2008A&A...478..311S

We present the results of a near-infrared imaging study of high redshift (z∼3) quasars using the ESO-VLT. Our targets were selected to have luminosities among the highest known (absolute magnitude M_B≲-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~-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 10¹¹M_☉, 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∼5x10⁹M_☉ . With respect to the known correlation between host galaxy luminosity L_V,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≃0)M_BH–Mbulge relation and taking also the low stellar mass-to-light ratios into account, we find tentative evidence for an excess in the M_BH/Mbulge mass ratio at z∼3.