The Dichotomy in the Nuclear and Host Galaxy Properties of High-redshift Quasars

TL;DR

This study uses JWST data to reveal a dichotomy in high-redshift quasar host galaxies, showing distinct morphological and spectral properties linked to quasar luminosity and black hole growth, informing early galaxy evolution.

Contribution

It introduces a new multi-band forward-modeling method to analyze quasar hosts at high redshift, uncovering a fundamental dichotomy in their properties and black hole growth patterns.

Findings

Luminous quasars are in bulge-dominated hosts with narrow UV slopes.

Fainter quasars reside in disk-like hosts with broad UV slopes.

High-luminosity quasars have higher black hole-to-stellar mass ratios.

Abstract

The early growth of high-redshift quasars and their host galaxies raises critical questions about their cosmic evolution. We exploit the angular resolution and sensitivity of NIRCam to investigate the host galaxies of 31 quasars at $4\lesssim z\lesssim7$ drawn from multiple JWST surveys. Using a new multi-band forward-modeling code (\textsc{GalfitS}) that incorporates physically motivated priors, we securely detect and quantify the host emission in 30 objects, while simultaneously characterizing the nuclear spectral energy distribution. The host galaxies of high-redshift quasars are $\sim 0.3$~dex more compact than star-forming galaxies of comparable mass. A striking dichotomy emerges: luminous ``blue'' quasars ($L_{5100}\gtrsim10^{45}\,{\rm erg\,s^{-1}}$) reside in bulge-dominated galaxies ($n \approx 5$) and exhibit a narrow range of ultraviolet nuclear slopes (median $\beta_{\rm UV} \approx -1.4$), while fainter ``red'' quasars inhabit disk-like hosts ($n\approx 1$) and display a broad range of slopes ($\beta_{\rm UV}\approx-2$ to 4). These two populations differ markedly in their black hole-to-stellar mass ratios, with high-luminosity quasars showing $M_{\mathrm{BH}}/M_\ast = 1.2\%$ compared to $4.7\%$ for lower luminosity sources, placing them collectively $\sim$0.6~dex above the local $M_{\mathrm{BH}}-M_\ast$ relation. This offset likely reflects rapid black hole growth in early gas-rich environments, where feedback from the active galactic nucleus becomes effective only after substantial gas depletion. Our findings suggest that the observed dichotomy, whether due to intrinsic spectral differences or dust extinction, fundamentally shapes the coevolution of supermassive black holes and their host galaxies in the early Universe.