The Dual Nature of GHZ9: Coexisting Active Galactic Nuclei and Star Formation Activity in a Remote X-ray Source at z = 10.145

TL;DR

This study presents a joint JWST/NIRSpec and Chandra analysis of GHZ9, a $z=10.145$ X-ray–detected system that hosts both active galactic nucleus activity and star formation. The NIRSpec spectrum reveals strong UV high-ionization lines and extreme chemical peculiarities (N-enrichment, low C/O, low Z), supporting an AGN-dominated ionizing field that is finely probed through UV and optical diagnostics. Independent X-ray analysis confirms GHZ9 as the X-ray source, enabling a BH mass estimate of about $M_{BH}\sim(0.5-1.6)\times10^8\,M_\odot$ and a BH-to-stellar-mass ratio of at least $\sim2\%$, with a best-fit value around $\sim0.33$ under plausible SED contributions. The results imply accelerated SMBH growth relative to the stellar mass in the early universe and establish GHZ9 as a prime target for constraining BH seeding and coevolution with its host through future multi-wavelength observations, including high-resolution spectroscopy and ALMA dynamical mass measurements.

Abstract

We present James Webb Space Telescope (JWST)/NIRSpec PRISM spectroscopic characterization of GHZ9 at z= 10.145 $\pm$ 0.010, currently the most distant source detected by the Chandra X-ray Observatory. The spectrum reveals several UV high-ionization lines, including CII, SiIV, NIV], CIV, HeII, OIII], NIII], and CIII]. The prominent rest-frame equivalent widths (EW(CIV)$\simeq$65A, EW(OIII])$\simeq$28A, EW(CIII])$\simeq$48A) show the presence of a hard active galactic nucleus (AGN) radiation field, while line ratio diagnostics are consistent with either AGN or star formation as the dominant ionizing source. GHZ9 is nitrogen-enriched (6--9.5 (N/O)$_{\odot}$), carbon-poor (0.2--0.65 (C/O)$_{\odot}$), metal-poor (Z = 0.01--0.1 Z$_{\odot}$), and compact ($<$ 106 pc), similarly to GN-z11, GHZ2, and recently discovered N-enhanced high redshift objects. We exploited the newly available JWST/NIRSpec and NIRCam data set to perform an independent analysis of the Chandra data confirming that GHZ9 is the most likely JWST source associated with X-ray emission at 0.5-7 keV. Assuming a spectral index $Γ$ = 2.3 (1.8), we estimate a black hole (BH) mass of 1.60 $\pm$ 0.31 (0.48 $\pm$ 0.09) $\times$ 10$^8$M$_{\odot}$, which is consistent either with Eddington-accretion onto heavy ($\geq$ 10$^6$ M$_{\odot}$) BH seeds formed at z=18, or super-Eddington accretion onto a light seed of $\sim$ 10$^2-10^4$ M$_{\odot}$ at z = 25. The corresponding BH-to-stellar mass ratio M$_{BH}$/M$_{star}$= 0.33$\pm$0.22 (0.10$\pm$0.07), with a stringent limit $>$0.02, implies an accelerated growth of the BH mass with respect to the stellar mass. GHZ9 is the ideal target to constrain the early phases of AGN-galaxy coevolution with future multi-frequency observations.

Figures (5)

• Figure 1: Left: Observed 2D (top panel) and 1D spectrum of GHZ9 (bottom panel). The horizontal green and cyan lines enclose the customized extraction regions where we extract the 1D spectra for GHZ9 and ID=29686, respectively. The pipeline error spectrum is reported in gray. Emission lines with an integrated S/N $>$ 3 are marked in blue, while the positions of lines where we have a 3$\sigma$ upper limit are marked in red. The α Ly$\alpha$-break feature is shown in blue. Upper Right: 25$\times$25 arcsec$^2$ Chandra image in the 0.5--7 keV band centered on GHZ9, Gaussian smoothed with a 1$^{\prime\prime}$ FWHM. The source extraction region (radius of 1$"$) is shown in red. Lower right: 2$\times$2 arcsec$^2$ zoom-in F200W image showing the NIRSpec/Prism MSA shutter positions for GHZ9, ID=29686 (z$_{spec}$=1.117), and ID=29852 (z$_{phot}$=0.575), as obtained from the APT tool. The first two visits are shown in blue, and the third visit in red.
• Figure 2: Spectral energy distribution of the host galaxy stellar and nebular component (dashed), the AGN component (dot-dashed), and the combined host and AGN fit (solid). Fits are color-coded by increasing AGN fraction used in the modeling. The combined fits are overplotted due to their degeneracy, as discussed in the main text. Photometric measurements (circles) and 2$\sigma$ upper limits (triangles) are shown from left to right for the following bands: F435W, F606W, F814W, F090W, F105W, F115W, F125W, F150W, F160W, F200W, F277W, F356W, and F444W, as reported in the ASTRODEEP catalog from Merlin2024.
• Figure 3: Diagnostic diagrams based on flux ratios and EW. The top left panel shows the legend for each high-redshift AGN candidate included. AGN and SFG models from NM22 are plotted as red and blue triangles, respectively, while F16 and G16 models are shown as circles with shaded backgrounds following the same color scheme. Demarcation lines used to separate AGN-dominated, SFG-dominated, and composite sources follow the criteria from Hirschmann2023 and Mazzolari2024a.
• Figure 4: Left: BH mass as a function of observed redshift. We report evolutionary models of BH mass that differ based on the initial mass seed and accretion rate. Yellow, pink, and blue colors represent 10$^6$ M$_{\odot}$, 10$^4$ M$_{\odot}$, and 10$^2$ M$_{\odot}$, respectively. Solid, dashed, and dot-dashed lines refers to the 1, 1.5, and 2.0 Eddington accretion rates. We present the inferred BH mass of GHZ9 based on two different spectral indices, as discussed in the main text. Right: BH mass versus stellar mass of the host galaxy. We show the stellar mass of GHZ9 color-coded by increasing AGN fractions used in the SED fitting, while adopting the BH mass solution for a steep spectral index $\Gamma$ = 2.3. The error bars for GHZ9 show the statistical uncertainties for the extreme cases, where the stellar and BH masses are at their minimum and maximum values. The red star shows our stringent lower limit M$_{BH}$/M$_{star}=$0.02, as discussed in the main text. The M$_{BH}$ -- M$_{bulge}$ relation obtained by Kormendy2013 is indicated by the solid black line and gray shaded region. The gray symbols show estimates from observed JWST active galaxy at z $>$ 3.5 from the literature: Carnall2023, Goulding2023, Harikane2023b, Kocevski2023, Kokorev2023, Larson2023, Maiolino2023, Ubler2023, Chisholm2024, Furtak2024, Greene2024, Juodzbalis2024, Matthee2024, Taylor2024, Tripodi2024, Naidu2025A, and Taylor2025.
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