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16.22 Quasars as Probes of the Universe

Quasars continue to perplex astronomers over 50 years after their discovery. However, even though astronomers do not completely understand the power source, they can use quasars as cosmological probes of intervening material. If we accept that quasars are at the vast distances indicated by their red shifts, we can think of them as powerful flashlights for illuminating the universe. Each bright and distant quasar enables a different astrophysical experiment.

Gravitational lensing can affect the light of distant quasars. Among hundreds of thousands of known quasars, there are only a hundred or so that are multiply imaged. The lenses that cause the image splitting are elliptical galaxies in about 90% of the cases, and spiral galaxies in the other 10%. This result was anticipated, because only a very dense galaxy can bend light significantly, and elliptical are the densest galaxies. The study of gravitational lenses has revealed the properties of elliptical galaxies at red shifts of z = 0.5 to 1.5. Lensing shows that distant elliptically are embedded in halos made of dark matter, just as they are in the nearby universe.

Gravitational lenses are fascinating tools for studying the distant universe. Each system is like an optics experiment on a cosmic scale. We will mention one particularly interesting application here. The light output from most quasars varies. If the quasar is multiply imaged, then the light will take slightly different times to reach Earth from each different image. A variation will be seen first in the image whose light has been deflected the least — corresponding to the shortest path between the quasar and Earth. The variation will be seen last in the image whose light has been deflected the most. With a good model for the gravitational lens geometry, astronomers can use the time delay (Δt) to measure the difference in light paths between any two images (cΔt). This is a direct measurement of the distance scale of the universe, which is independent of the normal chain of reasoning that goes into the determination of the Hubble constant. The Hubble constant measured by gravitational lensing agrees well with the Hubble constant measured using Cepheids as distance indicators. This is a striking confirmation of the basic picture of an expanding universe.

Quasars can also be used as probes of the universe via the measurement of absorption lines. Quasar light travels for billions of years before reaching the Earth. If that light intercepts an intervening galaxy or cloud of gas along the way, the wavelength at which it will be absorbed depends on the red shift of this material. There are two types of absorption lines. One type is caused by heavy elements like carbon, magnesium, and silicon, presumed to originate in the disks and halos of normal galaxies. The other type consists of hydrogen lines and is believed to represent unprocessed gas clouds of primordial abundance. The positions of absorption lines in a quasar spectrum represent a map of the absorbers in red shift. Assuming cosmological red shifts, we can convert red shift into distance and produce a map reaching out billions of light years from Earth. By measuring the properties of absorbers using lines of several different elements, it is possible to trace the chemical evolution of galaxy halos back to z = 5 (since an average galaxy at that red shift is extremely faint, direct study would be impossible).

Another application of quasar absorbers is to study the clustering properties of the intervening material. Notice that a single line of sight can only yield the clustering in one dimension, but this is a start. With this data, astronomers determine if absorption lines are distributed randomly in red shift or if they are grouped with small separations in wavelength or red shift? By combining lines of sight, astronomers can reconstruct the size and shape of the absorbing material in three dimensions. To use a somewhat frivolous analogy, imagine a magician plunging swords into a closed box as a way of determining what (or who) is inside.

There is evidence that the heavy element absorbers associated with galaxy halos are clustered on enormous scales of 10 to 20 Mpc, providing unique data on the large-scale structure of the universe at high red shift. By contrast, the hydrogen absorbers are not clustered in space and do not appear to be associated with normal galaxies. Astronomers speculate that this gas might represent debris left over from galaxy formation. Using many quasar sight lines allows astronomers to estimate the total mass of hydrogen in the intergalactic medium. The result is impressive and important for cosmology: there is more diffuse and hot hydrogen in the space between galaxies that there is in all galaxies put together. The absorption line technique is extraordinarily sensitive. As little as a hundred solar masses of cold hydrogen can be detected at a distance of 10 billion light years! Large new telescopes are enabling astronomers to use absorption lines to learn much about the distant universe.