"How do disk population stars differ from spheroidal population stars? Briefly explain how the collapse of a protogalactic cloud is thought to lead to these two distinct populations of stars. How does this explain why the halo of our galaxy looks so much like an elliptical galaxy?"
Disk population stars are both young (hot and blue) and old (red and cool) whereas spheroidal population stars are just reddish and old. A protogalactic cloud will first collapse into spherical form and this is when the spheroidal population stars will be born. If the cloud has a fast enough rotation to form a disk, then disk population stars will also be created. The halo of the Milky Way looks like an elliptical galaxy because it began its formation like an elliptical galaxy would, the only difference being that it later went on to also form a disk.
"Describe the two scenarios in which the properties of a protogalactic cloud determine whether a galaxy becomes elliptical or spiral. What evidence supports these scenarios?"
The amount of angular momentum a protogalactic cloud has determines whether the galaxy will be elliptical or spiral. Per question 3, a protogalactic cloud that spins rapidly as it collapses is likely to become a spiral galaxy, whereas elliptical galaxies come from protogalactic clouds with little or no angular momentum.
The density a protogalactic cloud has is also important in determining which type of galaxy will form. Elliptical galaxies are commonly found in clusters and so they probably came from denser protogalactic clouds. These clouds would be able to cool more quickly, and so gravity would cause the gas to collapse into stars more quickly, and the gas would run out before a disk could be formed.
There is evidence for the latter scenario in the large and distant elliptical galaxies that appear very red even when redshifting is accounted for. There are no blue or white stars even those these galaxies would have formed with the universe was relatively young. Therefore the stars in these galaxies must be about the same age and were created before a disk could develop.
"Briefly describe the discovery of quasars. What evidence convinced astronomers that the high redshifts of quasars really do imply great distances? Why can we learn more about quasars by studying nearby active galactic nuclei?"
Quasars were discovered when Maarten Schmidt was trying to identify cosmic sources based on coordinates of radio sources reported by radio astronomers. One radio source looked like a blue star, but its emission lines didn't correspond to any known elements. Schmidt figured out that the emission lines were hydrogen but greatly redshifted. When he used Hubble's Law to determine the object's distance, he found it was surprisingly luminous. The name "quasar" originally came from "quasi-stellar radio sources" and is still used even those most quasars aren't really powerful radio emitters.
Astronomers were willing to accept that the unusually high redshifts had to do with distance and not some other factor when improved imaging showed that quasars are at the centers of extremely distant galaxies. Active galactic nuclei are similar to quasars, only less powerful, so we can use them to learn more. Like quasars, active galactic nuclei have huge luminosities in a small space and their spectra range from infrared to gamma rays.
"What is a radio galaxy? Describe jets and radio lobes. Why do we think that the ultimate energy sources of radio galaxies lie in quasarlike galactic nuclei?"
A radio galaxy is a galaxy that emits unusually large quantities of radio waves. Radio lobes are found on each side of such a galaxy and are the source of the radio wave emission. The jets transport energy from the galaxy's nuclei to the lobes. The nuclei are thought to be quasarlike because they have the same high luminosity in a small space and because quasars also seem to have a core-jet-lobe radio structure.
"Briefly explain the general picture of how supermassive black holes enable quasars to produce their huge luminosities. Summarize the evidence supporting the idea that supermassive black holes lie at the center of radio galaxies, quasars, and other active galactic nuclei."
When matter falls into a supermassive black hole, gravity converts the matter's potential energy to kinetic energy. When the matter collides it converts to thermal energy, which is carried away by photons. Ten to forty percent of this energy may show up as radiation before it reaches the black hole's event horizon. This accretion is much more efficient at producing light than nuclear fusion, so huge luminosities result.
Evidence for these supermassive black holes includes the M87 galaxy with its bright nucleus and radio/visible light jet. Enormous redshifts and blueshifts in the nuclei mean the gas is orbiting something 2-3 billion times more massive than the Sun, but this object isn't seen. This could only be a black hole. There is also the NGC 4258 galaxy, which also has a jet, and it has a ring of molecular clouds that orbit something less than a light year across. Their Doppler shifts tell us they're orbiting a 36 solar mass invisible object in this small space, which also is presumably a black hole.
"Briefly explain how intergalactic clouds between a distant quasar and Earth leave distinctive marks in the quasar's spectrum."
When photons leave a quasar they pass through intergalactic hydrogen clouds. The cloud's atoms absorb these photons and create an absorption line.
The lines believed to be produced by the most distant clouds are extremely wide, so these youngest galaxies are mostly made of gas. Nearer clouds have thinner absorption lines because more of their gas has been transformed into stars. Nearer clouds also have the absorption lines of heavier elements because they've experienced supernovae.
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