Direct pathway to the Early Supermassive Black Holes: A Red Super-Eddington Quasar in a Massive Starburst Host at $z=7.2$

TL;DR

This study investigates the formation of the first SMBHs by examining GNz7q, a dusty, rapidly accreting red quasar at $z=7.1899$ with JWST and NOEMA data. It measures a super-Eddington BH of $\log(M_{ m BH}/M_{\odot})=7.55\pm0.34$ and a massive host with $\log(M_*/M_{\odot})=10.5\pm0.4$, plus a star formation rate of $330\pm97\ M_{\odot}\,\rm yr^{-1}$, suggesting a rare transitional phase bridging dusty starbursts and luminous quasars. GNz7q lies on the local $M_{ m BH}$-$M_*$ relation and is predicted to evolve into a $z\sim 6$ quasar with $\log(M_{ m BH}/M_{\odot})\approx 9$ and $M_*\approx 10^{11}\,M_{\odot}$ due to rapid BH and host growth. This object contrasts with other JWST AGN populations that show subdued star formation, indicating GNz7q represents a direct growth pathway to the most massive black holes in the first billion years. Collectively, the findings support a rare, observable route for early BH–galaxy co-evolution toward the SMBH-dominated quasar population at high redshift.

Abstract

We present a panchromatic optical-mm characterization of GNz7q, a recently identified X-ray weak, rapidly growing red quasar embedded within a dusty starburst galaxy at $z=7.1899$, using the full suite of JWST/NIRCam, NIRSpec, MIRI, and archival NOEMA observations. Our deep NIRSpec/G395M spectroscopy reveals unambiguous broad Balmer emission (FWHM $=2221\pm20$kms$^{-1}$), confirming a super-Eddington accreting black hole ($λ_{\rm Edd}=2.7\pm0.4$) with a mass of $\log(M_{\rm BH}/M_{\odot})=7.55\pm0.34$, using accretion-rate corrected BH mass estimators. After subtracting the point source, we robustly detect stellar emission from the host galaxy across multiple NIRCam and MIRI filters. Out joint morphological-spectral analysis yields a stellar mass of $\log (M_*/M_\odot)=10.5\pm0.4$ and an intense star formation rate of ${\rm SFR}=330\pm97\,M_\odot\,\rm yr^{-1}$, confirming the host as a massive, dusty starburst galaxy. We find that GNz7q lies on the local $M_{\rm BH}$-$M_*$ relation ($M_{\rm BH}/M_*\simeq 0.001$) and is well positioned to evolve into the locus of massive SDSS quasars with $\log (M_{\rm BH}/M_\odot)\approx 9$ and $M_*\approx 10^{11}\,M_\odot$ at $z\sim 6$, owing to its remarkably rapid growth in both the black hole and its host galaxy. This stands in stark contrast to many recently reported JWST AGN populations at similar redshifts, including the little red dots (LRDs), whose weak or undetected star formation makes it difficult for them to grow into the massive galaxies hosting SDSS-like quasars. These results suggest that GNz7q marks as a rare, pivotal phase of early BH-galaxy co-eolution, plausibly providing a crucial direct pathway to the supermassive black hole systems within the first billion years of the Universe.