Transcript The Milky Way - Houston Community College System

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Chapter 17
Active Galaxies
and Supermassive
Black Holes
Guidepost
In the previous two chapters, you have explored our own
and other galaxies, and now you are ready to stretch your
scientific imagination and study some of the most
powerful objects in nature. Supermassive black holes at
the centers of galaxies are common but extreme. To
study them, you will be combining many of the ideas you
have discovered so far to answer four important
questions:
• What makes some galaxy cores active?
• How do supermassive black holes erupt?
• How did supermassive black holes form and evolve?
• How do supermassive black holes affect the evolution of
their host galaxies and galaxy clusters?
Guidepost (continued)
The formation and evolution of supermassive black holes
leads your astronomical curiosity outward into space and
backward in time to the era of galaxy formation. In the
next chapter, you will take the next step and try to
understand the birth and evolution of the entire universe.
Outline
I. Active Galactic Nuclei
A. Seyfert Galaxies
B. Double-Lobed Radio Sources
C. Quasars
II. Supermassive Black Holes
A. Disks and Jets
B. The Search for a Unified Model
C. Triggering Eruptions
D. Supermassive Black Holes Through Time
Active Galaxies
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!
The Spectra of Galaxies
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.
Seyfert Galaxies
Unusual spiral galaxies:
• Very bright cores
• Emission line spectra.
• Variability: ~ 50 % in
a few months
Most likely power
source:
Accretion onto a
supermassive black
hole (~107 – 108 Msun)
Interacting Galaxies
Active galaxies are often associated with
interacting galaxies, possibly the result of
recent galaxy mergers.
Seyfert galaxy NGC 7674
Often: gas outflowing at high velocities, in opposite directions
Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets
Radio image
of Cygnus A
Hot spots: Energy in
Material in the jets moves
with almost the speed of
light (“Relativistic jets”)
the jets is released in
interaction
with
surrounding
material
Radio Galaxies
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
Radio Galaxies (2)
NGC 1265: Evidence for
the galaxy moving through
intergalactic material
Radio image
of 3C 75
3C 75: Evidence for
two nuclei  recent
galaxy merger
Radio Galaxies (3)
3C31: Member of a chain of galaxies
Twisted jets, probably because two
galactic nuclei are orbiting each other
Formation of Radio Jets
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
Active Galaxies in Galaxy Clusters
The powerful radio lobes of radio galaxies can
push away intergalactic gas in galaxy clusters.
Even hundreds of millions of years after the Galaxy’s
activity has calmed down, there are still “ghost
cavities” in the X-ray emission from intergalactic gas.
Quasars
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
The Spectra of Quasars
The Quasar 3C 273:
Spectral lines show a large red shift of
z = Dl / l0 = 0.158
Studying Quasars
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!
Evidence for Black Holes in AGNs
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.
Other Types of AGN and AGN Unification
Cyg A (radio emission)
Radio Galaxy:
Powerful “radio lobes”
at the end points of the
jets, where power in the
jets is dissipated.
Other Types of AGN and AGN Unification (2)
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”)
AGN Unification
Components of a Seyfert
Galaxy or Quasar
Narrow Line
Region
Broad Line Region
Bursts of Activity
of Supermassive
Black Holes
A star wandering too close
to a supermassive black
hole can be disrupted and
trigger an X-ray outburst.
Gallery of Quasar Host Galaxies
Elliptical galaxies; often merging / interacting galaxies
Quasars Through Time
Quasar activity in the Universe was most
abundant at redshifts z ~ 2 – 3.
The highest-redshift quasars are seen at
z > 6, but those are very rare.