Active Galaxies and Supermassive
Black Holes
Chapter 17
In the last few chapters, you have explored our own and other galaxies, and 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 essential 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
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.
Guidepost (continued)
I. Active Galaxic NucleiA. Seyfert GalaxiesB. Double-Lobed Radio SourcesC. Quasars
II. Supermassive Black HolesA. Disks and JetsB. The Search for a Unified ModelC. Triggering EruptionsD. Supermassive Black Holes Through Time
Outline
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 GalaxiesTaking 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 GalaxiesUnusual 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
Seyfert galaxy NGC 7674
Active galaxies are often associated with interacting galaxies, possibly the result of
recent galaxy mergers.
Often: gas outflowing at high velocities, in opposite directions
Cosmic Jets and Radio LobesMany active galaxies show powerful radio jets
Radio image of Cygnus A
Material in the jets moves with almost the speed of light (“Relativistic jets”).
Hot spots: Energy in the jets is released in
interaction with
surrounding material
Radio GalaxiesCygnus A: A giant pair of radio jets.
Jet visible in radio and X-rays; show bright spots in similar locations
Centaurus A
(= “Cen A” = NGC 5128):
Infrared image reveals warm gas near the
nucleus.
Radio Image
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 JetsJets are powered by accretion of matter onto
a supermassive black hole.
Black Hole
Twisted magnetic fields help to confine the material in the jet and to produce synchrotron radiation.
Accretion Disk
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 very strong, broad emission lines in their spectra
Also show strong variability over time scales of a few
months
The Spectra of Quasars
The Quasar 3C 273:
Spectral lines show a large red shift of
z = = 0.158
Studying QuasarsThe 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 AGNsNGC 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
Radio Galaxy:
Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated.
Cyg A (radio emission)
Observing direction
Other Types of AGN and AGN Unification (2)
Emission from the jet pointing towards us is enhanced (“Doppler boosting”) compared to the jet moving in the other direction (“counter jet”).
Quasar or BL Lac object (properties very similar to quasars, but no emission lines)
Observing direction
AGN Unification
Components of a Seyfert Galaxy or Quasar
Broad Line Region
Narrow 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 TimeQuasar 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.