Surveys for High Redshift QSOs

We present recent results from surveys for radio-loud QSOs. These surveys cover most of the sky to a limiting radio flux density of S = 0.2 - 0.25 Jy. In addition we have used the GB6 and VLA FIRST catalogue to survey a smaller area (1600sq deg) to a fainter limit of S = 25mJy. Digitised plate material was used to identify the majority of the radio sources and CCD images to identify the optically fainter sources. The Northern surveys have produced more than 25 z > 3 QSOs, one of which, at z = 4.72, is the most distant radio source known. In the South redshift information has been obtained for a complete sample of 442 flat-spectrum Parkes QSOs covering the range z ~0 to z ~4.5. These QSO samples show a clear drop-off in space density at z > 3. The form of this decline is remarkably similar to that seen in optically-selected samples. Since radio emission is unaffected by dust this implies that dust has a minimal effect on the observed drop-off seen in optical samples. Our radio-selected samples show evidence for a decline in space density at high redshift which is more pronounced for less powerful radio sources. We have started a new survey that will be sensitive to bright QSOs with redshifts up to 6.