[Paper Review] GA-NIFS: A massive black hole in a low-metallicity AGN at $z\sim5.55$ revealed by JWST/NIRSpec IFS

Hannah Übler, R. Maiolino+21 more|arXiv (Cornell University)|Feb 13, 2023

Astrophysical Phenomena and Observations10 references15 citations

TL;DR

JWST/NIRSpec IFS observations of GS_3073 at z=5.55 reveal a massive black hole in a low-metallicity AGN, showing a broad-line region and a fast ionized outflow, with metallicity Z_gas/Z_sun~0.21 and a central BH mass ~10^8.2 M_sun.

ABSTRACT

We present JWST/NIRSpec Integral Field Spectrograph rest-frame optical data of the compact $z=5.55$ galaxy GS_3073. Its prominent broad components in several hydrogen and helium lines (while absent in the forbidden lines), and the detection of a large equivalent width of He II$\lambda4686$, EW(He II) ~ 20A, unambiguously identify it as an active galactic nucleus (AGN). We measure a gas-phase metallicity of $Z_{ m gas}/Z_\odot\sim0.21^{+0.08}_{-0.04}$, lower than what has been inferred for both more luminous AGN at similar redshift and lower redshift AGN. We empirically show that classical emission line ratio diagnostic diagrams cannot be used to distinguish between the primary ionisation source (AGN or star formation) for such low-metallicity systems, whereas different diagnostic diagrams involving He II$\lambda4686$ prove very useful, independent of metallicity. We measure the central black hole mass to be $\log(M_{ m BH}/M_\odot)\sim8.2\pm0.4$ based on the luminosity and width of the broad line region of the H$α$ emission. While this places GS_3073 at the lower end of known high-redshift black hole masses, it still appears to be over-massive compared to its host galaxy properties. We detect an outflow with projected velocity $\gtrsim700$ km/s and infer an ionised gas mass outflow rate of about $100\ M_\odot/$yr, suggesting that GS_3073 is able to enrich the intergalactic medium with metals one billion years after the Big Bang.

Motivation & Objective

• Investigate the rest-frame optical emission of a z~5.55 galaxy to identify AGN signatures and characterize the ionizing source.
• Measure the black hole mass and host dynamics using BLR and narrow/outflow line components.
• Assess gas-phase metallicity and ionization conditions in a very early, low-metallicity AGN.
• Quantify feedback via outflow properties and implications for IGM enrichment.
• Evaluate how high-z, low-metallicity AGN compare to local scaling relations and diagnostic diagrams.

Proposed method

• Use JWST/NIRSpec IFS (G395H) to obtain rest-frame optical spectra of GS_3073 at z=5.55.
• Decompose emission lines into narrow (host/NLR), outflow, and broad-line region (BLR) components for Hα, Hβ, He II, He I, [O III], [N II], [S II].
• Fit multi-component Gaussian profiles with tied velocities and widths to constrain BLR and outflow properties.
• Derive BH mass from virial relations using Hα BLR luminosity and FWHM following Reines et al. (2013) with ε≈1.075±0.325.
• Compute bolometric luminosity from narrow line luminosities and compare calibrations (Netzer 2009; alternative methods).
• Construct kinematic maps (flux, velocity, dispersion) for narrow and outflow components to assess dynamics and feedback.

Experimental results

Research questions

• RQ1 What is the black hole mass in GS_3073 as inferred from the BLR in rest-frame optical lines?
• RQ2 What are the kinematic properties (rotation, outflow) of the host galaxy and its ionized gas?
• RQ3 What is the gas-phase metallicity of GS_3073, and how does it compare to higher-redshift and local AGN?
• RQ4 Can classical emission line ratio diagnostics distinguish AGN from star formation at z>3, particularly in low-metallicity systems?
• RQ5 What are the implications of the outflow for metal enrichment of the IGM at z~5.6?

Key findings

• Detection of a broad-line region in Hα, Hβ, He II, and He I with BLR FWHM up to ~3370 km/s, confirming an AGN in GS_3073.
• Estimated central black hole mass log(M_BH/M_sun)=8.2±0.4 from Hα BLR properties.
• Gas-phase metallicity Z_gas/Z_sun~0.21^{+0.08}_{-0.04}, lower than for more luminous or lower-redshift AGN.
• Identification of an ionized outflow with projected velocities up to ~700 km/s and ionized gas outflow rate ≈100 M_sun/yr.
• Outflow energetics inferred to contribute to IGM metal enrichment ~1 Gyr after the Big Bang, with Eddington-limited considerations.
• BH mass appears over-massive relative to host galaxy properties when placed on local scaling relations.