1	Astr 1050 Fri., Apr 2, 2004 Today: Extra Credit Articles Start Chapter 14, Active Galaxies
2	Chapter 14: Galaxies with Active Nuclei Discovery of Active Galactic Nuclei (AGN) Seyfert Galaxies and Radio Sources The Unified Model Black Holes in Galaxies, disks, orientation, + Quasars Distances and Relativistic Redshifts Quasars as extreme AGN Evolution of Quasars/Galaxies Gravitational Lensing
3	The (slightly) active nucleus of our galaxy Probable Black hole High velocities Large energy generation At a=275 AU P=2.8 yr Þ 2.7 million solar masses Radio image of Sgr A* about 3 pc across, with model of surrounding disk
4	Active Galactic Nuclei: AGNs A small fraction of galaxies have extremely bright “unresolved” star-like cores (active nuclei) Shown here is an HST image of NGC 7742, a so-called “Seyfert galaxy” after Carl Seyfert who did pioneering work in the 1940s
5	NGC4151 with a range of exposures
6	Spectra of Stars, Spectra of AGNs
7	Active Galactic Nuclei: AGNs Small fraction of galaxies have extremely bright “unresolved” star-like nuclei Very large energy generation Brightness often varies quickly Implies small size (changes not smeared out by light-travel time) High velocities often seen (> 10,000 km/s in lines) Emission all over the electro-magnetic spectrum Jets seen emerging from galaxies
8	3C31
9	Many Views of Radio Galaxy Centaurus A
10	Active Galactic Nuclei: AGNs Many galaxies have extremely bright “unresolved” star-like nuclei Very large energy generation Brightness often varies quickly Implies small size (changes not smeared out by light-travel time) High velocities often seen (> 10,000 km/s in lines) Emission all over the electro-magnetic spectrum Jets seen emerging from galaxies More common in colliding galaxies More common at large distances (redshift): Quasars! So more common in distant past (look-back time)
11	Relativistic Doppler (Red) Shift Classical Doppler Effect: Also refer to Δλ/λ as the “redshift” or “z” What if z is so large that v => c? v (z + 1)² - 1 c (z + 1)² + 1
12	Relativistic Doppler (Red) Shift
13	Black Holes: Review Escape velocity from the surface at radius R is At small enough R we have V_Escape= c (speed of light) That R is by definition the Schwarzschild radius Far from the black hole gravity is the same as for any ordinary mass M Stars will just orbit around it like any other mass Gas orbiting it collides, tries to slow down, (just like reentering satellite) As gas falls inward it ends up speeding up and heating up Gas will be moving at close to speed of light by the time it reaches R_S Light emitted by hot gas just outside R_S can still escape Provides a way to release about 10% of E=mc² of energy Fusion releases only about 1% Signature of black hole: Very high energy release, very high velocity
14	Accretion Disks Black hole is “active” only if gas is present to spiral into it Isolated stars just orbit black hole same as they would any other mass Gas collides, tries to slow due to friction, and so spirals in (and heats up) Conservation of angular momentum causes gas to form a disk as it spirals in
15	Different Views of the Accretion Disk The torus of gas and dust can block part of our view Seyfert 2 galaxies: Edge on view Only gas well above and below disk is visible See only “slow” gas Þ narrow emission lines Seyfert 1 galaxies: Slightly tilted view Hot high velocity gas close to black hole is visible High velocities Þ broad emission lines BL Lac objects: Pole on view Looking right down the jet at central region Extremely bright – vary on time scales of hours Quasars: Very active AGN at large distances Can barely make out the galaxy surrounding them Were apparently more common in distant past
16	Different Views of the Accretion Disk The torus of gas and dust can block part of our view Seyfert 2 galaxies: Edge on view Only gas well above and below disk is visible See only “slow” gas Þ narrow emission lines Seyfert 1 galaxies: Slightly tilted view Hot high velocity gas close to black hole is visible High velocities Þ broad emission lines BL Lac objects: Pole on view Looking right down the jet at central region Extremely bright – vary on time scales of hours Quasars: Very active AGN at large distances Can barely make out the galaxy surrounding them Were apparently more common in distant past
17	Different Views of the Accretion Disk The torus of gas and dust can block part of our view Seyfert 2 galaxies: Edge on view Only gas well above and below disk is visible See only “slow” gas Þ narrow emission lines Seyfert 1 galaxies: Slightly tilted view Hot high velocity gas close to black hole is visible High velocities Þ broad emission lines BL Lac objects: Pole on view Looking right down the jet at central region Extremely bright – vary on time scales of hours Quasars: Very active AGN at large distances Can barely make out the galaxy surrounding them Were more common in distant past
18	Quasar Images 1
19	Nice, early Quasar Quote:
20	Quasar Images II
21	Quasar Images III: “Starburst-Quasar”
22	What makes an AGN active? Need a supply of gas to feed to the black hole (Black holes from 1 million to >1 billion solar masses! Scales as a few percent of galaxy bulge mass.) Collisions disturb regular orbits of stars and gas clouds Could feed more gas to the central region Galactic orbits were less organized as galaxies were forming, also recall the “hierarchical” galaxy formation Expect more gas to flow to central region when galaxies are young => Quasars (“quasar epoch” around z=2 to z=3) Most galaxies may have massive black holes in them They are just less active now because gas supply is less
23	Gravitational Lensing
24	For more information, movies: A nice website: http://www.mssl.ucl.ac.uk/www_astro/agn/agn_beginners.html Other links on the website will take you to movies showing quasar structure, and discussing unified models.
25	Chapter 14: Galaxies with Active Nuclei Discovery of Active Galactic Nuclei (AGN) Seyfert Galaxies and Radio Sources The Unified Model Black Holes in Galaxies, disks, orientation, + Quasars Distances and Relativistic Redshifts Quasars as extreme AGN Evolution of Quasars/Galaxies Gravitational Lensing