New quasars with z = 3.4 and 3.7 and the surface density of very high redshift quasars

Abstract

It has been suggested that there is a sharp decline in the space density of quasars beyond redshifts z∼3.5¹. We describe here the discovery on a UKST (UK Schmidt telescope) low-dispersion IIIaF objective prism plate of a QSO (quasistellar object) with z = 3.7, which is the second highest redshift QSO known and, moreover, lies in the same region in which a QSO with z = 3.61 has already been reported². We also note a QSO with z = 3.4. Before these observations, only seven QSOs with z≥3.4 were known throughout the whole sky. When compared with the results of a search for QSOs with 2.7<z<3.3 on the same plate, these observations do not support suggestions of a z∼3.5 cut-off in the QSO distribution¹, but rather support the suggestion of a steady decline from z∼2. We show here that deep searches over small areas of sky are unlikely to be successful in discovering QSOs at z>4 and that such searches must be carried out over very large areas, even if only to a moderate limiting magnitude of m(R)∼19.