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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 a High-Energy Astronomer.
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