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

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$\alpha$-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 $>3\sigma$ 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.