Host galaxies of bright high redshift quasars: luminosities and colours

M. Schramm; L.Wisotzki; K. Jahnke

doi:doi:10.1051/0004-6361:20077319

Free access

Issue		A&A Volume 478, Number 2, February I 2008


Page(s)		311 - 319
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20077319

A&A 478, 311-319 (2008)
DOI: 10.1051/0004-6361:20077319

Host galaxies of bright high redshift quasars: luminosities and colours

M. Schramm¹, L.Wisotzki¹, and K. Jahnke²

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 ( $z\sim 3$ ) quasars using the ESO-VLT. Our targets were selected to have luminosities among the highest known (absolute magnitude $M_B\la -28$ ). 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 $K_{\rm s}$ , and with somewhat lower significance also in H; one object is resolved only in $K_{\rm s}$ . At these redshifts, H and $K_{\rm s}$ 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 $M_V \sim -25$ . 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 $M_\star$ in the range of several $10^{11}\:M_\odot$ , placing them within the high-mass range of recent high-redshift galaxy surveys. We present optical spectra and use $\ion{C}{iv}$ line width measurements to predict virial black hole masses, obtaining typical values of $M_{\rm BH}\sim 5\times10^{9}\,M_{\odot}$ . With respect to the known correlation between host galaxy luminosity $L_{V,{\rm host}}$ and $M_{\rm BH}$ , our measurements reach higher luminosities and redshifts than previous studies, but are consistent with them. Comparing our objects with the local ( $z\simeq 0$ ) $M_{\rm BH}$ - $M_{\rm bulge}$ relation and taking also the low stellar mass-to-light ratios into account, we find tentative evidence for an excess in the $M_{\rm BH}/M_{\rm bulge}$ mass ratio at $z\sim 3$ .

Key words: galaxies: active -- galaxies: high redshift -- quasars: general

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