Host galaxies of bright high redshift quasars: luminosities and colours

We present the results of a near-infrared imaging study of high redshift (z similar to 3) quasars using the ESO-VLT. Our targets were selected to have luminosities among the highest known (absolute magnitude M-B less than or similar to -28). We searched for resolved structures underlying the bright point-source nuclei by comparing the QSO images with stars located in the same fields. Two QSOs (HE 2348-1444 at z = 2.904 and HE 2355-5457 at z = 2.933) are clearly resolved in K-s, and with somewhat lower significance also in H; one object is resolved only in K-s. At these redshifts, H and K-s correspond almost exactly to rest-frame B and V, respectively, with virtually no K-correction. We also report the non-detection of some additional QSOs. The detected host galaxies are extremely luminous with M-V similar to -25. Their rest-frame B - V colours, however, are close to zero in the Vega system, indicating substantial contributions from young stars and a stellar mass-to-light ratio below 1 (in solar units). Tentatively converting M-V and B - V into rough estimates of stellar mass, we obtain values of M-* in the range of several 10(11) M-circle dot, placing them within the high-mass range of recent high-redshift galaxy surveys. We present optical spectra and use CIV line width measurements to predict virial black hole masses, obtaining typical values of M-BH similar to 5 x 10(9) M-circle dot. With respect to the known correlation between host galaxy luminosity L-V,L-host and M-BH, our measurements reach higher luminosities and redshifts than previous studies, but are consistent with them. Comparing our objects with the local (z similar or equal to 0) M-BH-M-bulge relation and taking also the low stellar mass-to-light ratios into account, we find tentative evidence for an excess in the M-BH/M-bulge mass ratio at z similar to 3.