An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

3.2 QSO spectra

It only became apparent that these quasi-stellar objects were not stars when their spectra were examined. At first astronomers could not interpret their spectra because the spectral lines did not appear at appropriate wavelengths for atoms of any known chemical element. The spectrum shown in Figure 4 provided the breakthrough: Maarten Schmidt identified the three indicated lines as common emission lines of hydrogen, but redshifted by a factor

where z is redshift;

∆λ is wavelength shift;

λ0 is rest wavelength.

Figure 4
Figure 4 An historic QSO optical spectrum, which was taken using a photographic plate. The darker areas are regions where the intensity of light is higher. The top rectangle is the spectrum of the radio galaxy 3C 273, while the lower rectangle is the spectrum of a comparison emission line source. In both spectra three common lines of hydrogen have been indicated. The lines in the spectrum of 3C 273 appear at a redshift z = 0.158

If this redshift is interpreted as cosmological (i.e. due to the overall expansion of the Universe described by Hubble's law) then the distances to these QSOs are huge, and their luminosities prodigious. For example, Schmidt's redshift measurement would imply that the point optical source 3C 273 is 100 times as luminous as the entire Milky Way Galaxy! Naturally there was some reluctance to adopt this interpretation, and a few sceptics vigorously persisted into the 1980s in advocating the view that the QSOs were much closer than Hubble's law would imply.

SAQ 1

Question: How would these sceptics have explained the measured redshifts of QSOs?

Answer

They asserted that the QSOs had large motions through space, rather than receding as part of the overall cosmological expansion of space. One suggestion was that QSOs were ejected from nearby galaxies at relativistic velocities.

Just as in democratic politics, the scientists who argue a minority viewpoint perform an invaluable function by ensuring that the accepted view indeed stands up to all possible rational criticism. Any scientist who has never been proved wrong has probably had a very dull career.

S381_1

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