Introduction to Active Galactic Nulcei AGN

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Introduction to Active Galactic Nulcei (AGN)

• Historical Background

• Taxonomy - Classes of AGN

• Brief overview of continuum and spectral
  characteristics

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Background AGN

• Point sources radio surveys in 1950s

• 1960 first optical counterpart found

• 1963 Marten Schmidt identifies spectrum

• High z implies high L (1013 L8)

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Background
Optical Spectra of Galaxies
Nearby AGN
Something very different is going on!
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AGN were quickly seen to show emission in all
astrophysically accessible wavelengths regimes
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Radio features
Lobes
Jet
Hotspot
Core
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Definition
What are Active Galactic Nuclei?
Active Galactic Nuclei (AGN) are nuclei of
galaxies which show energetic phenomena that
cannot be clearly and directly attributed to
stars.
Signs of Activity

• Luminous UV emission from a compact region in the
  center of a galaxy

• Strongly Doppler-broadened emission lines

• High variability

• Strong non-thermal emission polarized emission

• Compact radio core

• Extended, linear radio structures (jetshotspots)

• X-ray, g-ray, and TeV-emission

In some luminous AGN (quasars) the radiation from
a region comparable to the solar system can be
several hundred times brighter than the whole
galaxy.
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Historical Perspective

• First detections
• NGC 1068 (E.A. Fath 1908, Lick Obs.) and V.M.
  Slipher (Lowell Obs.) - strong emission lines
  similar to planetary nebulae, line widths of
  several hundred km/sec
• Detection of an optical jet in M87 (Curtis 1913)
• Same period Einstein develops GR, Schwarzschild
  metric (1916), but no connection seen yet
• Hubble (1926) - Nebulae are extragalactic
  (galaxies)
• Carl Seyfert (1943) - found several galaxies
  similar to NGC1068 (henceforth named Seyfert
  galaxies), i.e. galxies with a bright nucleus and
  strong emission lines (NGC1275, NGC3516, NGC
  4051, NGC 4151, NGC7469)
• Detection of radio emission from NGC1068
  NGC1275 (1955)
• Woltjer (1959) - Timescale for Seyfert-activity
  108 years (1 of Galaxies are luminous Seyferts),
  Mass of nucleus is very high 108-10 M?(velocity
  dispersion/line width several thousandkm/sec at
  base in unresolved nucleus)

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Historical Perspective
Radio Surveys Quasars

• Early radio surveys were very important for the
  discovery of quasars. For example
• 3C and 3CR The third Cambridge (3C) catalog
  (Edge et
• al. 1959) at 158 MHz. Revision 3CR catalog
  (Bennett 1961)
• at 178 MHz down to limiting flux density of 9 Jy
  (1 Jy 10-26 W m-2 Hz-1) - Discovery of first
  quasars (e.g. 3C273,279)
• PKS Parkes (Australia, Ekers 1969) survey of
  southern sky at
• 408 MHz (gt 4 Jy) and 1410 MHz (gt 1 Jy).

• Sources found
• Normal Galaxies
• Stars with weird broad emission lines!?

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Detection of Quasars
Radio ID of quasars by Hazard et al. (1963),
Maarten Schmidt (1963) The lines in 3C273 are
highly redshifted (z0.158) emission lines
! first detection of quasars (3C 48, 3C273), from
Hubbles law follows (h0 H0/(100 km s-1 Mpc-1)
(100 times larger than normal galaxy) quasar
quasi-stellar radio source, QSO quasi-stellar
object (no longer distinguished)
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Quasars are much bluer than stars
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AGN were quickly seen to show emission in all
astrophysically relevant wavelengths regimes
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AGN Taxonomy

• Seyfert galaxies 1 and 2
• Quasars (QSOs and QSRs)
• Radio Galaxies
• LINERs
• Blazars
• Related phenomena

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Seyferts

• Lower-luminosity AGN
• MB gt -21.5 5log(H0/100) (Schmidt Green
  1983)
• Quasar-like nucleus host galaxy clearly seen
• Seyferts occur mainly in spirals
• Nucleus has strong, high-ionization emission
  lines in the optical

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Optical spectra of Seyferts

• Broad lines
• FWHM500-10,000 km/s
• permitted lines
• high-density gas (negt 109 cm-3)
• pc distance from center
• Narrow lines
• FWHM hundreds km/s
• forbidden lines
• low-density gas (ne 103-6 cm-3)
• 50-100 pc distance
• Absorption features due to stars in the host
  galaxy

