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SHELLQs-JWST perspective on the intrinsic mass relation between supermassive black holes and their host galaxies at z > 6

John Silverman, Junyao Li, Xuheng Ding, Masafusa Onoue, Michael Strauss, Yoshiki Matsuoka, Takuma Izumi, Knud Jahnke, Tommaso Treu, Marta Volonteri, Camryn Phillips, Irham Andika, Kentaro Aoki, Junya Arita, Shunsuke Baba, Sarah Bosman, Anna-Christina Eilers, Xiaohui Fan, Seiji Fujimoto, Melanie Habouzit, Zoltan Haiman, Masatoshi Imanishi, Kohei Inayoshi, Kazushi Iwasawa, Nobunari Kashikawa, Toshihiro Kawaguchi, Chien-Hsiu Lee, Alessandro Lupi, Tohru Nagao, Jan-Torge Schindler, Malte Schramm, Kazuhiro Shimasaku, Yoshiki Toba, Benny Trakhtenbrot, Hideki Umehata, Marianne Vestergaard, Fabian Walter, Feige Wang, Jinyi Yang

TL;DR

This study leverages JWST observations of nine $z>6$ quasars from the SHELLQs sample to test how supermassive black holes (SMBHs) co-evolve with their host galaxies in the early universe. By applying a forward-modeling framework that accounts for selection biases and measurement uncertainties, the authors infer the intrinsic $M_{BH}$–$M_*$ relation and its scatter, finding consistency with the local relation from Greene2020 and a relatively large dispersion of about 0.8 dex. They show that the apparent offset of high-$z$ quasars above the local relation can be explained by biases rather than a truly elevated high-$z$ relation, and they infer a low active fraction of UV-unobscured AGN ($p_{active} oughly 2$–$3 imes10^{-2}$). The work anticipates a substantial, as-yet-undiscovered population of lower-mass BHs at $z>6$ and highlights the need for larger JWST samples to constrain the slope and dispersion of the mass relation, informing black hole seeding and growth scenarios.

Abstract

The relation between the masses of supermassive black holes (SMBHs) and their host galaxies encodes information on their mode of growth, especially at the earliest epochs. The James Webb Space Telescope (JWST) has opened such investigations by detecting the host galaxies of AGN and more luminous quasars within the first billion years of the universe (z > 6). Here, we evaluate the relation between the mass of SMBHs and the total stellar mass of their host galaxies using a sample of nine quasars at 6.18 < z < 6.4 from the Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs) survey with NIRCam and NIRSpec observations. We find that the observed location of these quasars in the SMBH-galaxy mass plane (log M_BH/Msun ~ 8-9; log M_*/Msun ~ 9.5-11) is consistent with a non-evolving intrinsic mass relation with dispersion (0.80_{-0.28}^{+0.23} dex) higher than the local value (~0.3-0.4 dex) of their more massive descendants. Our analysis is based on a forward model of systematics and includes a consideration of the impact of selection effects and measurement uncertainties with an assumption on the slope of the mass relation. While degeneracies between parameters persist, the best-fit solution has a reasonable AGN fraction (2.3%) of galaxies at z ~ 6 with an actively growing UV-unobscured black hole. In particular, models with a substantially higher normalisation in M_BH would require an unrealistically low intrinsic dispersion (~0.22 dex). Consequently, our results predict a large population of AGNs at lower black hole masses, as are now just starting to be discovered in focused efforts with JWST.

SHELLQs-JWST perspective on the intrinsic mass relation between supermassive black holes and their host galaxies at z > 6

TL;DR

This study leverages JWST observations of nine quasars from the SHELLQs sample to test how supermassive black holes (SMBHs) co-evolve with their host galaxies in the early universe. By applying a forward-modeling framework that accounts for selection biases and measurement uncertainties, the authors infer the intrinsic – relation and its scatter, finding consistency with the local relation from Greene2020 and a relatively large dispersion of about 0.8 dex. They show that the apparent offset of high- quasars above the local relation can be explained by biases rather than a truly elevated high- relation, and they infer a low active fraction of UV-unobscured AGN (–). The work anticipates a substantial, as-yet-undiscovered population of lower-mass BHs at and highlights the need for larger JWST samples to constrain the slope and dispersion of the mass relation, informing black hole seeding and growth scenarios.

