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

First listed 2025-07-30 | Last updated 2025-12-08

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.

Short digest

Using nine SHELLQs quasars at 6.18<z<6.4 with NIRCam host detections and NIRSpec Balmer-line M_BH, the authors infer the intrinsic M_BH–M_* relation via forward modeling that accounts for selection and measurement systematics, with the slope fixed to 1.61 and acknowledging parameter degeneracies. The sample (log M_BH≈8–9; log M_*≈9.5–11) is consistent with a non-evolving relation but with markedly larger intrinsic scatter, σ≈0.80 dex. The preferred model implies an active UV-unobscured AGN fraction of ~2.3% at z≈6 and predicts many lower-mass AGN soon to be found. An overmassive normalization is disfavored, as it would require unrealistically small scatter (~0.22 dex) and a very low AGN fraction (~0.6%).

Key figures to inspect

• Figure 1: Inspect the SHELLQs selection completeness versus M1450 and redshift to see how the Cycle 1 targets populate 6.18<z<6.4 and how selection enters the forward model of biases.
• Figure 2: Read the M_BH–M_* plane for the nine objects—note the best-fit line and 1σ band (slope fixed to 1.61), and compare directly to G20/KH13 and the high-z JWST assessments; the inset shows each quasar’s offset within 1–3σ.
• Figure 3: Check the bi-variate model versus data in the M_BH–M_* plane and the observed M_BH–Eddington ratio distribution; confirm that the forward-model contours reproduce both spaces simultaneously.
• Figure 4: Examine the forced overmassive scenario (normalization above KH13, slope 1.17) and note it only matches the data with σ≈0.22 dex and AGN fraction ≈0.6%, illustrating why such a normalization is disfavored.