Significant Evidence of an Active Galactic Nucleus Contribution in GHZ2 at z = 12.34

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Abstract
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 λ 1548+1551 emission (rest-frame equivalent width of 45 Å). 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 χ 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 λ 1548+1551 and C III ] λ 1908 emission lines, respectively. We estimate the black hole mass using the accretion luminosity ( L acc ) from the best-fit BEAGLE -AGN model, inferring a value of log 10 ( M BH / M ⊙ ) = 7.19 − 0.03 + 0.03 , assuming an Eddington ratio of η = 0.5 (with a much larger systematic uncertainty of ∼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.
Funding Information
  • National Aeronautics and Space Administration (22-ASTRO22-0224)
  • National Aeronautics and Space Administration (0NSSC22K0489)

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