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Galaxies With Active Nuclei Chapter 17 Galaxies With Active Nuclei

Guidepost This chapter is important for two reasons. First, it draws together ideas from many previous chapters to show how nature uses the same basic rules on widely different scales. Matter flowing into a protostar, into a white dwarf, into a neutron star, or into the heart of a galaxy must obey the same laws of physics, so we see the same geometry and the same phenomena. The only difference is the level of violence. Second, this chapter is important because the most distant objects we can see in the universe are the most luminous galaxies, and many of those are erupting in outbursts and are thus peculiar. By studying these galaxies, our attention is drawn out in space to the edge of the visible universe and back in time to the earliest stages of galaxy formation. In other words, we are led to think of the origin and evolution of the universe, the subject of the next chapter.

Outline I. Active Galaxies A. Seyfert Galaxies I. Active Galaxies 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

“Active Galactic Nuclei” (= AGN) 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. Unusual spiral galaxies: 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 result of recent galaxy mergers. 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 Centaurus A (“Cen A” = NGC 5128): the closest AGN to us. Jet visible in radio and X-rays; show bright spots in similar locations. 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

3C31: Member of a chain of galaxies. 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.

The Jets of M 87 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.

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. NGC 7052: Stellar velocities indicate the presence of a central black hole.

Model for Seyfert Galaxies 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

Dust Torus is directly visible with Hubble Space Telescope The Dust Torus in NGC 4261 Dust Torus is directly visible with Hubble Space Telescope

Other Types of AGN and AGN Unification 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.

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”). Observing direction

Black Holes in Normal Galaxies X-ray sources are mostly accreting stellar-mass black holes. The Andromeda galaxy M 31: No efficient accretion onto the central black hole

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.

Spectral lines show a large red shift of The Spectra of Quasars Spectral lines show a large red shift of z = Dl / l0 = 0.158 The Quasar 3C 273

Quasar Red Shifts z = Dl/l0 Quasars have been detected at the highest red shifts, up to z ~ 6 z = 0 z = 0.178 z = 0.240 z = Dl/l0 z = 0.302 This indicates distances of several Gigaparsec z = 0.389

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!

Probing Dark Matter with High-z Quasars: Gravitational Lensing 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.

Evidence for Quasars in Distant Galaxies Quasar 0351+026 at the same red shift as a galaxy  evidence for quasar activity due to galaxy interaction

Host Galaxies of Quasars The radio image of the quasar 3C175 shows a double-lobe jet structure, indicating its association with an active galactic nucleus.

Gallery of Quasar Host Galaxies Elliptical galaxies; often merging / interacting galaxies

Superluminal Motion Individual radio knots in quasar jets: Individual radio knots in quasar jets: Sometimes apparently moving faster than speed of light! Light-travel time effect: Material in the jet is almost catching up with the light it emits