K-band imaging of 52 B3-VLA quasars: Nucleus and host properties

We present K-band imaging and photometry of a sample of 52 radio-loud quasars (RQs) selected from the B3 survey with flux densities greater than 0.5 Jy at 408 MHz. The optical completeness of the sample is 90%, and the quasars cover the redshift range 0.4-2.3. For similar to 57% of the sources for which the quality of the images allowed a detailed morphological study (16/28), resolved extended emission was detected around the QSO, and its K flux was measured. Interpreting this "fuzz" as starlight emission from the host galaxy, its location on the K-z plane at z < 1 is consistent with radio quasars being hosted by galaxies similar to radio galaxies (RGs) or giant ellipticals (gE's). At higher redshifts the detected host galaxies of RQs are more luminous than are typical RGs and gE's, although some weak detections or upper limits are consistent with a similar fraction of RQs being hosted by galaxies with the expected luminosities for RGs or gE's. The study of the B-K color distribution of the QSO nuclei, after removing the contribution of K emission from the host galaxy, confirm that these sources are not reddened by large amounts of dust, with an estimated extinction A(V) < 1.0 mag at z similar or equal to 1. We find a significant correlation between radio power and nuclear infrared luminosity, indicating a direct link between the radio synchrotron emission and the nuclear emission in K. This correlation is more tight for the steep-spectrum sources (99.97% significance). In addition, a trend is found between radio power and infrared luminosity of the host galaxy (or mass), in the sense that the most powerful quasars inhabit the most luminous galaxies. The similarity of this tendency with that found for powerful FR II radio galaxies is consistent with the unification model for radio sources.