The host galaxies and black-hole:galaxy mass ratios of luminous quasars at z~4

TL;DR

This study uses deep Ks-band imaging of luminous quasars at z~4 to analyze their host galaxies and black hole mass ratios, revealing that these early universe quasars have unusually high black-hole-to-galaxy mass ratios compared to low-redshift counterparts.

Contribution

It provides the deepest high-quality Ks-band images of z~4 quasars and estimates their host galaxy properties and black hole mass ratios, highlighting evolutionary differences.

Findings

Host galaxies are very massive, similar to radio galaxies at the same epoch.

Quasar hosts are smaller than low-redshift quasar hosts by a factor of ~5.

Black-hole to galaxy mass ratio is 0.01-0.05, higher than in the local universe.

Abstract

We present and analyse the deepest, high-quality Ks-band images ever obtained of luminous quasars at z~4, in an attempt to determine the basic properties of their host galaxies less than 1 Gyr after the first recorded appearance of black holes with Mbh > 10^9 Msol. To maximise the robustness of our results we have carefully selected two SDSS quasars at z~4. These quasars are representative of the most luminous quasars known at this epoch but they also, crucially, lie within 40 arcsec of comparably-bright foreground stars (required for accurate PSF definition), and have redshifts which ensure line-free Ks-band imaging. The data were obtained in excellent seeing (<0.4-arcsec) at the ESO VLT with integration times of ~5.5 hours per source. Via carefully-controlled separation of host-galaxy and nuclear light, we estimate the luminosities and stellar masses of the host galaxies, and set constraints on their half-light radii. The quasar host galaxies have K-band luminosities similar to radio galaxies at comparable redshifts, suggesting that these quasar hosts are also among the most massive galaxies in existence at this epoch. However, the quasar hosts are a factor ~5 smaller than the host galaxies of luminous low-redshift quasars. We estimate the stellar masses of the z~4 host galaxies to lie in the range 2-10x10^11 Msol, and use the CIV emission line in the Sloan spectra to estimate the masses of their black holes. The results imply a black-hole:host-galaxy mass ratio Mbh:Mgal~0.01-0.05. This is an order of magnitude higher than typically seen in the low-redshift Universe, and is consistent with existing evidence for a systematic growth in this mass ratio with increasing redshift, at least for objects selected as powerful AGN.