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BLR/NLR Diagnostic

• Dynamics gives clouds location
• Types of lines observed/not observed give info on
  temperature, density
• Example
• OIII ?5007 not observed from BLR
• critical de-excitation density ne106cm-3
  lower limit
• CIII?1909 is observed from BLR
• critical de-excitation density ne1010cm-3
  upper limit

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Seyferts 1 and 2

• Seyferts 1 both broad and narrow lines
• Balmer lines, OIII ?4959,5007
• Seyferts 2 only narrow lines
• OIII ?4959,5007
• Intermediate types 1.1-1.9
• decreasing intensity of the broad
• line component

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Blue Continuum
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Reddened continuum
Seyfert 2
LINER
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Quasars

• Most luminous AGN
• MBlt -21.5 5log(H0/100)
• Unresolved (lt7) on Palomar Plates
• Weak fuzz on deep observations (HST)
• Similar nuclear spectra to Seyferts, but
• weaker abs lines and narrow/broad ratios
• QSOoptically bright (most)
• QSRradio bright (5-10)

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Radio Galaxies

• Occur in giant ellipticals
• Bright radio emission with extended features
• (jets, lobes) and compact core
• Broad-Line Radio Galaxies (BLRGs)
• Narrow-Line Radio Galaxies (NLRGs)
• QSRs

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Cygnus A in the radio
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LINERs

• Low Ionization Nuclear Emission Lines
• Very faint, and numerous may be 50 of local
  extragalactic population
• Similar to Sy2 but stronger low-ionization lines
• What are LINERs?
• Ratios of line fluxes depends on
• shape of ionizing continuum

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Starlight vs AGN light

• Ionization potential for O to OIII
• 54.93 eV
• AGN produce many photons at these energies
• Are LINERs powered by faint AGN?

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LINER diagnostic
Ho et al. 1989

xHII regions
LINERs
Seyferts
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Blazars

• High luminosity, non-thermal continuum from radio
  to gamma-rays
• Flat or inverted radio spectrum
• Rapid variability T day- hours
• Large Polarizations 10
• BL Lacs weak emission lines, nearby
• OVVs/FSRQs strong emission lines, distant
  

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Comparison of Optical Spectra
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Related Phenomena

• Starbursts Intense starformation, often nuclear
• bright in optical, X-rays,
  radio
• Markarian galaxies from Markarian survey at
  Byurakan Observatory, Armenia
• UV-excess galaxies
• 11 Seyferts, 2 QSOs and BL Lacs
• ULIRGs from IRAS survey in the 80s at ?gt10µ
• L(8-1000µ) gt 1012Lsolar
• due to dust heated by
  AGN/Starburst

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What powers AGN?

• It is very difficult to explain the observed set
  of characteristics of AGN without invoking a
  supermassive central black hole.

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AGN What are they good for?

• Study black hole physics
• Study high energy physics
• Background sources on cosmological scales
• -Ly-a forrest in optical spectra (absorbing gas
  in cosmic walls?)
• -Are gravitationally lensed by clusters, get
  Hubble constant from time delay
• -Background radiation to detect absorption lines
  in host galaxies at large z
• -Produce cosmic background radiation, e.g. at
  X-ray wavelengths
• Find galaxies at highest redshifts (z gt 5.8!)
• Constrain cosmology
• History of our Galaxy impact on galaxy
  evolution

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The Black Hole Paradigm
Basic question early on what powers quasars,
Seyferts, and radio galaxies?
Remember the characteristic signatures of
quasars are

• high luminosity (L gt 1044 erg/sec, i.e.
  1010.5-14.5 L?)

• high compactness (variability on time scales of
  years, compact
• radio cores lt 1pc)

Such a high luminosity will produce an enormous
radiation pressure. Hence, for material to be
gravitational bound to the center of the galaxy,
we can calculate a minimum central mass - the
Eddington mass - independent of a particular
model.
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Eddington Limit

• The momentum of photons is l E/c (one photon E
  hn)

• the force F of photons is F dl/dt E/c

• the pressure is force per area P F/A

Hence, the radiation pressure at distance r of an
isotropically radi- ating point source of
luminosity L is (L luminosity, F flux density)
The force exerted on a single electron is
obtained by multiplying with the electron
scattering cross-section (dimension cm2)
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Here se is the Thomson cross-section (from
classical electron radius)
The inward directed gravitational force of a
central mass M is given by
NB The radiation pressure is mainly acting on
the electrons while gravitation mainly acts on
protons (since they have bigger mass). Coulomb
forces will keep them together!
In order for matter to be gravitationally bound
(to fall inward), we need
which is independent of the radius.
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This condition is called the Eddington Limit and
states that fora given luminosity a certain
minimum central mass is required (Eddington mass)
or that for a given central mass the luminosity
cannot exceed the Eddington luminosity.
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Schwarzchild Radius
Equating kinetic and potential energy in a
gravitating system yields
setting v c-the speed of photons-and canceling
m we get
This is called the Schwarzschild radius and
defines the event horizon in the Schwarzschild
metric (non-rotating black hole).

