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
公開日: 2025/8/29
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.