Dec 9 Our three main topics (first two are really the same topic) were I. The [non-stellar-evolution] massive black hole at the center of the Milky Way. II. The massive black holes at the center of other nearby galaxies III. Quasars, a model for the luminosity of quasars, evolution of quasars Go to the December 4 figures site. There, you'll find the pictures/movies we showed. You'll also find good writeups about the context/results of the science at the Galactic Center (Genzel's site), and about black holes in nearby galaxies (Kormendy's site and "another great website"). Given that you'll read and study these websites as part of your studying, we don't have to write a lot here. The context: In the early 1960s, "radio stars", releasing lots of radio radiation were found. Just as in our discussion of gamma ray bursts (this is an anachronism, that is, my discussion is an anachornism, since the quasar stuff happened long before the gamma ray stuff), the positional accuracy of the radio sources was not well-enough determined to allow astronomers to find the "star" making the radio light. If you want some Christmas reading, try LONELY HEARTS OF THE COSMOS by Dennis Overbye (you should be able to find it used, it's copyright 1991). That book will explain the whole story to you, including why some very famous astronomers have hated each other for the past 40 years. Eventually, we got good enough coordinates, and an optical spectrum of the radio star was obtained. ALL OF THE SPECTRAL LINES WERE IN THE WRONG PLACES. How can that be? We taught you in this class that the spectral lines of atoms are like fingerprints. Eventually, Dr Maarten Schmidt realized that the spectral lines WERE in the right places. These objects aren't "stars" but are objects "at the other end of the Universe." So they're highly redshifted. In Astro 203 we told you not to think of the redshift as being a major effect for stars within the Milky Way. You can predict the maximum redshift of a star in the Milky Way by calculating the escape velocity (by knowing the mass of the Milky Way). It's a few-hundred-kilomaters-per-second effect, or about 10 Angstroms (where yellow light has a wavelength of 5000 Angstroms). So for stars, even though we're grateful that Nature has provided us with the Doppler Effect, since it allows us to measure velocities, this effect is tiny. But since the Universe is expanding, for distant objects, this effect can be substantial. When Maarten Schmidt make the "aha" breakthrough, the first quasar whose lines were identified, 3C273, had a redshift of 0.16c, sixteen percent of the speed of light. the star in the previous paragraph has a redshift of 0.002c, two tenths of a percent of the speed of light. You can see why "all the lines are in the wrong places." Astronomers had never found objects so far away! The second quasar whose redshift was measured had a redshift of 0.37c, thirty-seven percent the speed of light. So, if these objects are the "most distant objects in the Universe," and they're reasonably bright here on Earth, they must be monstrously bright where the light is being formed. A quick and dirty calculation shows that they must be brighter than entire galaxies. The next chore is to find a model for the source of all that light. MAterial falling onto black holes seemed to be the best bet. Well, these black holes need to be monstrously massive. The figures websites will tell you the observations that make this a true story. So, if there are massive black holes at the centers of quasars, what does that tell us? Well: 1) better pictures of quasars revealed "faint fuzz", ie, regular galaxy, surrounding the quasars. So quasars are the unbelievably bright centers of special galaxies. Why are they special? 2) There was an "epoch of lots of quasars" around [to invent a VERY CRUDE number] 10 billion light years away (10 billion years old). 3) there were fewer quasars before that time, and fewer after that time, and almost none today. So something makes a quasar, then shuts it off. Does the black hole model deal with this fact in a natural way? Well: How does a black hole shine? It doesn't! the accretion disk does. So how can we make an accretion disk here? Remember, we're dealing with an object smaller than our solar system inside an object, a galaxy, spread out over tens of thousands of light years. Well, some small percentage of the gas and stars will have orbits that takes them close enough to the black hole to be swallowed up. [THE REST DOESN'T.] So we have a source of food, thus light, for the monster. When this gas is done, no more food, thus no more light. So far, so good. Well: In the 1980s, the IRAS satellite (Infrared Astronomical Satellite) was launched. It's the precursor to the SIRTF satellite that was launched on about the first day of this semester, and whose first scientific results will be announced the morning of your final exam. [Jane is heavily involved, and her thesis advisor BUILT one of the instruments on SIRTF.] IRAS saw a set of "infrared luminous" objects. What this means is that there were places in the sky where there was lots of infrarted but "no" optical. Olszewski was in the right place as these results were announced, namely, at a telescope. So he observed these places in the sky with bigger telescopes and better detectors, and discovered that there were ugly looking galaxies in these places. Shortly thereafter, others discovered that the galaxies are ugly because they're often in the process of colliding with other galaxies. So apparently, collding galaxies can reawaken the monster. How? Well, when galaxies collide, gravity changes the orbits of the gas. So once again, some of the gas has orbits that takes it near the monster. Even though the million or billion solar mass black hole idea sounds preposterous, at least the story is consistent. The figures site (and we in class), soend time testing whether indded there are black holes at the centers of galaxies. The answer is YES. For the Milky Way, we see stars orbiting the black hole. From the laws of gravity, we can measure the mass (and density) of the black hole. In the Milky Way, the mass of this central object is something like 2-3 million solar masses. The for other nearby galaxies, you can make two observations. [You can't make the "see stars move" observation from the previous paragraph.] First observation is, do these galaxies have a relatively bright central object. YES. See Kormendy's website. Second observation is, even though we can't see motions, the Doppler Effect allows us to measure one component of this motion, the component towards us or away from us. Near the center of these galaxies, the speed of the stars and gas changes abruptly and in the sense expected for a massive black hole. Once again, using the Laws of Gravity, we can measure the mass of this central object. The answer turns out to be, for some galaxies, upwards of a billion Solar masses. So, the story and the observations hang together. Somehow, in the formation of ordinary galaxies, massive black holes are formed at their centers. When the monsters are "fed enough food", a quasar develops. If the monster is fed less food, an active galaxy with a different name is formed. When the monster is fed almost no food, something like the monster at the center of the Milky Way is seen. Even though these black holes have nothing to do with stellar evolution, they're an important part of astronomy, and they're subject to the same rules as are stellar mass black holes. They are the solution to the mystery of quasars.