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...

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...

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...

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...

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...

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...

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...

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.

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...

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...

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...

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...

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...

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...

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...

Transcript: We’ve discovered a supermassive black hole in the center of our galaxy the Milky Way. However, it would violate the Copernican principle if the Milky Way were unique in any way, so astronomers anticipated black holes in other galaxies or other evidence of nuclear activity. There are two main things that astronomers search for when they are trying to detect a supermassive black hole in another galaxy. The first is a sharp peak or cusp in the light distribution, and the second is...

Transcript: Seen from afar, our galaxy the Milky Way would be a beautiful but unremarkable spiral galaxy. However, we have a ringside seat at a distance of only thirty thousand lightyears from the nucleus. The central regions of our galaxy have a very compact radio source, a region of intense star formation, a lot of ionized gas, and stellar dynamics that strongly indicate the presence of a supermassive black hole a few million times the mass of the Sun. Thus the definition of an active...

Transcript: In the first billion or so years after the big bang or before the first epoch of galaxy formation, the universe was in the period called the dark ages. No stars had yet formed. Ironically, the universe was smaller, denser, and hotter than it is now, and much of the gas and intergalactic space was very highly ionized at temperatures of tens of thousands of degrees. But the gas could not cool and could not gravitationally collapse to form objects, so the universe was dark. At...

Transcript: Most galaxies are made of stars, gas, and dust plus the ubiquitous dark matter. Elliptical and spiral galaxies contain different proportions of these materials, less dust and less gas for elliptical galaxies than spiral galaxies, but they are basically made of the same thing. The morphology of the galaxies is determined by their stellar orbits and the evolution of stellar populations. Their spectra are simply the sum of the spectra of billions of individual stars. However,...

Transcript: Normally astronomers talk about the brightness or luminosity of galaxies. However, galaxies are not point sources. Their light is spread out or diffuse. Another measure of a galaxy brightness is its surface brightness or its flux per unit area. As measured on a fixed size of detector the surface brightness of galaxies in the local universe is independent of distance. As the distance to the galaxy increases the flux or apparent brightness goes down as the distance squared. ...

Transcript: Astronomers have used deep multi-wavelength observations of the sky to try and determine the history of star formation in the universe. That is the sum of the formation processes of all stars in all galaxies over cosmic time. This ambitious task is important in terms of deciding how the history of galaxies occurred. It’s a difficult procedure because measurements of the blue light of nearby galaxies correspond to something entirely different at high redshift. For example the...

Transcript: Astronomers have used deep field observations, pointed surveys with large telescopes in the Hubble Space Telescope, to estimate the number of galaxies in the universe. This is based on a sampling technique. Astronomers do not have to survey the sky in every direction. They use a small pencil beam survey punching deep through the observable universe to count the number of galaxies in a few directions and then multiply up to get the total number of galaxies on the full coverage...

Transcript: Astronomers have learned much about galaxies in cosmology by the intensive study of very small regions of sky. By looking with a variety of telescopes at a variety of wavelengths very hard at one region of the sky astronomers can punch through the universe reaching almost the entire span of galaxy and star formation, ten or eleven billion years of look-back time. The most famous of the deep fields have been done by the Hubble Space Telescope which had a northern and a southern...

Transcript: In the standard model of cosmology structure formation occurs in a top down way which means that the smallest objects, galaxies, form first and then subsequently cluster to form clusters of galaxies and eventually superclusters of galaxies. The largest structures therefore should be the youngest, and indeed the local supercluster of galaxies in the nearby universe is only just forming. Clusters in the local universe are observed to be relaxed, that is symmetric, and apparently...

Transcript: When clusters of galaxies are observed with microwaves something very interesting happens. The microwaves show a decrement or a hole where the cluster is. For awhile astronomers did not understand this effect, but it turns out to have a clean and clear theoretical explanation. What happens is that the hot, dense material at the center of clusters scatters the microwave photons of the background radiation from the big bang up to higher frequencies leaving a deficit of those...