An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

8.6 Line spectra: Activity 8 Quasar redshifts

Activity 8: Quasar redshifts

Read Peterson section 1.3.5 (pages 16 and 17) [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .

Discussion

Keywords: cosmological probes, luminosity function, quasar absorption lines

SAQ 13

Question: How can astronomers detect cool intergalactic gas in distant parts of the Universe?

Answer

By detecting redshifted absorption lines in the spectra of even more distant quasars.

As Peterson stressed in the section you have just read, high-redshift quasars are important for a variety of reasons. Consequently astronomers are currently investing time, energy and many nights of telescope usage on searching for ever more distant quasars. At the time this course was written, the most distant known quasar was that shown in Figure 29a. The identification of these distant quasars is done photometrically by looking for Lyman-break objects, where there is no detected flux in the blue part of the observed optical spectrum because it has been absorbed by high-redshift hydrogen gas, as shown in Figure 29b.

Figure 29
Figure 29 (a) The faint red source indicated by the arrow is a quasar with redshift z = 6.2. (b) Spectra of three very distant quasars. In each case the highest observed flux is the Lyα emission line, which has been redshifted into the red part of the optical spectrum. To shorter wavelengths the detected flux is zero because all these photons have undergone Lyman absorption by less redshifted neutral hydrogen gas between us and the quasar
S381_1

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