Significant Evidence of an Active Galactic Nucleus Contribution in GHZ2 at z=12.34
Journal article, 2026
GHZ2 is among the highest-redshift galaxies discovered to date, exhibiting a spectrum rich with prominent emission lines in the rest-frame ultraviolet (UV) and optical. These features raise critical questions about the mechanism powering this nebular emission, in particular the extremely strong C iv lambda 1548+1551 emission (rest-frame equivalent width of 45 & Aring;). Here we aim to quantify the active galactic nucleus (AGN) contribution within this system using the BEAGLE-AGN tool to simultaneously fit the spectrum and photometry of GHZ2. We consider a range of models with and without AGN components, allowing us to disentangle the stellar and AGN contribution of GHZ2 for the first time simultaneously using all observed emission lines. We conclude that a partial contribution by an AGN is significantly favored based on chi 2 comparisons between models with and without an AGN component, measuring an AGN contribution of 97 -2+1 %, and 64 -12+23 % for the C IV lambda 1548+1551 and C III] lambda 1908 emission lines, respectively. We estimate the black hole mass using the accretion luminosity (Lacc) from the best-fit BEAGLE-AGN model, inferring a value of log10(MBH/M circle dot) = 7.19 -0.03+0.03 , assuming an Eddington ratio of eta = 0.5 (with a much larger systematic uncertainty of similar to 1 dex). The inferred black hole mass to stellar mass ratio is 0.06 -0.02+0.02 (statistically), consistent with other high-redshift AGN systems. If the black hole interpretation is confirmed, GHZ2 would represent the most distant black hole identified to date, making it an ideal laboratory to study AGN growth and their role in shaping high-redshift galactic evolution.
Author
Oscar A. Chavez Ortiz
University of Texas at Austin
University of Texas
Steven L. Finkelstein
University of Texas
University of Texas at Austin
Adele Plat
Swiss Federal Institute of Technology in Lausanne (EPFL)
Maddie Silcock
University of Hertfordshire
Emma Curtis Lake
University of Hertfordshire
Anthony Taylor
University of Texas
Ansh R. Gupta
University of Texas
University of Texas at Austin
Lorenzo Napolitano
Sapienza University of Rome
Istituto nazionale di astrofisica (INAF)
Marco Castellano
Istituto nazionale di astrofisica (INAF)
Volker Bromm
University of Texas
University of Texas at Austin
Ikki Mitsuhashi
University of Colorado
Stephane Charlot
Sorbonne University
Adriano Fontana
Istituto nazionale di astrofisica (INAF)
Jorge A. Zavala
University of Massachusetts
Jacopo Chevallard
University of Oxford
Denis Burgarella
Aix Marseille University
Michaela Hirschmann
Swiss Federal Institute of Technology in Lausanne (EPFL)
Tom Bakx
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Alba Vidal-Garcia
Spanish National Observatory (OAN)
Antonello Calabro
Istituto nazionale di astrofisica (INAF)
Anna Feltre
Istituto nazionale di astrofisica (INAF)
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 1008 2 227Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
Subatomic Physics
DOI
10.3847/1538-4357/ae96a4