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Active Galactic Nuclei

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Seyferts (usually found in spiral galaxies) BL Lacs (normally found in ellipticals) Quasars (nucleus outshines its host galaxy) Quasars ... Jet collimation ... – PowerPoint PPT presentation

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Title: Active Galactic Nuclei


1
Active Galactic Nuclei
  • 4C15 - High Energy Astrophysics
  • jlc_at_mssl.ucl.ac.uk
  • http//www.mssl.ucl.ac.uk/

2
Introduction
  • Apparently stellar
  • Non-thermal spectra
  • High redshifts
  • Seyferts (usually found in spiral galaxies)
  • BL Lacs (normally found in ellipticals)
  • Quasars (nucleus outshines its host galaxy)

3
Quasars - Monsters of the Universe
  • Artists
    impression

4
AGN Accretion
  • Believed to be powered by accretion onto
    supermassive black hole

high luminosities
highly variable
Eddington limit gt large mass
small source size
Accretion onto supermassive black hole
5
Quasars - finding their mass
  • The Eddington Limit

Where inward force of gravity balances the
outward push of radiation on the surrounding
gas.
L
mass
Edd
So a measurement of quasar luminosity gives the
minimum mass assuming radiation at the
Eddington Limit
6
Measuring a Quasars Black Hole
  • Light travel time effects

If photons leave A and B at the same time, A
arrives at the observer a time t ( d / c )
later.
A
B
If an event happens at A and takes a time dt,
then we see a change over a timescale tdt. This
gives a maximum value for the diameter, d,
because we know that our measured timescale must
be larger than the light crossing time.
d c x t
c speed of light
d diameter
7
Accretion Disk and Black Hole
  • In the very inner regions, gas is believed to
    form a disk to rid itself of angular momentum
  • Disk is about the size of our Solar System
  • Geometrically thin, optically-thick
  • and radiates like a collection of
  • blackbodies
  • Very hot towards the centre
  • (emitting soft X-rays) and
  • cool at the edges (emitting
  • optical/IR).

8
Quasars
  • Animation of a quasar

This animation takes you on a tour of a quasar
from beyond the galaxy, right up to the edge of
the black hole.
It covers ten orders of magnitude, ie the last
frame covers a distance 10 billion times smaller
than the first.
9
Accretion Rates
  • Calculation of required accretion rate

.
10
More about Accretion Disks
  • Disk self-gravitation is negligible so material
    in differential or
  • Keplerian rotation with angular velocity WK(R)
    (GM/R3)1/2
  • If n is the kinematic viscosity
  • for rings of gas rotating,
  • the viscous torque
  • exerted by the outer
  • ring on the inner will be
  • Q(R) 2pR nS R2 (dW/dR) (1)
  • where the viscous force per unit length is acting
    on 2pR and
  • Hr is the surface density with H (scale height)
    measured
  • in the z direction.

11
More about Accretion Disks (Cont.)

The viscous torques cause energy dissipation of Q
W dR/ring Each ring has two plane faces of area
4pRdR, so the radiative dissipation from the disc
per unit area is from (1) D(R) Q(R) W/4pR
½ n S (RW)2 (2) and since W WK (G
M/R3)1/2 differentiate and then D(R) 9/8
n S Q(R) M/R3 (3)


12
More about Accretion Disks (Cont.)
From a consideration of radial mass and angular
momentum flow in the disk, it can be shown
(Frank, King Raine, 3rd ed., sec 5.3/p 202,
2002) that n S (M/3p) 1
(R/R)1/2 where M is the accretion rate and
from (2) and (3) we then have D(R) (3G M
M/8pR3) 1 (R/R)1/2 and hence the radiation
energy flux through the disk faces is
independent of viscosity



13
Accretion Disk Structure
  • The accretion disk (AD) can be considered as
  • rings or annuli of blackbody emission.

14
Disk Temperature
  • Thus temperature as a function of radius T(R)

and if
then for
15
Disk Spectrum
  • Flux as a function of frequency, n -

Total disk spectrum
Log nFn
Annular BB emission
Log n
16
Black Hole and Accretion Disk
For a non-rotating spherically symetrical BH, the
innermost stable orbit occurs at 3rg or
and when
17
High Energy Spectra of AGN
  • Spectrum from the optical to medium X-rays

Low-energy disk tail
Comptonized disk
Balmer cont, FeII lines
high-energy disk tail
Log (nFn)
optical UV EUV soft X-rays X-rays
14 15 16
17 18
Log n
18
Fe Ka Line
  • Fluorescence line observed in Seyferts from gas
    with temp of at least a million degrees.

FeKa
X-ray
e-
19
Source of Fuel
  • Interstellar gas
  • Infalling stars
  • Remnant of gas cloud which originally
    formed black hole
  • High accretion rate necessary if z cosmological -
    not required if nearby

20
The Big Bang and Redshift
  • All galaxies are moving
  • away from us.
  • This is consistent with
  • an expanding Universe,
  • following its creation
  • in the Big Bang.

