Fig 19.20.

Spectrum of the quasar Q0I03-294, at z = 3.11. [ESO]

wide. They are also mostly from the ground states of atoms, indicating a low temperature. All of this suggests that the absorption lines arise in material between us and the quasar.

At the time of this writing, the largest observed quasar redshift is 6.28. The light from the most distant quasars has been traveling over 90% of the age of the universe to reach us. This means that we are seeing the universe as it was before our own galaxy formed. Therefore, quasars provide us with an important link to our past.

19.4.3 Energy-redshift problem

The immediate problem that astronomers recognized with quasars was explaining their enormous energy output. What makes the problem even more difficult is the fact that the energy must be generated in a small volume. One way out of the problem is to say that quasars are not as far away as we think they are. After all, we only observe their apparent brightness. We infer their absolute brightness by knowing their distances, determined from the redshifts and Hubble's law. (It should be noted that even the factor of two uncertainty in the Hubble constant has no real bearing on this particular problem.) If we are saying that the distances are wrong, we are saying that the quasars do not obey Hubble's law. If that is the case, we must explain the large redshifts. This is the basic problem. Either we have to explain the large energy output, or we have to come up with another redshift mechanism. For this reason, we can think of this as the energy-redshift problem.

One possible source of redshift is gravitational. We have already seen that photons are redshifted as they leave the surface of any object. One problem with this explanation arises from the limited range of redshifts seen in the emission lines. This tells us that the emitting gas would have to be in a thin shell around some massive object. An analysis of such systems shows that quasars would have to be so close or so massive that our local part of the galaxy would be greatly affected by their presence. Even with a mass of 1011 M0, the objects would still be within our galaxy.

There is an additional problem. For the larger redshifts, we need black holes. Even with a black hole to get a redshift greater than 100%, the photons must be emitted from very close to the Schwarzschild radius. We cannot think of any way to come up with a hot radiating gas close to RS, especially given the narrow range of redshifts in a given quasar. The accretion disks responsible for X-ray emission around objects like Cyg X-1 are generally outside the photon sphere (defined in Section 8.4.2), which is at 1.5 RS.

Another possible source of redshift is called kinematic. This means that we are observing a high velocity due to something other than the expansion of the universe. For example, they might be shot out of galaxies. However, if this were the case, we might expect to see some blueshifts comparable to the redshifts seen in quasars. To get them all moving away from us, we might think that they have been ejected from our galaxy, but the kinetic energy becomes quite large.

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