A direct black hole mass measurement in a Little Red Dot at the Epoch of Reionization

Ignas Juodžbalis, Cosimo Marconcini, Francesco D'Eugenio, Roberto Maiolino, Alessandro Marconi, Hannah Übler, Jan Scholtz, Xihan Ji, Santiago Arribas, Jake S. Bennett, Volker Bromm, Andrew J. Bunker, Stefano Carniani, Stéphane Charlot, Giovanni Cresci, Pratika Dayal, Eiichi Egami, Andrew Fabian, Kohei Inayoshi, Yuki Isobe, Lucy Ivey, Gareth C. Jones, Sophie Koudmani, Nicolas Laporte, Boyuan Liu, Jianwei Lyu, Giovanni Mazzolari, Stephanie Monty, Eleonora Parlanti, Pablo G. Pérez-González, Michele Perna, Brant Robertson, Raffaella Schneider, Debora Sijacki, Sandro Tacchella, Alessandro Trinca, Rosa Valiante, Marta Volonteri, Joris Witstok, Saiyang Zhang

First listed 2025-08-29 | Last updated 2025-09-01

Abstract

Recent discoveries of faint active galactic nuclei (AGN) at the redshift frontier have revealed a plethora of broad \Halpha emitters with optically red continua, named Little Red Dots (LRDs), which comprise 15-30\% of the high redshift broad line AGN population. Due to their peculiar spectral properties and X-ray weakness, modeling LRDs with standard AGN templates has proven challenging. In particular, the validity of single-epoch virial mass estimates in determining the black hole (BH) masses of LRDs has been called into question, with some models claiming that masses might be overestimated by up to 2 orders of magnitude, and other models claiming that LRDs may be entirely stellar in nature. We report the direct, dynamical BH mass measurement in a strongly lensed LRD at $z = 7.04$. The combination of lensing with deep spectroscopic data reveals a rotation curve that is inconsistent with a nuclear star cluster, yet can be well explained by Keplerian rotation around a point mass of 50 million Solar masses, consistent with virial BH mass estimates from the Balmer lines. The Keplerian rotation leaves little room for any stellar component in a host galaxy, as we conservatively infer $M_{\rm BH}/M_{*}>2$. Such a ''naked'' black hole, together with its near-pristine environment, indicates that this LRD is a massive black hole seed caught in its earliest accretion phase.

Short digest

Strong lensing plus JWST/NIRSpec IFS resolves narrow Hα kinematics and spectroastrometric shifts in the lensed Little Red Dot Abell2744‑QSO1 at z=7.04, enabling a direct rotation curve from ~10–200 pc. The curve is Keplerian and rules out compact extended mass profiles (e.g., MW‑like nuclear star cluster, Plummer, or a DM cusp), implying a central point mass of order 5×10^7 M⊙ consistent with Balmer‑line virial estimates. The inferred stellar content is minimal, with MBH/M* > 2, pointing to a near‑“naked” black hole in a pristine environment. This result validates virial BH masses for LRDs at the EoR and captures a massive BH seed in its earliest growth phase.

Key figures to inspect

• Fig. 1 (narrow Hα maps): Inspect the flux, velocity, and dispersion maps to see the ~200 pc spatial extent, the ~10 km s−1 velocity gradient, and the offset between the broad‑line centroid and the dynamical center—evidence the gradient is real (not slicer PA artefacts).
• Fig. 2 (top, rotation curve): Compare binned narrow‑Hα node velocities with the spectroastrometric points; note the strong preference for a point‑mass fit (χR^2≈0.8) over an MW‑like nuclear star cluster (χR^2≈3.8), yielding log MBH/M⊙≈6.94 as a lower limit due to unknown inclination.
• Fig. 2 (bottom, residuals): The full 2D MOKA3D fits show far smaller residuals for Keplerian rotation than for extended‑mass models—key visual proof that an NSC/Plummer profile cannot reproduce the field.
• Extended Data Fig. 1 (spectroastrometry): The +/−50 km s−1 Hα centroids are separated by 24.9±9.4 pc, implying rspec≈12.5 pc; with FWHM≈52±14 km s−1 this encloses log M/M⊙≈6.90—anchoring the inner mass scale.
• Extended Data Fig. 2 (cluster concentration limits): Upper limit Rc≲0.2 pc places any putative NSC >1 dex more concentrated than known NSCs or early star clusters, reinforcing that the mass must be a BH.