1 Active Galaxies & Quasars Normal Galaxy Active Galaxy Galactic Nuclei Bright Active Galaxy NGC 5548 Galaxy Nucleus: Exact center of a galaxy and its immediate surroundings. If a spiral galaxy, it is the center of rotation. Normal Galaxies: Dense central star cluster A composite stellar absorption-line spectrum May also show weak nebular emission lines. HST Image Variability ( ) OSU AGN Watch Discovery of Active Galaxies 1943: Seyfert Galaxies Carl Seyfert identified 6 galaxies with strong, broad emission lines coming from a compact, bright galaxy nucleus. 1950s: Radio Galaxies First radio telescopes found faint galaxies at the location of intense radio emission. Show broad emission-lines in their spectra. Radio Galaxy M84 at 6cm wavelength HST Image
2 Quasar 3C273 Quasar spectra with broad emission lines Palomar Observatory Photo (c. 1968) The Riddle of the Quasars Quasar spectra with broad emission lines 1960s: Radio astronomers found intense, pointlike sources of radio emission. Photographs revealed slightly fuzzy or quasi-stellar objects at these locations. The spectra were bizarre and full of broad, unrecognized emission lines. Quasars: Quasi-Stellar Radio Sources The Riddle of Quasars: Solved Composite Quasar Spectrum Sloan Digital Sky Survey 1963: Maarten Schmidt (Caltech) Recognized that the emission lines were Hydrogen lines with extreme redshifts. Luminous objects located very far away. The fuzz is the host galaxy lost in the glare of an intense active nucleus. Problem: Great distances imply extreme luminosities
3 Quasars have non-thermal continuum emission Active Galactic Nuclei About 1% of all galaxies have active nuclei Bright, compact nucleus: Sometimes brighter than the entire galaxy. Strong, broad emission lines from hot, dense, highly excited gas. Rapidly Variable: Small: only a few light days across. What powers AGNs? Properties that need to be explained: Powerful: Luminosities of Billions or Trillions of suns. Emit everything from Radio to Gamma rays Compact: Visible light varies on day timescales X-rays can vary on a few hour timescales The Black Hole Paradigm A plausible energy source is accretion of matter by a supermassive Black Hole. Supermassive = M sun Schwarzschild Radii: ~ AU Matter releases gravitational energy as it falls in. Infalling gas settles into an accretion disk. The hot inner parts of the disk shine brightly.
4 The Central Engine Black Hole accretion can be very efficient: up to ~10% efficiency ~1 M sun /year of matter needed for bright AGN Get fuel from surrounding gas and stars The broad emission lines The emission lines arise in gas rotating around the central black hole. The lines are broad because of the large velocities. Rapidly Spinning Black Hole: Leads to the jets seen in radio-loud AGNs The Active Galaxy Zoo M87: Elliptical with an AGN and a Jet Most Active Galaxies are related: Radio Loud: powerful radio sources Low Power: Radio Galaxies High Power: Quasars Radio Quiet: very weak radio sources Low Power: Seyfert Galaxies High Power: Quasi-Stellar Objects (QSOs). HST Images of Quasar Host Galaxies Cosmic Beacons Quasars are the most luminous objects in the Universe: Brightest are ~10 14 L sun Brightest Quasars: Among the most distant objects in the Universe Most distant is ~2 Gpc Probes of the Universe on very large scales
5 Very Distant Quasar (Sloan Digital Sky Survey) Some Nagging Questions: How do supermassive black holes form? we don t really know for sure... How are they fueled? Galaxy interactions might dump gas into the nuclear regions to feed the Black Hole. Stellar bars might be able to funnel gas into the nucleus from the disk of the galaxy. Cannibalism of a gas-rich dwarf? Do most galaxies harbor a central black hole? Nearly all spirals show some level of activity if perhaps only very faintly. Dynamical evidence for massive black holes in many nearby inactive galaxies. Milky Way has a M sun Black hole, but lacks strong activity. Many more AGNs in the distant past, but few today - where are all the dead quasars? Finding answers is an active area of research.