Galaxy EvolutionSeeing Galaxies of thePastBecause we see distantobjects as they were in the past, all we have to do to learn how galaxiesevolve is to look at more and more distant galaxies, which show us whatthey were like earlier and earlier in the history of the universe.Galaxies 5 billion light years away must be at most 14 billion - 5 billion= 9 billion years old. Galaxies 10 billion light years away mustbe at most 4 billion years old, and so on. What is remarkable aboutgalaxy formation is that we can see galaxies all the way to the limit ofour present telescopes, about 12 billion light years away. Thesegalaxies are only 1 or 2 billion years old, which means that galaxies formedvery early in the universe. We see little evidence that galaxiesare still forming -- apparently whatever caused their formation took placein only a short time. Observationally, we have several problems withseeing galaxy formation by looking farther into space (and farther backin time):Very distant galaxies are veryfaint, and we cannot see them very well even with the largest of today"stelescopes. New, super-sized telescopes are planned, to allow usto see even farther into the past.Very distant galaxies have verylarge recession velocities -- they are moving away at a good fraction ofthe speed of light. This causes extreme doppler redshifts, whichmeans that distant galaxies are brightest in the infrared part of the spectrum.To observe these, we need to put large infrared telescopes into space.Galaxies formed so quickly thatit is not easy to see differences in galaxies until we reach close to thelimit of our current capabilities.We would like to tell you thecomplete story of galaxy formation, but much still remains hidden to us.Instead, we can only outline the current ideas, and tell how it might havehappened. We will see that some of the brightest objects in the earlyuniverse are objects called quasars.Luckily, quasars are so bright that we can see them nearly to the edgeof the universe. The light from quasars pass through many galaxiesbetween us and the quasars, and this lets us study these intervening galaxiesby looking at the spectral lines they absorb from the quasar light.Quasars appear to be something that happened only in the early universe,and we see no nearby quasars today.Galaxy FormationWe can learn a lotabout galaxy formation by studying the parts of our own galaxy. Recallthe schematic view of a spiral galaxy like our own:Here we see that there are objects(stars and globular clusters) in a more-or-less spherically symmetric "halo,"which becomes thicker toward the center to make up the central bulge, butthere are other objects (young stars, dust and gas, etc.) that are concentratedin a thin disk. From this we envision that our galaxy formed froma more-or-less spherical cloud of hydrogen and helium that collapsed dueto gravity, just as our solar system formed from a far smaller cloud.In the early history of the collapsing cloud, the cloud would have remainedcool due to radiating its heat away, so particularly dense regions wouldhave formed the globular clusters and halo stars.It is important to realizethat once a star forms, it will not change its orbit to form a disk ofstars. It will forever orbit at whatever inclination angle it waswhen it formed. By studying the orbits of globular clusters and halostars, we can see that the gas of the original protogalaxy cloud was oncemore spherically distributed. However, the gas that remained didcontinue to collapse toward a thin disk, because the gas molecules couldinteract and lose their random motions. The law of conservation ofmomentum was at work to ensure that the remaining gas clouds orbited closeto the plane of the disk. Stars that formed later, when the gas wasmore and more nearly a disk, kept their inclinations just as the halo starsdid, so stars in the thick disk should be older than stars in the thindisk. In fact, stars in the halo and thick disk should be among theoldest stars in the galaxy, and because the original cloud was only hydrogenand helium (no heavier elements) the oldest stars should have no heavyelements (metals). In fact, we do see that the halo and thick diskstars are older and have fewer metals, while disk stars are younger andhave more metals.
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Why Do Galaxies Differ?Last time we learnedabout the different types of galaxies, especially spiral galaxies and ellipticalgalaxies, which are very different in their characteristics. Thecloud collapse idea we just covered can explain spiral galaxies, but howcan we explain the ellipticals, which never form disks at all?There are several differentexplanations to account for ellipticals, and perhaps some ellipticals formedin one way and others formed in another way. One idea is that thecloudsthat formed ellipticals were somehow different from the cloudsthat formed spirals. Since the clouds are all made of the same stuff(hydrogen and helium), something else would have to be different.There are two scenarios:Lack of Rotation: Perhpasthe initial cloud simply had very little angular momentum (very littlerotation). If this were the case, no disk would form and the starswould remain rather evenly distributed in the cloud. However, thiswould suggest that ellipticals would have dust and gas from generationsof stars, but the gas would be distributed throughout the elliptical.In fact, ellipticals seem not to have any dust at all.Rate of Cooling: If theprotocloud were cool enough, so many stars would form early on that therewould have been little dust left to form a disk. One way to makethe cloud extra cool would be to make it have a high starting density.Evidence for this scenario comes from very distant ellipticals, whichseem to be redder than their recessional redshift would give. Thesegalaxies must have no young blue or white stars, indicating that thereis no new star formation going on, even though they are only a few billionyears old.There is another possibilitythat could explain ellipticals that does not rely on the initial cloudbeing different. This scenario suggests that ellipticals form dueto galactic collisions and mergers.
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Shaped by Collisions:We know that collisions among galaxies are quite common. Recall thatgalaxies are very different from stars in this respect. Stars areso far apart relative to their size that they essentially nevercollide. Recall that if we scale things down so that the Sun werethe size of an orange, Alpha Centauri (the next nearest star) would beanother orange separated by 3000 miles. If we then think about shrinkingthe galaxy to the size of an orange, then the Andromeda galaxy would beanother orange only a few meters away! So relative to their sizes,galaxies are much much closer together. And recall that the universeis expanding. In the distant past, when the universe was smaller,the distance between galaxies was far smaller than it is today.What would a collision do toa pair of galaxies? All of the stars in the two galaxies would passcompletely by each other, without collisions, but the gas and dust wouldinteract and be left behind. The two stellar populations could eventuallymerge into a more-or-less spherical shape due to their mutual gravitationalinteractions, while the gas and dust might be left behind, or might sinkto the center of the galaxy and form a huge region of new stars.Such a huge region would eventually produce supernovas that could generateshocks and wind that could clear out the gas.Let"s look at afew galactic collisions in progress.One thing that is clear,is that the giant ellipticals at the centers of clustersof galaxies show lots of evidence of mergers and collisions.The interiors of some giant ellipticals show that stars are orbiting inopposite directions! They also show multiple nuclei, as if severallarge galaxies had been gobbled up. Such giant ellipticals are 10or more times more massive than individual spiral galaxies. You can watch a video that describes the future of our own galaxy, when we collide with the Andromeda Galaxy. See also M82and starburst galaxies.Quasars and Active GalaxiesSome galaxies arecalled "active galaxies" because they have some sort of engine in theirnucleus that is producing huge amounts of energy, sometimes 100s of timeshigher than a normal galaxy. The very brightest and most energeticare called quasars, which is a shortening of the term quasi-stellarobject. As we mentioned earlier, quasars and other activegalactic nuclei are a product of the early universe, and nearby (and hencepresent-day) galaxies do not seem to show such activity. It may bethat the engine is present in nearby galaxies (even our own), but is currentlynot active.For many years the causeof the phenomena seen in active galactic nuclei was completely unknown,but now we believe that all can be explained by a central, super-massiveblack hole in the nucleus of some galaxies. These giant black holes,of sometimes billions of solar masses, are active when new matter is beingfed to them in accretion disks. The accretion disks produce lotsof X-rays and ultraviolet light. The magnetic fields threading thedisks create jetsof particles that zoom far out into space to form radiogalaxies.Usingquasars to probe the universe.