Chapter 17 Active Galaxies and Supermassive Black Holes
Guidepost 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 (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 Galaxic 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 (~10 7 10 8 M sun )
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 the jets is released in interaction with surrounding material Material in the jets moves with almost the speed of light ( Relativistic jets ).
Radio Galaxies Cygnus A: A giant pair of radio jets. Radio Image 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 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. Accretion Disk Black Hole 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 = = 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.