--- DOAJ_listed: '1' OA_place: publisher OA_type: diamond PlanS_conform: '1' _id: '21934' abstract: - lang: eng text: "An accurate characterisation of the physical properties of galaxies at cosmic dawn is key to understanding\r\nthe origin of the high abundance of UV-bright galaxies at z≳10. We exploit deep (9.1-hour exposure time)\r\nNIRSpec PRISM observations of GHZ2 to constrain the sources of ionising radiation and the properties of the\r\ninterstellar medium (ISM) in this bright, compact, and highly ionising galaxy at z=12.3. We measure with\r\nhigh significance the prominent N IV, C IV, He II, O III, C III, O II, and Ne III emission features previously\r\ndetected in shallower observations, and confirm the detection of the N III] λ1750 multiplet, yielding tight\r\nconstraints on the N/O ratio, which is found to be ≃2 times the solar value. We also detect the Mg II λ2800,\r\n[Fe IV] λ2833 and Si II λ1812 doublets, the H8+HeI λλ3889 blend, and the Si IV+O IV] λλ1400 absorption\r\ncomplex. The O III λ3133 fluorescence line is only detected in the first observing epoch, implying variability\r\non a rest-frame time span of 19 days, strongly suggesting the presence of an active nucleus. Combining the\r\nNIRSpec dataset with available optical and far-infrared constraints from MIRI and ALMA, we show that the\r\nemission spectrum of GHZ2 cannot be reproduced by single-density spectro-photometric models, even under\r\nextreme assumptions on the ionisation parameter and electron density. Multi-zone photoionisation modelling\r\nperformed with the HOMERUN code demonstrates that star formation must be occurring in a strongly stratified\r\nISM, where both low-/intermediate-density gas and high-density regions (log(ne/cm−3\r\n) ≳ 4) coexist. The\r\nGHZ2 emission landscape is consistent with either a composite star-formation plus AGN scenario, or with\r\nstar formation occurring in a combination of radiation- and matter-bounded regions. Purely radiation-bounded\r\nstellar models fail to reproduce the observed He II emission, making an additional hard ionising component\r\nunavoidable." acknowledgement: "We thank the referee for the constructive comments that\r\nhelped us improve the manuscript. We thank S. Finkelstein,\r\nY. Harikane, C. Mason, and D. Stark for the useful comments.\r\nWe thank Tony Roman (Program Coordinator) and Glenn\r\nWahlgren (NIRSpec reviewer) for the assistance in the\r\npreparation of GO-3073 observations. This work is based\r\non observations made with the NASA/ESA/CSA James\r\nWebb Space Telescope (JWST). The JWST data presented in this article were obtained from the Mikulski Archive for\r\nSpace Telescopes (MAST) at the Space Telescope Science\r\nInstitute. The specific observations analysed are associated with program JWST-GO-3073 and can be accessed\r\nvia https://doi.org/10.17909/4r6b-bx96 (first pointing) and\r\nhttps://doi:10.17909/zq4g-r525 (second pointing). We\r\nacknowledge financial support from NASA through grant\r\nJWST-ERS-1324 and JWST-GO-3073. Support was also\r\nprovided by the PRIN 2022 MUR project 2022CB3PJ3 –\r\nFirst Light And Galaxy aSsembly (FLAGS) funded by the\r\nEuropean Union – Next Generation EU, by INAF GO Grant\r\n2024 ”Revealing the nature of bright galaxies at cosmic\r\ndawn with deep JWST spectroscopy”, by INAF Mini-grant\r\n2022 “Reionization and Fundamental Cosmology with\r\nHigh-Redshift Galaxies”, and by INAF Large Grant 2022\r\n“Extragalactic Surveys with JWST”. L.N. acknowledges\r\nsupport from grant “Progetti per Avvio alla Ricerca - Tipo\r\n1, Unveiling Cosmic Dawn: Galaxy Evolution with CAPERS” (AR1241906F947685). EV acknowledges financial\r\nsupport through grants INAF GO Grant 2024 “Mapping Star\r\nCluster Feedback in a Galaxy 450 Myr after the Big Bang”\r\nand by the European Union – NextGenerationEU within\r\nPRIN 2022 project n.20229YBSAN - Globular clusters\r\nin cosmological simulations and lensed fields: from their\r\nbirth to the present epoch. AM acknowledges support\r\nfrom project PRIN-MUR project “PROMETEUS” financed\r\nby the European Union - Next Generation EU, Mission 