The nuclear to host galaxy relation of high-redshift quasars

We present near-infrared imaging of the host galaxies of low-luminosity quasars at 1< z < 2, aimed at investigating the relationship between the nuclear and host-galaxy luminosities at high redshift. This work complements our previous study to trace the cosmological evolution of the host galaxies of high-luminosity quasars. The sample consists of nine radio-loud (RLQ) and six radio-quiet (RQQ) low-luminosity quasars. They have similar redshift and optical luminosity distributions, and together with the high- luminosity quasars, cover a large range of the quasar luminosity function. For all but two of the quasars, we have been able to derive the global properties of the surrounding nebulosity. The host galaxies of both types of quasars are massive inactive ellipticals between L* and 10L* , with RLQ hosts being significantly more luminous than RQQ hosts. This luminosity gap is independent of the rest-frame U-band luminosity but correlated with the rest-frame R-band luminosity. The color difference between the RQQs and the RLQs is likely a combination of an intrinsic difference in the strength of the thermal and nonthermal components in their SEDs, and a selection effect due to internal dust extinction. For the combined set of quasars, we find a reasonable correlation between the nuclear and the host luminosities. This correlation is less apparent for RQQs than for RLQs. If the R-band luminosity represents the bolometric luminosity, and the host luminosity is proportional to the black hole mass, as in nearby massive spheroids, quasars emit in a relatively narrow range with respect to their Eddington luminosity and with the same distribution for RLQs and RQQs.