JWST CEERS & JADES Active Galaxies at z = 4-7 Violate the Local $M_\bullet-M_\star$ Relation at $>3σ$: Implications for Low-Mass Black Holes and Seeding Models

TL;DR

This study uses JWST galaxies at z=4–7 with Hα-determined black hole masses to infer a high-z M•-M★ relation, finding a significant deviation (>3σ) from the local relation with BHs overmassive by ~10–100×. Employing an MCMC framework that incorporates JWST Hα sensitivity, the z=5 SMF, and a flux-limit bias, the authors derive log M• = -2.43^{+0.83}_{-0.83} + 1.06^{+0.09}_{-0.09} log M★, with ≈0.69 dex intrinsic scatter, implying a slope near unity. They predict many more low-mass BHs detectable by JWST than locally expected, especially at z≈4–6, and argue these findings inform seed models and early BH-galaxy co-evolution, while highlighting potential systematic uncertainties. The work suggests JWST surveys could reveal a substantial population of light BHs in relatively modest hosts, enabling robust tests of BH seeding and growth scenarios in the early universe.

Abstract

JWST is revolutionizing our understanding of the high-$z$ Universe by expanding the black hole horizon, looking farther and to smaller masses, and revealing the stellar light of their hosts. By examining JWST galaxies at $z=4-7$ that host H$α$-detected black holes, we investigate (i) the high-$z$ $M_\bullet-M_\star$ relation and (ii) the black hole mass distribution, especially in its low-mass range ($M_\bullet \lesssim 10^{6.5} M_\odot$). With a detailed statistical analysis, our findings conclusively reveal a high-$z$ $M_\bullet-M_\star$ relation that deviates at $>3σ$ confidence level from the local relation. The high-$z$ relation is: $\log(M_\bullet/M_\odot) = -2.43^{+0.83}_{-0.83} + 1.06^{+0.09}_{-0.09} \log(M_\star/M_\odot)$. Black holes are overmassive by $\sim 10-100\times$ compared to their low-$z$ counterparts in galactic hosts of the same stellar mass. This fact is not due to a selection effect in surveys. Moreover, our analysis predicts the possibility of detecting in high-$z$ JWST surveys $5-15\times$ more black holes with $M_\bullet \lesssim 10^{6.5} M_\odot$, and $10-30\times$ more with $M_\bullet \lesssim 10^{8.5} M_\odot$, compared to local relation's predictions. The lighter black holes preferentially occupy galaxies with a stellar mass of $\sim 10^{7.5}-10^8 M_\odot$. We have yet to detect these sources because (i) they may be inactive (duty cycles $1\%-10\%$), (ii) the host overshines the AGN, or (iii) the AGN is obscured and not immediately recognizable by line diagnostics. A search of low-mass black holes in existing JWST surveys will further test the $M_\bullet-M_\star$ relation. Current JWST fields represent a treasure trove of black hole systems at $z = 4-7$; their detection will provide crucial insights into their early evolution and co-evolution with their galactic hosts.

Figures (5)

• Figure 1: Overview of the data set used and the local $M_\bullet-M_\star$ relation from Reines_Volonteri_2015. The dark-shaded region denotes the $1\sigma$ uncertainty from the linear fit, and the light-shaded region denotes the 0.55 dex root-mean-square deviation of the local observations from the local relation. The black hole masses are estimated from the $\rm H\alpha$ virial relation from Reines_2013 with the parameters chosen by Reines_Volonteri_2015. At four different redshifts ($z=4, 5, 6, 7$), the horizontal lines show the limiting sensitivities for a $3\sigma$ detection of the $\rm H\alpha$ line with NIRSpec (see Sec. \ref{['subsec:JWST_sensitivity']} for a detailed description). Error bars indicate the $1\sigma$ uncertainty in the stellar and black hole masses. As a comparison, the dotted, brown line displays the $M_\bullet-M_\star$ relation by Greene_2020_review.
• Figure 2: Left panel: in red, the high-$z$$M_\bullet-M_\star$ relation derived from JWST data at $z=4-7$. The dark-shaded 1$\sigma$ uncertainty region is derived from sampling $n=1000$$M_\bullet-M_\star$ relations randomly chosen within the 1$\sigma$ three-dimensional contour of the fit parameters. The red dotted lines and light-shaded region represent the best-fit intrinsic scatter of 0.69 dex (defined as $\sqrt{\nu} \sec \theta$, see Sec. \ref{['subsec:likelihood']}). Right panel: the posterior distribution of the parameters defining the $M_\bullet-M_\star$. The solid blue line and black dashed lines of the one-dimensional distributions denote the median and 1$\sigma$ uncertainties of the fit parameters. The solid contour lines in the two-dimensional distributions denote the 1$\sigma$, 2$\sigma$, and 3$\sigma$ joint uncertainties.
• Figure 3: Left panel: expected surface density of sources with a mass $M_\bullet \lesssim 10^{6.5} \,{\rm M_\odot}$, i.e., lighter than the smallest SMBH in our dataset: JADES 62309 Maiolino_2023_new, assuming the local $M_\bullet-M_\star$ relation. The shaded regions account for uncertainties in the black hole mass measurement using the $\rm H\alpha$ line (Eq. \ref{['mass_Ha']}) and the SMF (Eq. \ref{['smf']}). The expected surface density is provided for different redshifts $z \sim 4, 5, 6$, with a redshift binning of $\Delta z = 1$. Right panel: same as the left panel, but using our inferred high-$z$ relation. The peaks of the distributions are shifted to hosts with lower stellar masses, and the surface density of observable light-mass black holes is higher.
• Figure 4: Left panel: expected surface density of sources with a mass $<M_\bullet$, for $z=4,5,6$, assuming the local $M_\bullet-M_\star$ relation. The two vertical lines indicate the lightest (JADES 62309, Maiolino_2023_new) and heaviest (GS 3073, Ubler_2023) black holes included in the data set. The horizontal line indicates the surface density of sources identified by Harikane_2023 in CEERS ($8$ sources in $\approx 72 \, \rm arcmin^{-2}$). The 1$\sigma$ error bars account for uncertainties in the black hole mass measurement using the $\rm H\alpha$ line (Eq. \ref{['mass_Ha']}) and the SMF (Eq. \ref{['smf']}). Right panel: same as the left panel, but using the inferred high-$z$ relation. Again, the surface density of observable black holes is higher for all redshifts.
• Figure 5: Qualitative representation of the current horizon within which black holes are detectable with JWST/NIRSpec. The galaxy + black hole systems used in this work are shown in blue. The farthest galaxy hosting a SMBH detected, GN-z11, is shown in red. The contours indicate typical heavy and light seeding distributions at $z > 10$. Some growth lines are also shown from the center of the seed distributions and assume several Eddington ratios (as indicated). The general distribution of lower-redshift quasars is displayed as a reference SDSS_2020Fan_2022_review.