[Paper Review] EIGER V. Characterizing the Host Galaxies of Luminous Quasars at $z\gtrsim6$

Minghao Yue, Anna–Christina Eilers+7 more|arXiv (Cornell University)|Sep 8, 2023

Galaxies: Formation, Evolution, Phenomena87 references8 citations

TL;DR

The study uses JWST/NIRCam imaging and grism spectroscopy to detect and characterize host galaxies and SMBH masses for six luminous quasars at z~6–7, finding overmassive black holes relative to their hosts and evidence for M_BH–M_* trends already at z≳6.

ABSTRACT

We report {\em JWST}/NIRCam measurements of quasar host galaxy emissions and supermassive black hole (SMBH) masses for six quasars at $5.9

Motivation & Objective

• Assess the rest-frame optical emission of quasar host galaxies at z≳6 with JWST/NIRCam imaging and spectroscopy.
• Measure SMBH masses via Hβ line profiles from grism spectra.
• Estimate host-galaxy stellar masses through SED fitting and compare to SMBH masses.
• Investigate whether local M_BH–M_* relations hold in the reionization era.
• Evaluate the reliability of host detections against PSF modeling uncertainties.

Proposed method

• Obtain deep NIRCam imaging in F115W, F200W, F356W and F356W grism spectroscopy for six quasars (5.9<z<7.1).
• Construct empirical PSF models from bright unsaturated stars and propagate PSF uncertainties via PSF error maps.
• Fit quasar images with a point source (AGN) plus an exponential disk (host) using psfMC with MCMC to derive fluxes and morphologies.
• Use grism-derived Hβ line profiles and continuum to estimate SMBH masses.
• Perform SED fitting with Prospector (nebular emission included) to derive host stellar masses, assuming delayed-τ SFH, Chabrier IMF, and Calzetti dust law.

Experimental results

Research questions

• RQ1 What are the fluxes and morphologies of quasar host galaxies at z≳6 in rest-frame optical bands?
• RQ2 Can the SMBH masses inferred from Hβ in JWST grism spectra be robustly estimated for these high-z quasars?
• RQ3 What are the stellar masses of the host galaxies, and how do they compare to their SMBH masses (M_BH–M_*) at z≳6?
• RQ4 Is there evidence that the local M_BH–M_* relation already exists or is biased toward overmassive black holes in luminous quasars at z≳6?

Key findings

• Three of the six quasars have detected host galaxies with flux ratios F_host/F_QSO ≈ 1%–5%.
• Detected hosts imply stellar masses M_* ≳ 10^10 solar masses (from SED fitting).
• SMBH masses inferred from Hβ and continuum place M_BH/M_* ≈ 0.15, about 1–2 dex higher than local relations.
• Quasars with detections show rest-frame optical host morphologies consistent with exponential disks.
• Upper limits on stellar masses for non-detections still constrain the M_BH–M_* relation at z≳6.