EIGER V. Characterizing the Host Galaxies of Luminous Quasars at $z\gtrsim6$

TL;DR

This study uses JWST/NIRCam imaging and grism spectroscopy to measure quasar host-galaxy stellar masses and SMBH masses for six luminous quasars at $5.9<z<7.1$. By PSF-based image decomposition, three hosts are detected with host-to-quasar flux ratios of $\sim1\%-5\%$, yielding $M_*\gtrsim10^{10}\,M_\odot$, while H$\beta$-based SMBH masses from grism spectra imply a high $M_\mathrm{BH}/M_*$ ratio of $\sim0.15$. The results indicate that the $M_\mathrm{BH}$–$M_*$ relation is already in place at $z\gtrsim6$ and suggest possible redshift evolution beyond selection effects. This provides direct evidence for SMBH–host co-evolution in the reionization era and constrains models of early black hole growth.

Abstract

We report {\em JWST}/NIRCam measurements of quasar host galaxy emissions and supermassive black hole (SMBH) masses for six quasars at $5.9<z<7.1$ in the \textit{Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization} (EIGER) project. We obtain deep NIRCam imaging in the F115W, F200W, and F356W bands, as well as F356W grism spectroscopy of the quasars. We use bright unsaturated stars to construct models of the point spread function (PSF) and estimate the errors of these PSFs. We then measure or constrain the fluxes and morphology of the quasar host galaxies by fitting the quasar images as a point source plus an exponential disk. We successfully detect the host galaxy of three quasars, which have host-to-quasar flux ratios of $\sim1\%-5\%$. Spectral Energy Distribution (SED) fitting suggests that these quasar host galaxies have stellar masses of $M_*\gtrsim10^{10}M_\odot$. For quasars with host galaxy non-detections, we estimate the upper limits of their stellar masses. We use the grism spectra to measure the {\hb} line profile and the continuum luminosity, then estimate the SMBH masses for the quasars. Our results indicate that the positive relation between SMBH masses and host galaxy stellar masses already exists at redshift $z\gtrsim6$. The quasars in our sample show a high black hole to stellar mass ratio of $M_\text{BH}/M_*\sim0.15$, which is about $\sim2$ dex higher than local relations. We find that selection effects only contribute partially to the high $M_\text{BH}/M_*$ ratios of high-redshift quasars. This result hints at a possible redshift evolution of the $M_\text{BH}-M_*$ relation.