Transcript: The fact that quasars are at large distances and have huge luminosities depends on the cosmological interpretation of their redshift.  There are some crucial distinctions between galaxies and quasars as far as redshift goes.  For galaxies they follow a Hubble relation where distance indicators such as Cepheids within the galaxies or supernovae in more distant galaxies reliably indicate distance and are correlated well with redshift.  Quasars have no property that correlates well with redshift.  The luminosity varies by a factor of thousands between different objects, and the light from the quasar is variable on timescales of weeks, months, and years.  So the redshift itself is used as a distance indicator.  In many cases the redshift is high enough that the host galaxy cannot be seen.  Quasar redshifts begin at a few tenths, and beyond a redshift of a half the host galaxy is usually not visible.  The highest quasar redshifts are six or seven, an age when the universe was only ten percent of its current age and seven times smaller than it is now.

Redshift of Quasars

Transcript: Quasars were mysterious when they were first discovered in the 1960s.  But careful work showed that the quasar is surrounded by nebulosity, and eventually spectroscopy of the nebulosity showed that it was the light of stars in a normal galaxy.  Thus quasar stands for quasi-stellar objects.  They are not truly stellar but do show fuzz when observed with high resolution for example with the Hubble Space Telescope.  Thus quasars are point-like nuclei in a host galaxy seen at large or cosmological distances.  Quasars are normal galaxies with an extraordinarily luminous source of radiation in their centers.  For example 3C273 is at a distance of six hundred megaparsecs or two billion lightyears.  At its center is optical radiation with a power of ten to the power fourteen or a hundred trillion suns.  This is an extraordinary amount of energy, and it emerges from a tiny region in the heart of the quasar.

Nature of Quasars

Transcript: Astronomers at Caltech became interested in the newly accurate radio positions of strong sources in the sky.  They focused in particular on two sources, 3C48 and 3C273 which appeared to be associated with bluish stars.  Since normal stars like the Sun do not emit strong radio waves this was a mystery.  The mystery deepened when Martin Schmidt used the Palomar 200-inch to take spectra of the two stellar counterparts.  He saw a series of strong broad lines that he could not identify with any known element.  Weeks later, playing with their spectrum, he made the remarkable discovery that the lines corresponded to the spectral series of hydrogen but redshifted by a huge amount.  In 3C48 the lines were redshifted by a speed corresponding to sixteen percent the speed of light, a hundred million miles per hour, and in 3C273 by twice that amount.  This was an extraordinary mystery.  What was the nature of strong radio emitting stars that were moving at speeds of hundreds of millions of miles per hour?

Discovery of Quasars

Transcript: In the 1940s Grote Reber used amateur astronomy radio equipment in his backyard to discover the first cosmic sources of radio radiation.  The first three sources he discovered were in the constellation of Sagittarius from the center of our own galaxy, and the constellation Cassiopeia from a supernova remnant, and in the constellation Cygnus which was a radio galaxy at a distance of seven hundred and fifty million lightyears.  By the 1950s hundreds of radio sources were known due to a series of large radio telescopes being built in the United States and in England.  But there were no accurate positions because these large telescopes used long wavelength radiation, and so their diffraction limits were a few arcminutes.  The positional accuracy was no better than optical astronomy before the invention of the telescope.  By the early 1960s astronomers used the clever technique of occultation by the Moon to pinpoint the location of several radio sources from the third Cambridge catalog.  With these accurate positions it was possible to identify optical counterparts.  In some cases the counterparts looked to be a distant galaxy, but in other cases it appeared to be a stellar source.

Discovery of Radio Sources

Transcript: Galaxies in close proximity are called interacting galaxies.  Interacting galaxies might be gravitationally bound to each other, or they may simply be passing on trajectories through the universe that bring them close together.  Interacting galaxies are affected by each other’s gravity.  Tidal forces can act to cause mass or gas to flow towards the centers, and gas can pass from one galaxy to the other.  There’s good evidence that interacting galaxies show increased incidence of nuclear activity as manifested by Seyfert nuclei, radio emission, or x-ray emission.  Thus interaction is a trigger for nuclear activity.

Interaction Galaxies

Transcript: An accretion disk is a hallmark of an active galactic nucleus.  Supermassive black holes accrete gas from the surrounding galaxy mostly coming from normal mass loss from stellar processes or from infall from the intergalactic medium.  When this gas eventually works its way to within the central parsec, it forms a hot, dense, thick disk which shares the rotation of the embedded black hole.  The characteristic temperature of this gas mixed with dust is a few tens of thousands of degrees Kelvin which means that its thermal radiation peaks in the far ultraviolet.  This peak radiation is not visible from the ground and so must be observed from space.  Astronomers have seen the characteristic hallmark of accretion disks in a number of AGN.  The UV emission emerges from a distance of ten to a thousand times the Schwarzschild radius of the supermassive black hole.

Emission from Accretion Disks

Transcript: Inverse Compton radiation can occur in an active galactic nucleus when the energy density is very high, as can occur in the vicinity of a black hole.  High energy electrons emit synchrotron radiation.  In a situation of high energy density the photons that result almost immediately scatter by the electrons again gaining further energy to push them to x-ray frequencies or wavelengths.  Thus, intense x-ray emission from active galactic nuclei can be caused by the Inverse Compton process, although sometimes it is the synchrotron process.

Inverse Compton Radiation

Transcript: Synchrotron radiation is radiation caused when particles, usually electrons, are accelerated in the presence of a magnetic field.  The acceleration can be caused for example by the death of a star,  supernova remnants show synchrotron emission, or by black hole physics in an active galactic nucleus.  The hallmark of synchrotron emission is linear polarization which is imprinted by the magnetic field itself.  The radio emission from active galactic nuclei is synchrotron radiation.