4\r\nComponent 1 CUP B53D2300475000. AM acknowledges\r\nsupport from Ricerca Fondamentale INAF under Mini Grant\r\n2023 ”Quantitative Spectroscopy of Ionized Nebulae and\r\nGalaxies (QSING)” and under Data Analysis Grant 2024\r\n“Accurate measurements of metallicity in galaxies with a new\r\napproach to photoionization modelling”." article_processing_charge: No article_type: original arxiv: 1 author: - first_name: M. full_name: Castellano, M. last_name: Castellano - first_name: L. full_name: Napolitano, L. last_name: Napolitano - first_name: B. full_name: Moreschini, B. last_name: Moreschini - first_name: A. full_name: Calabrò, A. last_name: Calabrò - first_name: L. full_name: Christensen, L. last_name: Christensen - first_name: M. full_name: Llerena, M. last_name: Llerena - first_name: T. J.L.C. full_name: Bakx, T. J.L.C. last_name: Bakx - first_name: F. full_name: Belfiore, F. last_name: Belfiore - first_name: D. full_name: Bevacqua, D. last_name: Bevacqua - first_name: M. full_name: Dickinson, M. last_name: Dickinson - first_name: A. full_name: Fontana, A. last_name: Fontana - first_name: G. full_name: Gandolfi, G. last_name: Gandolfi - first_name: T. full_name: Gasparetto, T. last_name: Gasparetto - first_name: A. full_name: Marconi, A. last_name: Marconi - first_name: Sara full_name: Mascia, Sara id: edaf889c-c7cd-11ef-ab1b-bb28c431bd29 last_name: Mascia - first_name: E. full_name: Merlin, E. last_name: Merlin - first_name: T. full_name: Morishita, T. last_name: Morishita - first_name: T. full_name: Nanayakkara, T. last_name: Nanayakkara - first_name: D. full_name: Paris, D. last_name: Paris - first_name: L. full_name: Pentericci, L. last_name: Pentericci - first_name: B. full_name: Pérez-Díaz, B. last_name: Pérez-Díaz - first_name: G. full_name: Roberts-Borsani, G. last_name: Roberts-Borsani - first_name: S. full_name: Rojas-Ruiz, S. last_name: Rojas-Ruiz - first_name: P. full_name: Santini, P. last_name: Santini - first_name: T. full_name: Treu, T. last_name: Treu - first_name: E. full_name: Vanzella, E. last_name: Vanzella - first_name: B. full_name: Vulcani, B. last_name: Vulcani - first_name: X. full_name: Wang, X. last_name: Wang - first_name: I. full_name: Yoon, I. last_name: Yoon - first_name: J. full_name: Zavala, J. last_name: Zavala citation: ama: 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:10.33232/001c.160281 apa: 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. https://doi.org/10.33232/001c.160281 chicago: 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. https://doi.org/10.33232/001c.160281. ieee: 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. ista: 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. mla: 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:10.33232/001c.160281. short: 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). date_created: 2026-05-31T22:02:14Z date_published: 2026-04-09T00:00:00Z date_updated: 2026-06-02T06:39:53Z day: '09' ddc: - '520' department: - _id: JoMa doi: 10.33232/001c.160281 external_id: arxiv: - '2512.08490' file: - access_level: open_access checksum: ec33ca56b8836c61cb01e26893d43cbf content_type: application/pdf creator: dernst date_created: 2026-06-02T06:38:59Z date_updated: 2026-06-02T06:38:59Z file_id: '21935' file_name: 2026_OpenJourAstrophysics_Castellano.pdf file_size: 4855934 relation: main_file success: 1 file_date_updated: 2026-06-02T06:38:59Z has_accepted_license: '1' intvolume: ' 9' language: - iso: eng month: '04' oa: 1 oa_version: Published Version publication: The Open Journal of Astrophysics publication_identifier: eissn: - 2565-6120 publication_status: published publisher: Maynooth Academic Publishing quality_controlled: '1' scopus_import: '1' status: public title: Investigating ionising sources and the complex interstellar medium of GHZ2 at z=12.3 tmp: image: /images/cc_by.png legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode name: Creative Commons Attribution 4.0 International Public License (CC-BY 4.0) short: CC BY (4.0) type: journal_article user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87 volume: 9 year: '2026' ...