• For the mass of the earth we have RS 1cm

• For a quasar with M 108 M? we have RS 3 x
  1013 cm 2 AU.

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When mass falls onto a black hole, potential
energy is converted into kinetic energy. This
energy is either advected into and beyond
the event horizon or released before.
The potential energy of a mass element dm in a
gravitational field is
The available energy (luminosity) then is
where we call the mass accretion rate
The characteristic scale of the emitting region
will be a few gravitational radii, i.e. r rinRs
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Efficiency is just a function of how compact the
object is
The compactness of the accretion disk will
depend on the spin (a) of the black hole for a
0 (Schwarzschild) we have 6 and for a 1
(extreme Kerr) we have 40! Note that for
nuclear fusion we only have 0.007.
For a quasar with L1046 ergs/s and ?0.1, we
have M 2 M? yr-1.
Accretion is the most efficient energy-generation
process currently known
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Where does the luminosity come from?
Accretion Disks
Luminosity of AGN derives from gravitational
potential energy of gas spiraling inward through
an accretion disk. Mass streams are in orbit
around the BH viscosity, an internal force that
converts the directed KE of the bulk mass motion
into random thermal motion, causes orbiting gas
to lose angular momentum and fall inwards.
Energy is dissipated in the disk and is radiated
away.
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Angular momentum transport
If the disk is thin (this means that the orbital
velocity is much greater than the sound speed),
then orbital velocity of the gas is Keplerian
Specific angular momentum vfR is
i.e. increasing outwards. Gas at large R has too
much angular momentum to be accreted by the black
hole.
To flow inwards, gas must lose angular momentum,
either By redistributing the angular momentum
within the disk (gas at small R loses angular
momentum to gas further out and flows inward)
By loss of angular momentum from the entire
system.e.g. a wind from the disk could take away
angular momentum allowing inflow
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Redistribution of angular momentum within a thin
disk is a diffusive process - a narrow ring of
gas spreads out under the action of the disk
viscosity
Gas surface density
Radius, R
With increasing time Mass all flows inward to
small R and is accreted Angular momentum is
carried out to very large R by a vanishingly
small fraction of the mass
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Radiation from thin disk accretion
Consider gas flowing inward through a thin disk.
Easy to estimate the radial distribution of
temperature. The total energy felt by a mass m
of orbiting gas is
Conservation of energy required that energy dE
radiated in time t be equal to energy difference
between the two boundaries in picture above
R-dR
R
If the luminosity of the ring is dLring, then
Using Stefan-Boltzmann law with A2(2prdr)
Solving for T
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Correct dependence on mass, accretion rate, and
radius, but wrong prefactor. Need to account
for Radial energy flux through the disk
(transport of angular momentum also means
transport of energy) Boundary conditions at the
inner edge of the disk
Correcting for this, radial distribution of
temperature is
where Rin is the radius of the disk inner edge.
For large radii R gtgt Rin, we can simplify the
expression to
with Rs the Schwarzschild radius as before. For a
hole accreting at the Eddington limit
Accretion rate scales linearly with mass
Schwarzschild radius also increases linearly with
mass Temperature at fixed number of Rs decreases
as M-1/4
Disks around more massive black holes are cooler.
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For a supermassive black hole, rewrite the
temperature as
Accretion rate at the Eddington limiting
luminosity (assuming h0.1)
A thermal spectrum at temperature T peaks at a
frequency
An inner disk temperature of 105 K corresponds
to strong emission at frequencies of 1016 Hz.
Wavelength 50 nm.
Expect disk emission in AGN accreting at close
to the Eddington limit to be strong in the
ultraviolet origin of the broad peak in quasar
SEDs in the blue and UV.
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Disk has annuli at many different temperatures
spectrum is weighted sum of many blackbody
spectra.
Consistent with the broad spectral energy
distribution of AGN in the optical and UV regions
of the spectrum.