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1.3.5 Quasar Redshifts
The first few quasars discovered had redshifts that were comparable to
those of the most distant known-clusters of galaxies. As more and more
quasars were discovered with the refinement of techniques for
isolating them (see the next section and Chapter 10), the maximum
measured redshifts continued to increase dramatically. By the
mid-1970s, several quasars with z 3 had been found. The
distributions of known quasar redshifts and apparent magnitudes as of
1993 are shown in Figs. 1.7 and
1.8, respectively. Aside from interest
in how these sources produce such copious radiation over a broad
spectral range, it was also recognized that quasars provide a possibly
unique probe of the early Universe - the light that we are now
detecting from the most distant known quasars was emitted by them when
the Universe was only a small fraction of its current age, and has
been in transit since. Quasars are still the only discrete objects
that can be observed with relative ease at z
1, and thus they are a
potentially important cosmological probe. However, in the context of
cosmological studies, quasars must be used judiciously. For example,
early attempts at producing a Hubble diagram for quasars (as in
Fig. 1.9) were not very enlightening because the
luminosity function for
quasars is very broad, and evolves with time (there are more luminous
quasars at high redshift; Chapter 11). An important early finding was
that the number of quasars per unit volume reaches a maximum somewhere
around z
2, even after correction for the Ly
selection effect
mentioned in §1.3.4; at earlier
epochs (i.e., higher redshifts), they
are comparatively rare. Detection of very high-redshift quasars
remains of great interest because their existence provides an
important constraint on the formation of large structures in the early
Universe as well as on the formation of heavy elements, which are
clearly seen in the spectra of all quasars.
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Figure 1.7. The redshift distribution of 7236 quasars in the Hewitt and Burbidge (1993) catalog. |
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Figure 1.8. The distribution in apparent V magnitude of 7110 quasars in the Hewitt and Burbidge (1993) catalog. |
By the late 1960s, it was also apparent from high-resolution, high signal-to-noise ratio spectra that quasars often have absorption lines in addition to the strong emission features (Chapter 12). The absorption lines are generally much narrower than the emission lines, and are usually detected at redshift lower than the emission-line redshift of the quasar itself. These absorption features are thought for the most part to arise in material unassociated with the quasars which lies at lower cosmological redshifts. Thus, quasars also provide an important probe as luminous background sources against which otherwise possibly undetectable structures can be observed.
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Figure 1.9. The Hubble diagram (apparent magnitude vs. redshift) for 7031 quasars in the Hewitt and Burbidge (1993) catalog. The vertical width of the distribution is due to the broad quasar luminosity function, i.e., at any redshift, the wide range of apparent magnitudes is due primarily to the wide range of quasar absolute magnitudes. |