Host galaxies of bright high redshift quasars: Luminosities and colours

Abstract

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 MB ≲-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 Ks, and with somewhat lower significance also in H; one object is resolved only in Ks. 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 MV and B-V into rough estimates of stellar mass, we obtain values of M* in the range of several 1011 M⊙, 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 MBH ∼ 5 × 109 M⊙. With respect to the known correlation between host galaxy luminosity LVihost and MBH, 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 MBH/Mbulge mass ratio at z ∼ 3. © ESO 2008.