Host galaxies of bright high redshift quasars: luminosities and colours^*

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: mschramm@aip.de
² Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 16 February 2007
Accepted: 6 September 2007

Abstract

We present the results of a near-infrared imaging study of high redshift () quasars using the ESO-VLT. Our targets were selected to have luminosities among the highest known (absolute magnitude ). 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 and HE 2355-5457 at ) 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 . Their rest-frame 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 into rough estimates of stellar mass, we obtain values of in the range of several , placing them within the high-mass range of recent high-redshift galaxy surveys. We present optical spectra and use line width measurements to predict virial black hole masses, obtaining typical values of . With respect to the known correlation between host galaxy luminosity and M_BH, our measurements reach higher luminosities and redshifts than previous studies, but are consistent with them. Comparing our objects with the local () 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 mass ratio at .