21
Cosmological Redshift
  • Continuity in luminosity from Seyferts to quasars
  • Absorption lines in optical spectra of quasars
    with

flux
l
22
Alternative Models
  • Supermassive star
    - 10 solar mass star radiating at 10
    J/s or less does not violate Eddington limit. It
    would be unstable however on a timescale of
    approx 10 million years.
  • May be stabilized by rapid rotation gt
    spinar - like a scaled-up pulsar

8
39
23
  • Also, general relativity predicts additional
    instability and star evolves into black hole.
  • Starburst nuclei
    - a dense cluster of massive, rapidly
    evolving stars lies in the nucleus, undergoing
    many SN explosions.
  • Explains luminosity and spectra of low-luminosity
    AGN

24
  • BUT SN phase will be short (about 1 million
    years) then evolves to black hole
  • radio observations demonstrate well-ordered
    motions (i.e. jets!) which are hard to explain in
    a model involving random outbursts

25
Radio Sources
  • Only few of galaxies contain AGN
  • At low luminosities gt radio galaxies
  • Radio galaxies have powerful radio emission -
    usually found in ellipticals
  • RG 10 - 10 erg/s 10 - 10 J/s
  • Quasars 10 - 10 erg/s 10 - 10 J/s

38
43
31
36
43
47
36
40
26
Radio Galaxies and Jets
  • Cygnus-A ?
  • VLA radio image at
  • n 1.4.109 Hz
  • the closest powerful
  • radio galaxy
  • (d 190 MPc)

? 3C 236 Westerbork radio image at n
6.08.108 Hz a radio galaxy of very
large extent (d 490 MPc)
Jets, emanating from a central highly active
galaxy, are due to relativistic electrons that
fill the lobes
27
Jets Focussed Streams of Ionized Gas
28
Electron lifetimes
For Synchrotron radiation by electrons
  • Calculating the lifetimes in AGN radio jets.
  • If nm 10 Hz (radio) 4.17x10 E B
  • E B 2.5x10 J Tesla
  • tsyn 5x10 B E sec
  • For B 10 Tesla, t 3x10 sec, 1 month
  • For B 10 Tesla, t 10 sec, 3x10 yrs

36
2
8
2
-29
2
-13
-2
-1
-3
6
syn
-8
14
6
syn
29
Shock waves in jets
  • Lifetimes short compared to extent of jets
    gt additional acceleration required.
    Most jet energy is ordered kinetic energy.
  • Gas flow in jet is supersonic near hot spot gas
    decelerates suddenly gt shock wave forms. Energy
    now in relativistic e- and mag field.

30
Equipartition of energy
  • Relative contributions of energy
  • What are relative contributions for minimum
    energy content of the source?

Energy in source
particles
magnetic field
31
  • Assume electrons distributed in energy according
    to power-law

Total energy density in electrons,
Must express k and E as functions of B.
max
32
  • Assume electrons distributed in energy according
    to power-law

Total energy density in electrons,
Must express k and E as functions of B.
max
33
  • We observe synchrotron luminosity density
  • And we know that

34
  • Hence

So
and the total energy density in electrons then
becomes
35
Finding Emax
  • Find E by looking for n

max
max
So
36
  • The energy density in the magnetic field is
  • Thus total energy density in source is
  • For T to be minimum with respect to B

37
  • Thus
  • So

particle
magnetic field
38
  • And finally,
  • This corresponds to saying that the minimum
    energy requirement implies approximate equality
    of magnetic and relativistic particle energy or
    equipartition.

energy density in particles
energy density in magnetic field
39
Equipartition in Radio Sources
For Cygnus A ? Lradio 5.1037 J/s
  • If dlobe 75 kPc 2.3.1021 m and vjet 103
    km/s, then
  • tlife 2.3.1021/106 2.3.1015 s 7.107
    years
  • Rlobe 35 kPc 1021 m and hence Vlobe 4/3 p
    Rlobe3
  • 5.1063 m3
  • Total energy requirement 5.1037 x 2.3.1015
    1053 J
  • and energy density 1053/1064 10-11
    J/m3
  • So from equipartition ? B2/2mo 10-11 or B
    5.10-9 Tesla

40
11
  • Maximum frequency observed is 10 Hz.

Thus electron acceleration is required in the
lobes.
41
Relativistic Beaming
  • Plasma appears to radiate preferentially along
    its direction of motion
  • Thus observer sees only jet pointing towards her
    - other jet is invisible.

Photons emitted in a cone of radiation and
Doppler boosted towards observer.
42
Jet collimation
  • Nozzle mechanism
    hot gas inside large, cooler cloud which is
    spinning hot gas escapes along route of least
    resistance rotation axis
    gt collimated jet
  • But VLBI implies cloud small and dense and
    overpredicts X-ray emission

43
Supermassive Black Hole
  • Black hole surrounded by accretion disk
  • Disk feeds jets and powers them by releasing
    gravitational energy
  • Black hole is spinning gt jets are formed
    parallel to the spin axis, perhaps confined by
    magnetic field

44
Geometrically-thick disk
  • Black hole disk acc rate gt Eddington
  • Disk puffs up due to radiation pressure
  • Torus forms in inner region which powers and
    collimates jets
  • Predicted optical/UV too high however, but still
    viable

45
ACTIVE GALACTIC NUCLEI
  • END OF TOPIC

46
Q 4.d) If the high energy electron spectrum in
the galaxy is of the formN(E) ? E-3/2, express
the ratio of Inverse Compton-produced to
Synchrotron-produced X-ray intensities in terms
of gIC and gSynch.
  • Ratio (no of electrons with )
  • (no of electrons with )
  • But

Hence IIC/ISynch gIC/gSynch2-3/2
gIC/gSynch1/2
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