Investigating ionising sources and the complex interstellar medium of GHZ2 at z = 12.3
Journal article, 2026
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.
ionisation
interstellar medium
GHZ2
high-redshift
astro-ph.GA (Physics - Galaxy Astrophysics)
cosmic dawn
Author
M. Castellano
Osservatorio Astronomico di Roma
Lorenzo Napolitano
Osservatorio Astronomico di Roma
B. Moreschini
University of Florence
Arcetri Astrophysical Observatory
European Southern Observatory (ESO)
Antonello Calabrò
Osservatorio Astronomico di Roma
L. Christensen
Niels Bohr Institute
Mario Llerena
Osservatorio Astronomico di Roma
Tom Bakx
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
F. Belfiore
Arcetri Astrophysical Observatory
European Southern Observatory (ESO)
D. Bevacqua
Osservatorio Astronomico di Roma
Mark Dickinson
NOIRLab
Adriano Fontana
Osservatorio Astronomico di Roma
G. Gandolfi
Osservatorio Astronomico di Roma
T. Gasparetto
Osservatorio Astronomico di Roma
A. Marconi
Arcetri Astrophysical Observatory
University of Florence
Sara Mascia
Institute of Science and Technology Austria
E. Merlin
Osservatorio Astronomico di Roma
Takahiro Morishita
California Institute of Technology (Caltech)
Themiya Nanayakkara
Swinburne University of Technology
Diego Paris
Osservatorio Astronomico di Roma
L. Pentericci
Osservatorio Astronomico di Roma
B. Pérez-Díaz
Osservatorio Astronomico di Roma
Guido Roberts-Borsani
University College London (UCL)
S. Rojas-Ruiz
University of California
P. Santini
Osservatorio Astronomico di Roma
Tommaso Treu
University of California
Eros Vanzella
Istituto nazionale di astrofisica (INAF)
B. Vulcani
Istituto nazionale di astrofisica (INAF)
X. Wang
Chinese Academy of Sciences
Beijing Normal University
I. Yoon
National Radio Astronomy Observatory
J. A. Zavala
University of Massachusetts
Open Journal of Astrophysics
25656120 (eISSN)
Vol. 9Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Astronomy, Astrophysics, and Cosmology
DOI
10.33232/001c.160281
Related datasets
Observations first pointing [dataset]
DOI: 10.17909/zq4g-r525 URI: https://doi.org/10.17909/4r6b-bx96