Investigating ionising sources and the complex interstellar medium of GHZ2 at $z=12.3$

TL;DR

This work analyzes GHZ2 at $z=12.3$ with deep JWST spectroscopy to identify the sources of its extreme ionisation. By combining BAGPIPES spectro-photometric fits and HOMERUN multi-zone photoionisation modeling, the authors show that a simple stellar population cannot reproduce the full emission spectrum, and that a composite star formation plus AGN (or dense, matter-bounded) ISM scenario best explains the data. They measure a strong nitrogen enhancement (N/O) and a prominent high-density gas component, with the O III $\lambda 3133$ Bowen line showing variability on a ~19-day timescale, indicative of AGN activity. The analysis also reveals a substantial neutral gas reservoir via the Ly$\alpha$ damping wing, and suggests GHZ2 hosts a stratified ISM where both star formation and AGN influence the emission, highlighting the need for multi-instrument follow-up to resolve the ISM structure and chemical abundances at cosmic dawn.

Abstract

An accurate characterisation of the physical properties of galaxies at cosmic dawn is key to understanding the origin of the high abundance of UV-bright galaxies at z$\gtrsim$10. We exploit deep NIRSpec PRISM observations of GHZ2 to constrain the sources of ionising radiation and the properties of the ISM in this bright, compact, and highly ionising galaxy at z=12.3. We measure with high significance the prominent N IV, C IV, He II, O III, C III, O II, and Ne III emission features previously detected in shallower observations, and confirm the detection of the N III] $λ1750$ multiplet, yielding tight constraints on the N/O ratio, which is found to be $\simeq$2 times the solar value. We also detect the Mg II $λ2800$, [Fe IV] $λ2833$ and Si II $λ1812$ doublets, the H8+HeI $λλ3889$ blend, and the Si IV+O IV] $λλ1400$ absorption complex. The O III $λ3133$ fluorescence line is only detected in the first observing epoch, implying variability on a rest-frame time span of 19 days, strongly suggesting the presence of an active nucleus. Combining the NIRSpec dataset with available optical and far-infrared constraints from MIRI and ALMA, we show that the emission spectrum of GHZ2 cannot be reproduced by single-density spectro-photometric models. Multi-zone photoionisation modelling performed with the HOMERUN code demonstrates that star formation must be occurring in a strongly stratified ISM, where both low-/intermediate-density gas and high-density regions (log($n_e$/cm$^{-3}) \gtrsim 4$) coexist. The GHZ2 emission landscape is consistent with either a composite star-formation plus AGN scenario, or with star formation occurring in a combination of radiation- and matter-bounded regions. Purely radiation-bounded stellar models fail to reproduce the observed He II emission, making an additional hard ionising component unavoidable.

Figures (11)

• Figure 1: Observed 2D (top) and 1D (bottom) NIRSpec PRISM spectra of GHZ2 acquired with a total exposure time of $\sim$9.1 hours. In the bottom panel the gray shaded area shows the 1$\sigma$ uncertainty, and red dashed lines highlight the wavelength of the UV features discussed in the present paper.
• Figure 2: NIRSpec 2D (top) and 1D (bottom) spectra in a region with width of 240 Å rest-frame centred at the position of the O III $\lambda 3133$ line. The red and blue lines and shaded areas in the bottom panel show respectively the first and second epoch spectra and relevant 1$\sigma$ uncertainties in each pixel. The vertical orange lines enclose the region where the signal-to-noise ratio (SNR) of the feature is evaluated from direct integration. The black line shows the UV continuum estimated on the stacked spectrum. The red dashed line marks the wavelength of the 3133Å O$\;$ Bowen fluorescence feature. The corresponding regions of the 2D spectra are shown on top.
• Figure 3: Top: Best-fit BAGPIPES template (yellow) obtained by performing a spectro-photometric fitting on the observed NIRSpec spectrum (blue) and NIRCam photometry (dark blue circles and errorbars) of GHZ2, using BPASS v. 2.2.1 stellar models, nebular emission computed with CLOUDY by assuming electron density log(n$_e$)=5, and a double power-law SFH. Bottom: comparison between the predicted line fluxes obtained by BAGPIPES (diamonds) and the observed ones (grey shaded areas). A minimum uncertainty of 10% is considered for the observed fluxes. Predicted fluxes are shown as green, yellow, orange, or red diamonds if they are at $\leq$1$\sigma$, $\leq$2$\sigma$, $\leq$3$\sigma$ or $>$3$\sigma$ from observed ones.
• Figure 4: Results for the HOMERUN M1 model of GHZ2 (radiation-bounded star-formation case). Top panel: the best-fit grid of single-cloud models in the log $U$ versus log(n$_e$/cm$^{-3}$) plane. The cells are colour-coded according to the assigned weight (when this is $>$0) of the relevant single-cloud model, as indicated by the colour bar. The black empty diamond represents the weighted density and ionization parameter of the single-cloud models. Central panel: relative contribution from high-density ($n>10^4\,\mathrm{cm}^{-3}$, yellow) and low-density (red) star-forming regions. Bottom panel: comparison between the predicted (diamonds) and observed (grey shaded areas) line fluxes (colour labels as in Fig. \ref{['fig_bagpipes_fit']}).
• Figure 5: Same as Fig. \ref{['fig_homerun_SF']} but for the two components of the M2 model (radiation-bounded star-formation plus AGN). The central panel shows the relative contribution of the star-forming (red) and AGN (yellow) component to the flux of the observed lines.