Significant Evidence of an AGN Contribution in GHZ2 at z = 12.34

TL;DR

GHZ2, a $z=12.34$ galaxy with unusually strong high-ionization UV lines, is analyzed with BEAGLE-AGN to quantify the AGN contribution to its nebular emission. The method performs full spectro-photometric fitting, separating AGN and stellar components and using Bayesian evidence to compare models with and without an AGN. The results indicate a significant AGN component, with a contribution of $54^{+1}_{-1}\%$ to the C IV $\lambda$1548 line and $26^{+4}_{-2}\%$ to the C III] $\lambda$1908 line, and yield a black hole mass of $\log_{10}(M_{\rm BH}/M_{\odot}) = 7.20^{+0.04}_{-0.04}$ for an Eddington ratio $\eta = 0.5$ (systematic uncertainty ~$1$ dex), implying a BH-to-stellar mass ratio of $0.05^{+0.02}_{-0.02}$. If confirmed, GHZ2 would be the most distant active black hole known, providing a unique laboratory for studying early AGN growth and their impact on high-redshift galaxy evolution.

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 emission (rest-frame EW $=$ 45 Angstrom). Here we aim to quantify the 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. We conclude that a partial contribution by an AGN is significantly favored based on the Bayes factor comparison to models without an AGN component, measuring an AGN contribution of 54$^{+1}_{-1}$% and 26$^{+4}_{-2}$% for the C IV$λ$1548 and C III]$λ$1908 emission lines, respectively. We obtain an estimate for the black hole mass using the accretion luminosity ($L_{acc}$) from the best fit BEAGLE-AGN model, computing a value of log$_{10}$(M$_{BH}$/M$_{\odot}$) = 7.20$^{+0.04}_{-0.04}$, for an Eddington ratio of $η$ = 0.5 (with a systematic uncertainty of $\sim$1 dex). The inferred black hole mass to stellar mass ratio is 0.05$^{+0.02}_{-0.02}$, 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.