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The Question

(Submitted February 16, 1998)

Do you truly believe that red-shifts can be utilized as a valid means of distance indication, and if so, on what grounds? I am encountering an increasing number of individuals who claim that large red-shifts do not occur in the spectra of quasars (though a few maintain that small red-shifts (z < 2x10^-3) could be present in the spectra)

The Answer

Regarding the redshift issue: in general, the galaxies with higher redshifts are fainter and subtend smaller angular sizes on the sky, so for galaxies, the redshift - distance relationship is reasonably well established. Regarding quasars, there is a rather substantial number of them that have redshifts determined from the absorption lines -- and these are similar to the redshifts measured from the emission lines normally present in quasars. These lines arise from atmospheres of stars in the host galaxy. Those often can be measured either when the quasar is faint (many quasars vary by quite a bit, in some cases by as much as a factor of 100!), or via a study of the spectrum with the central object masked off to reduce the "swamping" of the spectrum of the host galaxy by the intense light of the quasar. In some cases, we have absorption redshifts available up to z ~0.25. We also have a few cases of quasars residing behind intervening galaxies, and the gas in these intervening galaxies imprints an absorption redshift on the quasar spectrum. A well-known example is AO 0235+164; strictly speaking it is a BL Lac object, but this means that the emission lines are very weak, but still measurable at z = 0.94. The spectrum shows an absorption redshift at z=0.524, which is due to a clearly identifiable intervening galaxy. The quasar then must be behind the intervening galaxy!

It is important to note that the cosmological redshift of quasars is not believed by everyone, and in some cases, for good reasons. If you want to follow up on this debate, look for articles by Drs. H. Arp, or G. Burbidge.

Greg Madejski and Damian Audley
for Ask an Astrophysicist.

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