@ARTICLE{2026OJAp....960281C,
       author = {{Castellano}, M. and {Napolitano}, L. and {Moreschini}, B. and {Calabr{\`o}}, A. and {Christensen}, L. and {Llerena}, M. and {Bakx}, T.~J.~L.~C. and {Belfiore}, F. and {Bevacqua}, D. and {Dickinson}, M. and {Fontana}, A. and {Gandolfi}, G. and {Gasparetto}, T. and {Marconi}, A. and {Mascia}, S. and {Merlin}, E. and {Morishita}, T. and {Nanayakkara}, T. and {Paris}, D. and {Pentericci}, L. and {P{\'e}rez-D{\'\i}az}, B. and {Roberts-Borsani}, G. and {Ruiz}, S. Rojas and {Santini}, P. and {Treu}, T. and {Vanzella}, E. and {Vulcani}, B. and {Wang}, X. and {Yoon}, I. and {Zavala}, J.},
        title = "{Investigating ionising sources and the complex interstellar medium of GHZ2 at z = 12.3}",
      journal = {The Open Journal of Astrophysics},
     keywords = {Astrophysics of Galaxies},
         year = 2026,
        month = apr,
       volume = {9},
        pages = {60281},
     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$$10. We exploit deep
        NIRSpec PRISM observations of GHZ2 to constrain the sources of
        ionising radiation and the properties of the ISM in this bright,
        compact, and highly ionising galaxy at z=12.3. We measure with
        high significance the prominent N IV, C IV, He II, O III, C III,
        O II, and Ne III emission features previously detected in
        shallower observations, and confirm the detection of the N III]
        {\ensuremath{\lambda}}1750 multiplet, yielding tight constraints
        on the N/O ratio, which is found to be $$2 times the solar
        value. We also detect the Mg II {\ensuremath{\lambda}}2800, [Fe
        IV] {\ensuremath{\lambda}}2833 and Si II
        {\ensuremath{\lambda}}1812 doublets, the H8+HeI
        {\ensuremath{\lambda}}{\ensuremath{\lambda}}3889 blend, and the
        Si IV+O IV] {\ensuremath{\lambda}}{\ensuremath{\lambda}}1400
        absorption complex. The O III {\ensuremath{\lambda}}3133
        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(
        ne/cm) 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.}",
          doi = {10.33232/001c.160281},
archivePrefix = {arXiv},
       eprint = {2512.08490},
 primaryClass = {astro-ph.GA},
       adsurl = {https://ui.adsabs.harvard.edu/abs/2026OJAp....960281C},
      adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}