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3.2. Recent Large Optical Surveys

The above-mentioned questions on the evolution of quasars provided a significant part of the motivation for two surveys significantly larger than anything previously attempted, the 2dF survey (Boyle et al. 2000, and references therein) and the SDSS (York et al. 2000). The goal of the 2dF was to cover 750 deg2 of sky to B < 21 mag and find > 25, 000 quasars with z < 2.3 via the UVX technique. The SDSS objective was to survey 10,000 deg2 of sky in five colors, u'g'r'i'z', to find 100,000 quasars covering all redshifts up to 5.8.

Boyle et al. (2000) have estimated the luminosity function from the first 6684 quasars in the 2dF quasar survey. The survey was based on u, bJ, r UK Schmidt plates and is primarily a UVX technique. It was estimated to be 90% complete for z < 2. Their final sample included 5057 objects from 196 deg2 of sky with MB < - 23 mag, 18.25 < bJ < 20.85, 0.35 < z < 2.3. They combined their sample with 867 objects from the LBQS (Hewett, Foltz, & Chaffee 1995) and fitted a two-power law form to the luminosity function data. The data are shown in Figure 1 (right). They found that a polynomial evolution of L*B(z) fits the data well (Fig. 2, left). Thus, their main conclusion is that for 2dF+LBQS, -26 < MB < -23 mag(q0 = 0.5) and 0.35 < z < 2.3, pure luminosity evolution works fine, with the characteristic luminosity of quasars increasing by a factor of 40 at the peak of activity. However, the EQS (Edinburgh Quasar Survey, Miller et al., unpublished) and HEQS (Hamburg/ESO Quasar Survey, Köhler et al. 1997) do not fit as well, particularly for z < 0.5, where the LBQS has few or no data.

Figure 2a Figure 2b

Figure 2. Left: A linear plot of the characteristic luminosity of quasars from the 2dF (Boyle et al. 2000) and SSG samples versus look-back time, for the luminosity evolution model. Right: A i* - z* versus r* - i* plot of SDSS data, showing the stellar locus (black area and points) and how quasars with 4.6 < z < 5.0 separate from the stellar locus because of the presence of Lyalpha emission in the i* band and of Lyalpha forest absorption in the r* band (Fan et al. 1999).

The SDSS opened an important window for the search for high-redshift quasars through its large areal coverage of the sky and its use of the z* filter with lambdaeff approx 9100Å, which extended the discovery space in redshift to z = 5 and beyond. Figure 2 (right) (Fan et al. 1999) illustrates how the r*, i*, z* filters were used to discover the first quasars with z approx 5 by their clear separation from the stellar locus.

Fan et al. (2001a, b) then used the technique to compile a well-defined, color-selected sample of 39 quasars with 3.6 < z < 5.0 and i* leq 20 mag in 182 deg2 of sky. They estimated the luminosity function for objects with 27.5 < M1450 < - 25.5 and its evolution with redshift. Their results, which are shown in Figure 3 (left) agree within the errors at z approx 4 with the previous results of WHO, SSG, and KDC and give a value of the decline in space density of a factor of 3 per unit redshift for z > 3.6. They find a flatter slope for the luminosity function at z approx 4 relative to earlier surveys for z < 3. These results confirm that pure luminosity evolution does not match the data between redshifts 2 and 5.

Then, Fan et al. (2001c) extended their work to discover four quasars with z > 5.8 by using observation in the infrared J band to help eliminate nearby and numerous L and T dwarf stars (Fig. 3, right). They showed that the space density at z = 6 is about a factor of 2 below that at z = 5 and follows the decline with redshift just described. These objects provide the strongest evidence yet for a detection of the Gunn-Peterson absorption and thus evidence for reionization at redshifts in the vicinity of z approx 6 4.

Figure 3a Figure 3b

Figure 3. Left: The space density of quasars with MB < - 26 mag as a function of redshift for the 2dF, SDSS, SSG, and WHO surveys (Fan et al. 2001b). Right: The z* - J versus i* - z* diagram for the SDSS, showing how the infrared J band enables the separation of z > 5.8 quasars from the numerous L and T dwarfs (Fan et al. 2001c).

To summarize, the 2dF and SDSS results now cover the range 0.3 < z < 6.3 for high-luminosity, optically selected quasars and reach close to the epoch of reionization. They provide by far the best data on the evolution of such quasars that exist to date.

4 Note added in proof. Fan et al. (2003) announced the discovery of three additional quasars with z > 6. Back.

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