A QSO with redshift 3.8 found on a UK Schmidt telescope IIIa-F prism plate

Abstract

High-redshift QSOs (quasi-stellar objects) are important because of the information they provide on the early history and evolution of the Universe; the brighter ones are particularly valuable as probes of the intervening material. During the past 20 years much effort has been devoted to trying to discover QSOs at redshifts z > 3.5, but progress has been so slow that it has been suggested that there is a cutoff in the QSO distribution at z ≃ 3.5 (ref. 1). However, we have already demonstrated how unfiltered UK Schmidt telescope (UKST) IIIa-F low-dispersion objective prism plates can be used successfully in such searches up to at least z ≃ 3.7 (ref. 3). We report here the use of this technique to discover a QSO, 1208 + 1011, with a redshift z = 3.80. The highest redshift previously known was that of the radio-selected QSO, PKS2000–330 with z = 3.78 (ref. 5). In two UKST fields we have now discovered six QSOs with redshifts between 3.3 and 3.8, of which four have z ≥ 3.50; including PKS2000–330, only four other QSOs with z ≥ 3.50 are known over the whole sky. Our success up to z = 3.8 indicates that redshifts > 4 could soon be attained.

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