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

article Investigating ionising sources and the complex interstellar medium of GHZ2 at z=12.3 published yes M. Castellano author L. Napolitano author B. Moreschini author A. Calabrò author L. Christensen author M. Llerena author T. J.L.C. Bakx author F. Belfiore author D. Bevacqua author M. Dickinson author A. Fontana author G. Gandolfi author T. Gasparetto author A. Marconi author Sara Mascia author edaf889c-c7cd-11ef-ab1b-bb28c431bd29 E. Merlin author T. Morishita author T. Nanayakkara author D. Paris author L. Pentericci author B. Pérez-Díaz author G. Roberts-Borsani author S. Rojas-Ruiz author P. Santini author T. Treu author E. Vanzella author B. Vulcani author X. Wang author I. Yoon author J. Zavala author JoMa department 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≳10. We exploit deep (9.1-hour exposure time) NIRSpec PRISM observations of GHZ2 to constrain the sources of ionising radiation and the properties of the interstellar medium (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 ≃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, even under extreme assumptions on the ionisation parameter and electron density. 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(ne/cm−3 ) ≳ 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. https://research-explorer.ista.ac.at/download/21934/21935/2026_OpenJourAstrophysics_Castellano.pdf application/pdfno Maynooth Academic Publishing2026 eng The Open Journal of Astrophysics 2565-6120 2512.0849010.33232/001c.160281 9 M. Castellano et al., “Investigating ionising sources and the complex interstellar medium of GHZ2 at z=12.3,” The Open Journal of Astrophysics, vol. 9. Maynooth Academic Publishing, 2026. Castellano M, Napolitano L, Moreschini B, Calabrò A, Christensen L, Llerena M, Bakx TJLC, Belfiore F, Bevacqua D, Dickinson M, Fontana A, Gandolfi G, Gasparetto T, Marconi A, Mascia S, Merlin E, Morishita T, Nanayakkara T, Paris D, Pentericci L, Pérez-Díaz B, Roberts-Borsani G, Rojas-Ruiz S, Santini P, Treu T, Vanzella E, Vulcani B, Wang X, Yoon I, Zavala J. 2026. Investigating ionising sources and the complex interstellar medium of GHZ2 at z=12.3. The Open Journal of Astrophysics. 9. Castellano, M., et al. “Investigating Ionising Sources and the Complex Interstellar Medium of GHZ2 at Z=12.3.” The Open Journal of Astrophysics, vol. 9, Maynooth Academic Publishing, 2026, doi:<a href="https://doi.org/10.33232/001c.160281">10.33232/001c.160281</a>. Castellano, M., Napolitano, L., Moreschini, B., Calabrò, A., Christensen, L., Llerena, M., … Zavala, J. (2026). Investigating ionising sources and the complex interstellar medium of GHZ2 at z=12.3. The Open Journal of Astrophysics. Maynooth Academic Publishing. <a href="https://doi.org/10.33232/001c.160281">https://doi.org/10.33232/001c.160281</a> M. Castellano, L. Napolitano, B. Moreschini, A. Calabrò, L. Christensen, M. Llerena, T.J.L.C. Bakx, F. Belfiore, D. Bevacqua, M. Dickinson, A. Fontana, G. Gandolfi, T. Gasparetto, A. Marconi, S. Mascia, E. Merlin, T. Morishita, T. Nanayakkara, D. Paris, L. Pentericci, B. Pérez-Díaz, G. Roberts-Borsani, S. Rojas-Ruiz, P. Santini, T. Treu, E. Vanzella, B. Vulcani, X. Wang, I. Yoon, J. Zavala, The Open Journal of Astrophysics 9 (2026). Castellano M, Napolitano L, Moreschini B, et al. Investigating ionising sources and the complex interstellar medium of GHZ2 at z=12.3. The Open Journal of Astrophysics. 2026;9. doi:<a href="https://doi.org/10.33232/001c.160281">10.33232/001c.160281</a> Castellano, M., L. Napolitano, B. Moreschini, A. Calabrò, L. Christensen, M. Llerena, T. J.L.C. Bakx, et al. “Investigating Ionising Sources and the Complex Interstellar Medium of GHZ2 at Z=12.3.” The Open Journal of Astrophysics. Maynooth Academic Publishing, 2026. <a href="https://doi.org/10.33232/001c.160281">https://doi.org/10.33232/001c.160281</a>. 219342026-05-31T22:02:14Z2026-06-02T06:39:53Z