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 criterion L_r < 10²⁵ hso W Hz^-1. We combine this information with other radio surveys of quasars so as to break the degeneracy between redshift and 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 tentatively, for low-luminosity quasars in the range -22.5 < M_B < -25, the radio-loud fraction decreases with increasing redshift. We can infer from this that the radio luminosity function evolves more slowly than the optical luminosity function. The mechanism that leads to strong radio emission in only a small fraction of quasars at any epoch is still unknown.