The observational history of lensed quasars starts with a few lucky cases found "by accident" during surveys or follow-up observations of projects unrelated to gravitational lensing. The very first case was the double quasar Q 0957+561. When observed at optical wavelengths, the z = 1.405 quasar appeared as two point sources separated by 5.7" (Walsh et al. 1979). Spectra obtained with the Multi-Mirror-Telescope showed that both objects had almost identical spectral properties (Weymann et al. 1979), strongly supporting the hypothesis of gravitational lensing: two images of one single object were seen, due to the potential well created by a galaxy along the line of sight. In fact, not only the spectra were identical, but subtraction of the quasar images also revealed, for the first time, the lensing galaxy, hidden by the much brighter quasar images. With such observational material, no serious doubts could remain about the lensed nature of Q 0957+561.
Other cases were found soon after, such as the quadruply imaged quasar PG 1115+080 (Weymann et al. 1980) that we will use in this article to illustrate how lensed quasars can help use to determine H0. Even the famous "Einstein Cross", Q 2237+0305 (Huchra et al. 1985; see Fig. 1), was discovered during follow-up observations in the course of the CfA redshift survey: a spectrum obtained of the central parts of a z = 0.04 redshift galaxy, turned out to display the exact characteristics of a quasar at much higher redshift, z = 1.7. In addition, the total apparent luminosity of the bulge of the galaxy was far too high for a normal spiral. Indeed, it was in fact the combined light of the actual galaxy's bulge and of the four (unresolved) quasar images. High resolution images taken a few years later nicely confirmed that the object was composed of four separate quasar images almost aligned with the bulge of the spiral galaxy (Schneider et al. 1988; see Fig. 2).
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Figure 1. Left: ground based image of the lensing galaxy (z = 0.04) in the Einstein Cross. The spatial resolution is low. It does not allow to discriminate between the bulge of the galaxy and any background quasar image(s). Right: spectrum of the most central part of the galaxy. The spectrum is not the one of a low redshift galaxy, but that of a much more distant object: a quasar at z = 1.7 (Huchra et al. 1987). |
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Figure 2. Left: ground based image of the "Einstein cross" unveiling, for the first time, four separated quasar images. The field of view 10" wide (Schneider et al. 1988). Right: HST view of the Einstein cross, allowing for accurate astrometry and photometry of the system as well as detailed surface photometry of the bulge of the lensing galaxy. The length of the white bar at the bottom right is 1" (Image taken from Kochanek et al. 2003a). |
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