The Nuclear to Host Galaxy Relation of High‐Redshift Quasars

We present near-infrared imaging with ESO VLT+ISAAC of the host galaxies oflow luminosity quasars in the redshift range 1 < z < 2, aimed at investigatingthe relationship between the nuclear and host galaxy luminosities at highredshift. This work complements our previous study to trace the cosmologicalevolution of the host galaxies of high luminosity quasars (Falomo et al. 2004).The new sample includes 15 low luminosity quasars, nine radio-loud (RLQ) andsix radio-quiet (RQQ). They have similar distribution of redshift and opticalluminosity, and together with the high luminosity quasars they cover a largerange (~4 mag) of the quasar luminosity function. The host galaxies of bothtypes of quasars are in the range of massive inactive ellipticals between L*and 10 L*. RLQ hosts are systematically more luminous than RQQ hosts by afactor of ~2. This difference is similar to that found for the high luminosityquasars. This luminosity gap appears to be independent of the rest-frame U-bandluminosity but clearly correlated with the rest-frame R-band luminosity. Thecolor difference between the RQQs and the RLQs is likely a combination of anintrinsic difference in the strength of the thermal and nonthermal componentsin the SEDs of RLQs and RQQs, and a selection effect due to internal dustextinction. For the combined set of quasars, we find a reasonable correlationbetween the nuclear and the host luminosities. This correlation is lessapparent for RQQs than for RLQs. If the R-band luminosity is representative ofthe bolometric luminosity, and assuming that the host luminosity isproportional to the black hole mass, as observed in nearby massive spheroids,quasars emit with a relatively narrow range of power with respect to theirEddington luminosity and with the same distribution for RLQs and RQQs.