21. Deep Space and High-Energy Phenomena

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.

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.

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?

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.

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.

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.

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.

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 correspond to an equilibrium physical process. Non thermal radiation can be observed at all wavelengths from radio to gamma rays.

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.

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.

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.

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.

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.

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.

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 a high stellar velocity dispersion. The cusp in the light distribution is measured by imaging, the higher resolution the better, so usually this work is done from space with the Hubble Space Telescope. The spectroscopy is also best done from space where the smallest slit must be placed over the center of the galaxy to isolate the stellar motions in the very central regions. If a galaxy has a cusp in its light distribution and a high degree of stellar motion near the center then the implied mass to light ratio will be higher than any plausible stellar population, so dark, concentrated mass is indicated: a supermassive black hole.

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 galaxy is mostly a matter of degree. It may be that most or many galaxies have modest nuclear activity and a black hole about the size of the black hole in the galactic center whereas only the occasional galaxy has extreme nuclear activity and a black hole that may be thousands of times larger up to a billion times the mass of the Sun.

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 some point within the first billion years the first stars formed. Astronomers refer to these as Population III stars. They must have contained the tiniest amount of heavy elements because that’s when many of the first heavy elements were produced. Then starting with the stars, objects like globular clusters and small galaxies formed, and over the subsequent billions of years, by a series of mergers in the hierarchical structure formation scenario, galaxies assembled to produce the large galaxies we see around us today.

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, certain galaxies have violent phenomena taking place in their centers. This can manifest in a number of ways, by a fierce starburst, by active x-ray emission, by non-thermal radio emission, or by gas moving with very high velocities near the nucleus. Collectively such galaxies are called active galaxies or AGN for active galactic nuclei.

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. But the area of the galaxy covered by the detector increases as the distance squared, and the two factors cancel out. But in cosmological scales surface brightness is not constant. It reduces as one plus z to the fourth power. One plus redshift to the fourth power is a substantial number. At a redshift of one it’s two to the power four, or a factor of sixteen. This diminishing surface brightness of galaxies is what makes high redshift galaxies so difficult to detect, but it also becomes a test of the standard cosmology because only in the model of cosmological redshifts caused by the expansion of the space does surface brightness decline with this particular relationship.

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 blue light observation of a nearby galaxy corresponds to relatively young stars, but by high redshift that blue pass band is selecting far ultraviolet light which can correspond to very short-lived times of star formation. Thus a fixed filter pass band selects entirely different types of stellar populations at high and low redshift which is why astronomers must combine multi-wavelength observations in the optical and infrared spectral regions. In addition, dust can obscure galaxies, and in particular young galaxies are expected to be dust-enshrouded. Visible light cannot penetrate dust so infrared observations are needed to study high redshift galaxies.

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 of the sky. In a region that’s about the size of the head of a pin held at arms length the Hubble Space Telescope can count three thousand galaxies to the limit of its observation, ten billion times fainter than the eye can see. Multiplying this across the area of the sky yields an estimate of sixty billion galaxies in the observable universe each with billions of stars, so the total stellar content of the universe is ten to the power twenty stars, a hundred billion billion stars in the universe.

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 deep field and more recently an ultra-deep field. The Hubble Space Telescope stares in the deep field observation for hundreds of orbits in the same tiny patch of sky. The depth reach in these observations is phenomenal, twenty-five magnitudes fainter than the naked eye can see on a dark sky. Twenty-five magnitudes is a factor of ten to the ten, ten billion times fainter than the eye can see. These deep fields have been used to show that galaxies formed relatively early in the universe, about a billion years or so after the big bang.

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 gravitationally stable, the galaxies having had a number of orbits in and out of the cluster. But by high redshift, the time available for forming large structures is much less so astronomers anticipate that rich clusters should be very rare at high redshift. The density or number of clusters at high redshift is thus a test of the standard cosmological model, and astronomers put a large effort into trying to find high redshift clusters. It’s very difficult to find clusters beyond a redshift of one which means that mass concentrations that large were very rare in the first few billion years of the universe.

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 microwave photons in the direction of the cluster. This is a very important effect in cosmology because it’s proof that the clusters are at cosmological distances since the microwave background photons emerge from the entire universe at a redshift of a thousand. It’s called the Sunyaev-Zeldovich effect after the two Russian theorists who first predicted it, and it’s now been observed in dozens of clusters. It’s also a powerful but indirect way to measure the mass of a cluster.

Transcript: It was a surprise to astronomers twenty or so years ago when clusters of galaxies began to be detected in significant numbers in x-ray emission, a surprise because astronomers did not expect to find gas in clusters of galaxies. This is because the galaxies that are dominant in clusters, elliptical galaxies, tend to have very little gas, and what gas they would have will be swept out by the rapid motion of the galaxies through the cluster gravitational potential. But it turns out that there’s a mechanism called a cooling flow by which clusters can accumulate a large amount of gas. Due to the high degree of pressure and density, the temperature of the gas elevates to several million degrees Kelvin at which point it emits in x-rays. X-rays therefore have been used to detect clusters and their hot gas out to substantial redshifts, and this is in fact one of the most effective ways of finding high redshift clusters of galaxies.

Transcript: Purely by chance nature has created several hundred little optics experiments with gravity. This occurs whenever a single galaxy or cluster of galaxies lies directly along the sight line to a more distant object like a quasar, an active galaxy, or another galaxy. Gravitational optics is directly analogous to optics with light. The radiation can be bent, focused, magnified, or demagnified. Gravitational lensing always creates an odd number of images, but the central image is often demagnified and superimposed directly on the lensing galaxy. So astronomers typically see double images or quadruple images although a single eleven image lens has been seen. Magnification by lensing can help us see very distant objects in the universe, active galaxies or galaxies that would not be visible in any other way, and in addition to measuring the dark matter content of a galaxy, lensing can be used to measure the distance scale directly by the light travel time delay between the path of light taking two different routes around a galaxy.

Transcript: The most massive bound objects in the universe, clusters of galaxies, can also deflect light. Lensing by clusters produces the interesting phenomenon of multiple images of background galaxies along with distorted images of the galaxies where they form little arcs of light. Cluster lensing has now been observed in dozens of cases. It’s best seen with the Hubble Space Telescope. With its extraordinarily sharp images, the tiny little arcs are easily visible. Typically a relatively nearby cluster of galaxies at a redshift of a few tenths, 0.2 to maybe 0.5, and consisting mostly of massive red elliptical galaxies will cause distortion and multiple images of background distant galaxies that are often blue and are at a redshift of one or two. Many pairs of images or many arcs are seen in lensing clusters, and these little images can be used to reconstruct the mass distribution of the cluster. This analysis confirms that clusters of galaxies are overwhelmingly composed of dark matter.

Transcript: In Einstein’s theory of relativity mass bends light, and this leads to the phenomenon of gravitational lensing. A single massive galaxy can deflect light by a very small angle, about one arcsecond. However, this can be resolved from the ground and especially with the Hubble Space Telescope, and so many situations of gravitational lensing have been discovered. Elliptical galaxies are massive and concentrated enough in their centers to cause multiple image formation of background active galaxies or quasars. Astronomers know nearly one hundred situations where a single quasar image has been turned into multiple images by an intervening galaxy. Typically two or four images are seen; however, there is an odd numbered image, the fifth or the third, which is demagnified and superimposed on the lensing galaxy itself. The situation of gravitational lensing is important in astronomy because the gravity of all matter, visible and dark matter, causes the bending of the light and so astronomers can model the entire mass of a galaxy in this way.