Synchrotron Radiation

Transcript: All atoms or molecules are in constant motion or vibration.  The emission or radiation that results is called thermal radiation.  Thermal radiation is directly related to the temperature of a substance, and it has a peak wavelength of the emission or characteristic wavelength given by Wien’s law.  By contrast, non thermal radiation has no characteristic wavelength.  The radiation extends over a large frequency range in what’s called a power law, and non thermal radiation does not correspond to an equilibrium physical process.  Non thermal radiation can be observed at all wavelengths from radio to gamma rays.

Nonthermal Radiation

Transcript: Hundreds of radio galaxies have been found, studied, and identified using synthesis radio telescopes like the Very Large Array.  A typical radio galaxy has a radio morphology with an intense and compact core of radio emission.  On small scales the core can only be resolved with VLBI techniques with milliarcsecond resolution and in fact is about the size of the solar system.  Emerging from the core in two directions are radio jets.  These jets can extend beyond the distance of the galaxy itself which is typical of an elliptical galaxy.  On the largest scales these jets connect with diffuse, fuzzy lobes of radio emission.  The lobes can extend for millions of lightyears into the intergalactic medium.

Radio Galaxy Morphology

21. Deep Space and High-Energy Phenomena

Transcript: Active galaxies were discovered even before we knew the distance to galaxies.  In 1908, Edward Fath discovered strong emission lines from the central regions of the galaxy NGC 1068.  It indicated ionized gas, but a much larger amount of nuclear ionized gas than would exist in a normal star formation region.  Vesto Slipher and Edwin Hubble found other examples, and Carl Seyfert conducted the first systematic survey in the 1940s.  Seyfert also noted other common features of active galaxies:  a high-contrast, compact nucleus, broad emission lines, strong radio emission, and peculiar morphology of the galaxy itself.

Discovery of Active Galaxies

Transcript: About one percent of all galaxies and ten percent of all active galaxies have high levels of radio emission.  Stars like the Sun and all other normal stars have very low levels of radio emission.  So the sum of stellar populations can not produce such radio emission.  In 1944 the amateur astronomer Grote Reber detected sources of radio emission in the constellations of Sagittarius, Cassiopeia, and Cygnus.  The Sagittarius source was the galactic center.  The Cassiopeia source was a supernova remnant.  But the source in Cygnus could not be identified until 1951 when at Palomar Walter Baade and Rudolph Minkowski identified the intense radio emission with a faint galaxy at a distance of two hundred and thirty megaparsecs.  This galaxy Cygnus A has ten million times the radio emission of the nucleus of the Milky Way, yet it can be detected with the radio equipment of a backyard amateur radio astronomer.

Radio Galaxies

Transcript: The first systematic survey of active galaxies was carried out by Carl Seyfert in the 1940s.  The galaxies he identified, mostly blue mostly spiral galaxies, are named after him.  Gas in Seyfert galaxies is highly ionized by an amount that’s too large to be explained by the action of hot stars as in an HII region.  There is basically an intense source of ultraviolet photons that cannot be explained by normal stellar processes somewhere in the nucleus.  A normal spiral will show broadening of the emission lines based on the rotation of the galaxy, a few hundred kilometers per second in velocity width, but in Seyfert galaxies the width of the broad emission lines is thousands of kilometers per second.  Either the gas is being ejected from the nucleus or it’s bound by a large massive object.  Either way something unusual is going on in the centers of Seyfert galaxies.

Seyfert Galaxies

Transcript: Stellar mass black holes are a natural anticipated consequence of stellar evolution.  Evidence for their existence is strong but not beyond doubt, so most people are surprised when they hear astronomers routinely talking about the existence of supermassive black holes millions or billions of times more massive than the Sun.  Yet the existence of supermassive black holes is also anticipated theoretically.  A dense star cluster will naturally evolve to form a black hole with perhaps a seed mass of a hundred times the mass of the Sun.  Over a billion years or so this black hole can grow by accretion and by devouring stars whole to a mass of millions of times the mass of the Sun.  The dense center parts of galaxies are good environments for the growth of supermassive black holes.  The Schwarzschild radius of a supermassive black hole like that in M87 is about forty astronomical units.  Imagine three billion times the mass of the Sun crushed into a region the size of the solar system.  Yet the density of material inside a supermassive black hole is not extraordinary.  The density is only about a hundred times less than that of water, so the physical state in a supermassive black hole is not that extraordinary.

Properties of Supermassive Black Holes

Transcript: Careful studies from space have allowed us to make a census of the population of supermassive black holes in nearby galaxies.  M31, our nearest neighbor and similar galaxy to the Milky Way, has a black hole about ten million solar masses.  M87, the giant elliptical galaxy that dominates the Virgo cluster has a black hole that is much more massive, three times ten to the nine solar masses, three billion times more massive than the Sun.  One survey found that twenty-five percent of all nearby galaxies have black holes, but the most important thing that’s been found out about black holes in nearby galaxies is that their mass is proportional to the bulge mass or luminosity.  The bulge is the old stellar population that’s moving in elliptical orbits.  Elliptical galaxies are essentially purely bulge population and are large, so they have the most massive black holes.  Among spiral galaxies the black hole mass goes down according to the Hubble sequence.  Higher black hole masses for Sa’s, lower for Sb’s, and lower still for Sc’s.  The Milky Way fits this sequence with its black hole of a few million solar masses.

Black Holes in Nearby Galaxies

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