Abstract

The relation between the masses of supermassive black holes (SMBHs) and their host galaxies encodes information on their mode of growth, especially at the earliest epochs. The James Webb Space Telescope (JWST) has opened such investigations by detecting the host galaxies of AGN and more luminous quasars within the first billion years of the universe (z > 6). Here, we evaluate the relation between the mass of SMBHs and the total stellar mass of their host galaxies using a sample of nine quasars at 6.18 < z < 6.4 from the Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs) survey with NIRCam and NIRSpec observations. We find that the observed location of these quasars in the SMBH-galaxy mass plane (log M_BH/Msun ~ 8-9; log M_*/Msun ~ 9.5-11) is consistent with a non-evolving intrinsic mass relation with dispersion (0.80_{-0.28}^{+0.23} dex) higher than the local value (~0.3-0.4 dex) of their more massive descendants. Our analysis is based on a forward model of systematics and includes a consideration of the impact of selection effects and measurement uncertainties with an assumption on the slope of the mass relation. While degeneracies between parameters persist, the best-fit solution has a reasonable AGN fraction (2.3%) of galaxies at z ~ 6 with an actively growing UV-unobscured black hole. In particular, models with a substantially higher normalisation in M_BH would require an unrealistically low intrinsic dispersion (~0.22 dex). Consequently, our results predict a large population of AGNs at lower black hole masses, as are now just starting to be discovered in focused efforts with JWST.

Paper Structure

This paper contains 8 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. Data and measurements
  3. Sample selection
  4. Stellar and black hole masses with JWST
  5. Method to account for selection biases and measurement uncertainties
  6. Results
  7. Comparison to simulations
  8. Summary

Figures (5)

  • Figure 1: Properties of the Cycle 1 sample as drawn from SHELLQs Matsuoka2018ApJ: redshift, absolute UV magnitude, bolometric luminosity, and selection completeness as a function of absolute magnitude and redshift.
  • Figure 2: $Top$ Black hole mass ($M_\mathrm{BH}$) versus stellar mass ($M_*$) of the host galaxy for the 9 SHELLQs quasars. The best-fit relation (black line) and uncertainty (1$\sigma$; shaded area) are indicated which incorporate selection biases and measurement uncertainties. Local relations of Greene2020 (G20) and Kormendy2013 (KH13) are also indicated, along with the high-$z$ assessment of Pacucci2023 (P23) using JWST AGN in CEERS Kocevski2024 and JADES Maiolino2024b. The inset plot displays the location of the observed quasar sample with respect to our best-fit relation with shaded areas marking the intrinsic scatter (1--3$\sigma$). $Bottom$ Best-fit inference on model parameters (normalization, scatter of the linear mass relation, and AGN fraction) with the slope fixed to 1.61. The normalization of the G20 relation is indicated by the vertical red line in the top panel, while the local intrinsic dispersion of massive local galaxies Gueltekin2009Kormendy2013Greene2020Bennert2021 is shown by the red vertical band in the top-middle panel.
  • Figure 3: Model comparison to the observed sample. (a) Bi-variate mass distribution ($1\sigma$ to $3\sigma$ contours). Red curve and blue triangles show the mean mass relation of the observed model distribution and the observed data for the nine SHELLQs quasars, respectively. The recovered intrinsic mass relation is shown by the dashed line. (b) Observed distribution in the $M_\mathrm{BH}$ vs. Eddington ratio plane with the model prediction ($1\sigma$ to $3\sigma$ contours). (c) Best-fit model luminosity function compared to that from Matsuoka2018ApJ.
  • Figure 4: Same as Figure \ref{['fig:mass_rel']} but with the intrinsic mass relation forced to be larger than the Kormendy2013 relation (i.e., overmassive case) and the slope fixed to 1.17. As shown, this case demands a mass relation with very little dispersion (0.22, even less than the local relation) and a very low AGN fraction (0.6%); therefore, this solution is not preferred.
  • Figure 5: Best-fit value and $1\sigma$ confidence interval of the intrinsic mass ratio (red data point), relative to the local Greene2020 relation, for the ensemble of 9 SHELLQs-JWST quasars. The observed mass ratios for the individual quasars are shown as blue stars. The results from five cosmological hydrodynamic simulations and semi-analytic models at $\log M_*\xspace/\mathrm{M}_\odot\xspace\sim10.5$ are indicated Habouzit2021Dattathri2024 between 16th and 84th percentiles.