Radio-quiet quasars in the direction of the northern Hubble Deep Field

We match quasars discovered in a multicolor survey centered on the northern Hubble Deep Field (HDF) with radio sources from an ultradeep radio survey. Although three out of 12 quasars are detected at a level below 0.2 mJy at 1.4 GHz, all of the quasars in the search area are radio quiet by the crite rion L-r <10(25) h(50)(-1) W Hz(-1). We combine this information with other radio surveys of quasars so as to break the degeneracy between redshift an d luminosity. In the redshift range 0.02 < z <3.64, the radio-loud fraction increases with increasing optical luminosity, consistent with some degree of correlation between the nonthermal optical and radio emissions. More ten tatively, for low-luminosity quasars in the range -22.5 < M-B < -25, the ra dio-loud fraction decreases with increasing redshift. We can infer from thi s that the radio luminosity function evolves more slowly than the optical l uminosity function. The mechanism that leads to strong radio emission in on ly a small fraction of quasars at any epoch is still unknown.