2.  Galaxies with extremely violent energy release in their nuclei  Active Galactic Nuclei (AGN)  Up to many thousand times more luminous than the entire Milky Way.  Energy is released within a region approximately the size of our solar system.

3.  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.

4.  Seyfert Galaxies are one of the two largest groups of active galaxies; also an unusual type of spiral galaxy.  Make up about 2% of spiral galaxies.  Variability ~ 50% in a few months.  Type I Seyfert: very luminous at X-ray and UV wavelengths; typical broad emission lines with sharp, narrow cores.  Type II Seyfert: weaker X-ray emission and emission lines are narrower than Type I Seyfert but still broader than normal galaxy lines.

5.  Despite their small size, the cores of Seyfert galaxies produce tremendous amounts of energy.  The brightest emit a hundred times more energy than the entire Milky Way galaxy.  Lots of energy produced in a very small volume with extremely high temperatures and velocities … sound familiar?  Astronomers conclude the centers of these galaxies contain supermassive black holes into which matter is flowing from surrounding hot accretion disks. NGC 1068 (M77)

6.  Active galaxies are often associated with interacting galaxies, possibly the result of recent galaxy mergers.

7.  Many active galaxies show powerful radio jets.  Material in the jets moves at near the speed of light (“relativistic jets”).  Hot spots within the jets (bright red/orange colors) is energy released through interaction with the surrounding material. Cygnus A, the brightest radio source in Cygnus, is a pair of giant lobes with jets leading from the nucleus of a highly disturbed galaxy.

8.  Infrared image of Centuarus A (NGC 5128) reveals warm gas at its nucleus.

9. The radio jets from NGC 1265 are being left behind as the galaxy moves rapidly through the gas of the intergalactic medium. Twists in the tails are presumably caused by motions of the active nucleus. The radio source 3C 75 is produced by two galaxies experiencing a close encounter. As the active nuclei whip around each other, their jets twist and turn.

10. Radio galaxy 3C 31 is one of a chain of galaxies. It has ejected jets from its core that twist, presumably because the active nucleus is orbiting another object such as the nucleus of a recently absorbed galaxy.

11.  Jets are powered by accretion of matter onto a supermassive black hole.  Conservation of angular momentum forces the matter to form a whirling accretion disk around the black hole.  Twisted magnetic field confines the jets in a narrow beam and causes synchronous radiation.

12.  The powerful radio lobes of radio galaxies can push away intergalactic gas in galaxy clusters.  Hundreds of millions of years after the galaxy’s activity has calmed down, there are still “ghost” cavities in the X-ray emission from the intergalactic gas.

13.  Quasars are the other largest group of active galaxies; primarily in elliptical galaxies with even more powerful central sources than Seyfert galaxies.  Also show strong variability over time scales of a few months.  Show very strong, broad emission lines in their spectra.

14.  Spectral lines from Quasar 3C 273 show a large red-shift: z =   = 0.158  The original photographic plate holding a spectrum of 3C 273 contains three hydrogen Balmer lines, H δ, H γ, and H β. The spectrum is red-shifted by 15.8%.

15.  The study of high red-shift 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  Quasars can be far away, but are not very faint.  Ultra-luminous  10 – 1000 times the luminosity of a large galaxy.  How could they be this bright but also very small?  Overwhelming evidence now suggesting quasars are the active cores of very distant galaxies. ▪ Most extreme type of active galactic nuclei (AGN)

16. 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.

17.  Astronomers studying active galaxies have developed a unified model of active galaxy core that is well supported by evidence … a monster black hole is the centerpiece.  According to the unified model, what you see when you view the core of an active galaxy depends on how the black hole’s accretion disk is tipped with respect to your line of sight.  Unified model far from complete:  Actual structure of accretion disks is poorly understood.  Does not explain all differences among active galaxies and quasars.

18. 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

19. 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

20. Components of a Seyfert Galaxy or Quasar Broad Line Region Narrow Line Region

21. Orbiting X-ray telescopes observing active galaxies sometimes detect X-ray flares equaling the energy of a supernova explosion. Such flares are evidently caused when a star wanders too close to a supermassive black hole at the center of a galaxy and tidal forces rip the star apart.

22. Elliptical galaxies; often merging / interacting galaxies

23. 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. Evidently astronomers see few quasars at high red-shifts because they are looking back to an age when the universe was so young it had not yet formed many galaxies.