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

Lorenzo Napolitano, M. Castellano+34 more|arXiv (Cornell University)|Oct 24, 2024

Astrophysical Phenomena and Observations9 citations

TL;DR

JWST/NIRSpec reveals GHZ9 at z=10.145 hosting both AGN and star formation, with Chandra X-ray detection indicating a massive black hole and a high BH-to-stellar mass ratio.

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) $ imes$ 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.

Motivation & Objective

• Assess whether GHZ9's ionizing source is AGN, star formation, or a composite of both.
• Characterize the physical properties (metallicity, ionization, density) from rest-frame UV/optical lines.
• Constrain the black hole mass and its growth history in relation to the host galaxy.
• Verify the association between GHZ9 and the Chandra X-ray detection and explore implications for BH seeds and accretion.
• Evaluate the BH-to-stellar mass ratio and its evolution in the early universe.

Proposed method

• Obtain JWST/NIRSpec PRISM spectra of GHZ9 and perform line identification and Gaussian fits to measure fluxes and EWs.
• Use UV and optical emission-line diagnostics and EW-based indicators to distinguish AGN versus star-formation contributions (NM22, F16, and Hirschmann et al. lines).
• Apply SED fitting with CIGALE including an AGN component to estimate stellar mass, metallicity, and AGN fraction (f_AGN).
• Analyze Chandra X-ray data from Abell 2744 with a mosaic and source extraction to derive 0.5–7 keV counts, flux, and rest-frame luminosity under different photon indexes.
• Compute N/O and C/O abundances with PyNeb using observed line ratios, density/temperature grids, and ionization corrections (ICF).
• Correct all rest-frame quantities for lens magnification (μ = 1.36) and adopt a ΛCDM cosmology for distances and masses.

Experimental results

Research questions

• RQ1 Is GHZ9 primarily AGN-dominated, star-formation-dominated, or a composite of both according to UV and optical diagnostics?
• RQ2 What are the metallicity, ionization parameter, and density constraints for GHZ9, and how do they compare to other z>8.5 AGN candidates?
• RQ3 What is the black hole mass implied by the X-ray emission and bolometric corrections, and what does it imply about seed masses and accretion history?
• RQ4 What is the BH-to-stellar mass ratio in GHZ9, and what are the implications for BH-galaxy coevolution at z>10?

Key findings

• GHZ9 exhibits rest-frame UV high-ionization lines with large EWs (C IV ≈ 65 Å, He II ≈ 18 Å, C III] ≈ 48 Å), indicating a hard radiation field.
• UV and optical diagnostics place GHZ9 in a region consistent with both AGN and star-formation models, implying a composite nature with neither component clearly dominant.
• GHZ9 shows nitrogen enrichment (N/O ≈ 6–9.5× solar) and carbon-poor, metal-poor gas (Z ≈ 0.01–0.1 Z⊙), with a compact morphology (r_e ≲ 106 pc).
• Chandra X-ray data associate the X-ray source with GHZ9, yielding a rest-frame 2–10 keV luminosity of ≈1.8–3.8×10^44 erg s^-1 depending on the assumed photon index (Γ = 1.8–2.3).
• BH mass estimates are M_BH ≈ 1.60±0.31×10^8 M⊙ (Γ=2.3) or 0.48±0.09×10^8 M⊙ (Γ=1.8), implying Eddington-limited growth or possible super-Eddington scenarios.
• BH-to-stellar mass ratio is high, M_BH/M_star ≈ 0.33±0.22 (or 0.10±0.07 for alternative assumptions), with a stringent lower limit >0.02, suggesting accelerated BH growth relative to the host.