Galaxies%20with%20Active%20Nuclei

1
Galaxies with Active Nuclei
0

• Chapter 17

2
Guidepost
0

• You can imagine galaxies rotating slowly and
  quietly making new stars as the eons pass, but
  the nuclei of some galaxies are sites of powerful
  eruptions that eject high-speed jets in opposite
  directions. As you study these active galaxies,
  you will be combining many of the ideas you have
  discovered so far to answer five essential
  questions
• What evidence shows that some galactic nuclei
  are active?
• What is the energy source of this activity?
• What triggers the nucleus of a galaxy into
  activity?
• What are the most distant active galaxies?
• What can active galaxies reveal about the
  history of the universe?

3
Guidepost (continued)
0

• There are billions of the galaxies in the sky,
  and astronomers cant study every one. Rather
  they must use statistical evidence, and that
  raises a common question about the scientific
  method
• If statistics isnt certainty, how can
  scientists use it to understand nature?
• The active galaxies are the last pieces of
  evidence you need before you try to understand
  the birth and evolution of the entire universe
  and the galaxies that fill it. You will start
  that journey in the next chapter.

4
Outline
0
I. Active Galaxic Nuclei A. Seyfert Galaxies B.
Double-Lobed Radio Sources C. Exploring
Supermassive Black Holes D. The Search for a
Unified Model E. The Origin of Supermassive
Black Holes II. Quasars A. The Discovery of
Quasars B. The Distance to Quasars C. Evidence
of Quasars in Distant Galaxies D. Superluminal
Expansion E. A Model Quasar F. Quasars Through
Time
5
Active Galaxies
0
Galaxies with extremely violent energy release in
their nuclei (pl. of nucleus).
?Active Galactic Nuclei ( AGN)
Up to many thousand times more luminous than the
entire Milky Way energy released within a region
approx. the size of our solar system!
6
The Spectra of Galaxies
0
Taking a spectrum of the light from a normal
galaxy
The light from the galaxy should be mostly star
light, and should thus contain many absorption
lines from the individual stellar spectra.
7
Seyfert Galaxies

• Very bright cores

0

• Emission line spectra from core region.

Unusual spiral galaxies

• Variability 50 in a few months

Most likely power source Accretion onto a
supermassive black hole (107 108 Msun)
8
Interacting Galaxies
0
Seyfert galaxy NGC 7674
Active galaxies are often associated with
interacting galaxies, possibly result of recent
galaxy mergers.
Often gas outflowing at high velocities, in
opposite directions
9
Cosmic Jets and Radio Lobes
0
Many active galaxies show powerful radio jets
Radio image of Cygnus A
Hot spots Energy in the jets is released in
interaction with surrounding material
Material in the jets moves with almost the speed
of light (Relativistic jets).
10
Radio Galaxies
0
Cygnus A A giant pair of radio jets.
Jet visible in radio and X-rays show bright
spots in similar locations.
Radio Image
Centaurus A ( Cen A NGC 5128) Infrared
image reveals warm gas near the nucleus.
11
Radio Galaxies (2)
0
NGC 1265 Evidence for the galaxy moving through
intergalactic material
Radio image of 3C 75
3C 75 Evidence for two nuclei ? recent galaxy
merger
12
Radio Galaxies (3)
0
3C31 Member of a chain of galaxies.
Twisted jets, probably because two galactic
nuclei are orbiting each other.
13
Formation of Radio Jets
0
Jets are powered by accretion of matter onto a
supermassive black hole
Black Hole
Accretion Disk
Twisted magnetic fields help to confine the
material in the jet and to produce synchrotron
radiation.
14
The Jets of M 87
0
M 87 Central, giant elliptical galaxy in the
Virgo cluster of galaxies
Jet 2.5 kpc long
Optical and radio observations detect a jet with
velocities up to 1/2 c.
15
Evidence for Black Holes in AGNs
0
NGC 4261 Radio image reveals double-lobed jet
structure close-up view by Hubble Space
Telescope reveals a bright central source
embedded in a dust torus.
NGC 7052 Stellar velocities indicate the
presence of a central black hole.
16
Model for Seyfert Galaxies
0
Seyfert I Strong, broad emission lines from
rapidly moving gas clouds near the BH
Gas clouds
Emission lines
UV, X-rays
Seyfert II Weaker, narrow emission lines from
more slowly moving gas clouds far from the BH
Supermassive black hole
Accretion disk
Dense dust torus
17
The Dust Torus in NGC 4261
0
Dust Torus is directly visible with Hubble Space
Telescope
18
Other Types of AGN and AGN Unification
0
Observing direction
Cyg A (radio emission)
Radio Galaxy Powerful radio lobes at the end
points of the jets, where power in the jets is
dissipated.
19
Other Types of AGN and AGN Unification (2)
0
Quasar or BL Lac object (properties very similar
to quasars, but no emission lines)
Emission from the jet pointing towards us is
enhanced (Doppler boosting) compared to the jet
moving in the other direction (counter jet).
Observing direction
20
Black Holes in Normal Galaxies
0
X-ray sources are mostly accreting stellar-mass
black holes.
The Andromeda galaxy M 31
No efficient accretion onto the central black hole
21
Bursts of Activity of Supermassive Black Holes
0
A star wandering too close to a supermassive
black hole can be disrupted and trigger an X-ray
outburst.
22
Active Galaxies in Galaxy Clusters
0
The powerful radio lobes of radio galaxies can
push away intergalactic gas in galaxy clusters.
Even hundreds of millions of years after the
Galaxys activity has calmed down, there are
still ghost cavities in the X-ray emission from
intergalactic gas.
23
Quasars
0
Active nuclei in elliptical galaxies with even
more powerful central sources than Seyfert
galaxies
Also show strong variability over time scales of
a few months.
Also show very strong, broad emission lines in
their spectra.
24
The Spectra of Quasars
0
Spectral lines show a large red shift of
z Dl / l0 0.158
The Quasar 3C 273
25
Quasar Red Shifts
0
z 0
Quasars have been detected at the highest red
shifts, beyond z 6
z 0.178
z 0.240
z 0.302
z 0.389
z Dl/l0
This indicates distances of several Gigaparsec
26
Studying Quasars
0
The study of high-redshift quasars allows
astronomers to investigate questions of
1) Large scale structure of the universe
2) Early history of the universe
3) Galaxy evolution
4) Dark matter

• Observing quasars at high redshifts
• distances of several Gpc

• Look-back times of many billions of years
• The universe was only a few billion years old!

27
Probing Dark Matter with High-z
QuasarsGravitational Lensing
0
Light from a distant quasar is bent around a
foreground galaxy
? two images of the same quasar!
Light from a quasar behind a galaxy cluster is
bent by the mass in the cluster.
Use to probe the distribution of matter in the
cluster.
28
Evidence for Quasars in Distant Galaxies
0
Quasar 0351026 at the same red shift as a galaxy
? evidence for quasar activity due to galaxy
interaction
29
Host Galaxies of Quasars
0
The radio image of the quasar 3C175 shows a
double-lobe jet structure, indicating its
association with an active galactic nucleus.
30
Gallery of Quasar Host Galaxies
0
Elliptical galaxies often merging / interacting
galaxies
31
Superluminal Motion
0
Individual radio knots in quasar jets
Sometimes apparently moving faster than speed of
light!
Light-travel time effect
Relativity should be consider in the calculation
Material in the jet is almost catching up with
the light it emits (velocity is close to c)