Part 21: Active galaxies, black holes, quasars, and the early universe.
These short videos were created in August 2007 by Dr. Christopher D. Impey, Professor of Astronomy at the University of Arizona, for his students. They cover a broad range of terms, concepts, and princples in astronomy and astrobiology. Dr. Impey is a University Distinguished Professor and Deputy Head of the Astonomy Department. All videos are intended solely for educational purposes and are licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. The full list of collections follows below:
01. Fundamentals of Science and Astronomy
02. Ancient Astronomy and Celestial Phenomena
03. Concepts and History of Astronomy and Physics
04. Chemistry and Physics
05. Quantum Theory and Radiation
06. Optics and Quantum Theory
07. Geology and Physics
08. Solar Neighborhood and Space Exploration
09. Outer Planets and Planetary Atmospheres
10. The Solar System
11. Interplanetary Bodies
12. Formation and Nature of Planetary Systems
13. Particle Physics and the Sun
14. Stars 1
15. Stars 2
16. Stars 3
17. Galactic Mass Distribtuion and Galaxy Structure
19. Galaxies 2
20. Galaxy Interaction and Motion
21. Deep Space and High-Energy Phenomena
22. The Big Bang, Inflation, and General Cosmology
23. The Big Bang, Inflation, and General Cosmology 2
24. Chemistry and Context for Life
25. Early Earth and Life Processes
26. Life on Earth
27. Life in the Universe
28. Interstellar Travel, SETI, and the Rarity of Life
29. Prospects of Nonhuman Intelligences
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.
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: 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.
X-Ray Clustered Gas
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: 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.
High Redshift Clusters
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.