Decrease in the space density of quasars at high redshift

QUASARs have been found up to redshifts of almost five, corresponding to an epoch when the Universe was less than ten per cent of its present age. This leaves a rather short time for the formation of the galaxies in which quasars are believed to be embedded. Indeed, some optical studies(1-3) indicate that the space density of quasars does decline rapidly for redshifts z > 3, as expected if this is the epoch of galaxy formation. The interpretation of this decline is equivocal, however, as it could result simply from the obscuratron of distant quasars by dust in intervening galaxies(4). Radio emission from quasars, on the other hand, is unaffected by dust, and we show here that the space density of radio-loud quasars also decreases strongly fort > 3, demonstrating that the decline is real, at least for these objects. We argue that this conclusion probably applies to all quasars. If quasars are associated with galaxy formation and/or interactions between galaxies, the decline in their space density at high redshift provides a measure of the timescale for the onset of these processes.