What is an Active Galactic Nucleus (AGN)?
An active galactic nucleus is a small region, between about 1 and 100 light years in diameter, at the center of a galaxy that emits prodigious amounts of radiation over some or all of the electromagnetic spectrum, from the shortest gamma rays to the longest radio waves. Active galactic nuclei are among the most energetic phenomena in the universe, beating out even supernovae in terms of luminosity. Active galactic nuclei are powered by matter accreting onto supermassive black holes, with a mass between 106 and 1010 times that of the Sun. These black holes generally have a size about a light year in diameter. The infalling matter forms into an accretion disc, with gas particles moving and colliding with each other at velocities approaching the speed of light. This releases huge amounts of energy, often in the form of relativistic jets projecting perpendicular to the accretion disc. Depending on whether or not these jets point towards the Earth, an active galactic nucleus might have eith
Name several phenomena associated with an AGN. • Jets. What is the observational evidence for jets? In what kind of electromagnetic radiation are jets commonly observed? The radiation is mostly synchrotron radiation. What is synchrotron radiation? Two pieces of observations that at at least some jets are moving at very close to the speed of light are (1) superluminal motion, and (2) one-sidedness. Explain what each of these is, and why they are considered to be evidence for relativistic motion. • Supermassive Black Holes. What is a supermassive black hole? Where are they found? What is the evidence that supermassive black holes are indeed black holes? How are the masses of supermassive black holes determined? For approximately how many supermassive black holes have masess been measured? • Sagittario. Give an account of the object at the center of our own Galaxy, the Milky Way. What is SgrA*? How is the mass of Sagittario measured? What is its mass? • Penrose diagrams. What is a Penrose
Name several phenomena associated with an AGN. • Jets. What is the observational evidence for jets? In what kind of electromagnetic radiation are jets commonly observed? The radiation is mostly synchrotron radiation. What is synchrotron radiation? Two pieces of observations that at at least some jets are moving at very close to the speed of light are (1) superluminal motion, and (2) one-sidedness. Explain what each of these is, and why they are considered to be evidence for relativistic motion. • Supermassive Black Holes. What is a supermassive black hole? Where are they found? What is the evidence that supermassive black holes are indeed black holes? How are the masses of supermassive black holes determined? For approximately how many supermassive black holes have masess been measured? • Sagittario. Give an account of the object at the center of our own Galaxy, the Milky Way. What is SgrA*? How is the mass of Sagittario measured? What is its mass? • MAXIM. What does the MAXIM mission
Important clue: many AGN’s have a disk of very rapidly revolving gas at center – see Seyfert Type I galaxies above e.g. 1: Galaxy M87 – Giant Elliptical (E type) galaxy with an Active Galactic Nucleus (AGN) – central disk with radius = 25 parsecs (pc) – very compact – from spread of different Doppler shifts in spectral lines: disk gas revolving around galactic center with orbital velocity = 740 km/s – very fast – from Kepler’s Laws of orbital motion (see Planetary Motion chapter): Mass at center of gas disk = 3.2 billion – super-massive object eg. 2: Galaxy NGC 4261 – Radio galaxy – central disk with radius = 50 pc – Note: rotation axis of disk is aligned with direction of radio jets there is a connection between galactic center and radio-emitting lobes – disk is rotating very rapidly from Kepler’s Laws: central Mass = 1.2 billion The Super-massive Black Hole Model of Active Galactic Nuclei (AGN): AGN are galactic nuclei that have a super-massive black hole at galactic center Super-mas
