of strong radio sources (e.g. 3C
radio survey) in the 1960s. Appeared to be a new
type of radio star on the photographic plates of
the day (Quasi-stellar radio sources).
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Active galactic nuclei
A bit of history The discovery of quasars
redshifted Balmer series implies
cosmological distances and huge intrinsic
luminosities
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A bit of history The discovery of quasars
Active galactic nuclei
Fuzz
Determined in 1970s that low redshift quasars
had associated fuzz
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A bit of history The discovery of quasars
Active galactic nuclei
Watson et al. (2008)
Determined in 1970s that low redshift quasars
had associated fuzz
Spectroscopy of this fuzz in early 1980s
showed absorption lines of galaxy spectra
Galaxy absorption lines and quasar emission lines
had the same redshift
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Active galactic nuclei
It is now established that quasars are the
luminous cores of distant galaxies, many of which
outshine their host galaxies by 10-100 times. The
power source for such enormous luminosity is
accretion onto a supermassive black hole.
Supermassive means gt 106 M?
Evidence for Supermassive black holes
Variability
Early evidence demonstrated that quasars varied
in the optical on timescales ltlt 1 year
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Size constraint from variability
Variability timescale sets the physical scale of
the emission region
d
d c x ?t
Optical variability suggests emission region has
size of 1 light-week
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DERIVATION ON WHITEBOARD
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Size constraint from variability
Variability timescale sets the physical scale of
the emission region
d
d c x ?t
Optical variability suggests emission region has
size of 1 light-week
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Size constraint from variability
With the birth of x-ray astronomy, it was
realised that quasars vary on the timescale of
minutes in the x-ray
Light travel time from Sun to Earth is 8 mins
Implication is that the enormous luminosity of
quasars is being generated in a region not much
larger than the size of our own solar system.
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Evidence for Supermassive black holes
The black-hole at the centre of the Milky Way
Near IR observing allows you to penetrate the
dust obscuration
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Evidence for Supermassive black holes
The black-hole at the centre of the Milky Way
Infra-red flare from feeding black-hole at the
galactic centre
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Evidence for Supermassive black holes
The black-hole at the centre of the Milky Way
X-ray flare from feeding black-hole at the
galactic centre
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Evidence for Supermassive black holes
Gravitational motions
The black-hole at the centre of the Milky Way
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Evidence for Supermassive black holes
Gravitational motions
Orbits of stars allows the enclosed mass to be
measured
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Evidence for Supermassive black holes
Gravitational motions
Schödel et al. (2002)
All evidence points to a supermassive black-hole
with mass of 2.6x106M?
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Black holes in nearby galaxies
VIRGO SUPERCLUSTER 2000 Galaxies Distance of 50
million light-years
M87 is the most massive elliptical galaxy in the
Virgo supercluster
M84
M87
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The mass of M87s black hole
Mass of M87s central black-hole is 3 billion
solar masses
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The mass of M84s black hole
Mass of M84s central black-hole is 0.3 billion
solar masses
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Evidence for supermassive black holes
H20 Mega-masers
Example NGC4258
Orbital velocities of 1000 kms-1
Central black hole mass 4x107M?
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X-ray Astronomy
Lecture 6 - Active galactic nuclei (AGN)

• Last time
• Cosmology with galaxy clusters
• Constraining Om with x-ray cluster gas masses
• Constraining Om with x-ray cluster luminosity
  function
• Constraining H0 with the SZ effect

• This time
• History of the discovery of active galactic
  nuclei (AGN)
• Evidence for supermassive black holes as AGN
  power source
• The Eddington luminosity limit
• Where do x-rays come from in AGN?
• Broad emission lines in x-ray spectra of AGN
• X-ray background

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Active galactic nuclei
Accertion onto a supermassive black hole is the
only mechanism efficient enough (0.1mc2) to
produce the required luminosities from such a
small physical region
Limiting luminosity The Eddington Limit
Balance between gravity and radiation pressure
radiation pressure
gravity
gravity
rad pressure
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DERIVATION ON WHITEBOARD
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Active galactic nuclei
Accertion onto a supermassive black hole is the
only mechanism efficient enough (0.1mc2) to
produce the required luminosities from such a
small physical region
Limiting luminosity The Eddington Limit

• Eddington calculation provides useful because
• Provides benchmark limit to expected accretion
  rate
• Allows comparison of accretion rate in different
  AGN as fraction of Eddington
• Can use it to place good lower limit on
  black-hole mass

Typical quasar luminosity is 1039 Watts
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Where do the x-rays come from in AGN?
Question is, does the accretion disk get hot
enough to emit many x-rays?
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Where do the x-rays come from in AGN?
For an accretion disk, emitting as a blackbody
Schwarszchild radius
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Where do the x-rays come from in AGN?
If we assume that all sources are accreting a
same fraction of the Eddington limit
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Where do the x-rays come from in AGN?
Most of the emission from stellar mass black
holes occurs in the x-ray regime
lpeakTbb 3000µmK
AGN accretion disks peak in the UV