Investigating the ionization mechanisms powering the extreme emission lines in metal-poor galaxies
Abstract
Extreme emission metal-poor galaxies (EEMPGs) are excellent local laboratories for understanding the extreme physical conditions and ionizing radiation fields in the early Universe. However, the exact source of high-ionization emission lines in them, whether from intermediate-mass black holes (IMBHs), ultra-luminous X-ray sources (ULXs), or radiative shocks, remains debated. We characterize the fundamental properties and ionizing radiation field on metal-poor EEMPGs, aiming to identify the dominant sources of the highly ionizing photons that drive these lines. We selected a sample of 11 EEMPGs, among the most metal-poor galaxies with JWST/MIRI observations known so far (12+log(O/H) < 8.0, or Z/Z⊙ < 20%). Using optical and IR diagnostic diagrams for ions of both high and low ionization potential, we constrain the ionization source dominating their spectral features. We detect high-ionization lines, notably He IIλ4686 in all galaxies and [Ne V] 14.3 μm in 42% of a broader sample of local EEMPGs. The multi-wavelength diagnostic diagrams reveal that our EEMPGs are located in areas that normally are occupied by active galactic nuclei (AGNs) or between the star-formation and AGN sequences. Evaluating the energetics required to produce these lines, we find that radiative shocks and ULXs are not sufficient. Instead, our analysis suggests that a combination of sources with star-formation and a small contribution from an IMBH provide enough high-energy photon budget to explain the emission lines of ions with high-ionization potentials (> 54 eV). Our EEMPG sample, covering various properties, is ideal for studying the first galaxies. Their flux ratios of low- and high-ionization lines suggest stellar radiation as the likely ionization source, with a small contribution from an IMBH (4-8%). Pure shocks fail to explain the observed emission line ratios and ionizing photon budget.
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BibTeX
@misc{daoutis2026investigating,
title = {{Investigating the ionization mechanisms powering the extreme emission lines in metal-poor galaxies}},
author = {C. Daoutis and V. Lebouteiller and D. Schaerer and I. Morel and O. Bait and S. Satyapal and S. Doan and B. Trahin and A. Gurpide and Y. Izotov and N. Guseva and L. Ramambason and A. Verhamme and SpeXion Team},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.28329},
eprint = {2609.28329},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.28329},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Investigating the ionization mechanisms powering the extreme emission lines in metal-poor galaxies
AU - C. Daoutis
AU - V. Lebouteiller
AU - D. Schaerer
AU - I. Morel
AU - O. Bait
AU - S. Satyapal
AU - S. Doan
AU - B. Trahin
AU - A. Gurpide
AU - Y. Izotov
AU - N. Guseva
AU - L. Ramambason
AU - A. Verhamme
AU - SpeXion Team
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2609.28329
UR - https://arxiv.org/abs/2609.28329
AB - Extreme emission metal-poor galaxies (EEMPGs) are excellent local laboratories for understanding the extreme physical conditions and ionizing radiation fields in the early Universe. However, the exact source of high-ionization emission lines in them, whether from intermediate-mass black holes (IMBHs), ultra-luminous X-ray sources (ULXs), or radiative shocks, remains debated. We characterize the fundamental properties and ionizing radiation field on metal-poor EEMPGs, aiming to identify the dominant sources of the highly ionizing photons that drive these lines. We selected a sample of 11 EEMPGs, among the most metal-poor galaxies with JWST/MIRI observations known so far (12+log(O/H) < 8.0, or Z/Z_⊙ < 20%). Using optical and IR diagnostic diagrams for ions of both high and low ionization potential, we constrain the ionization source dominating their spectral features. We detect high-ionization lines, notably He IIλ4686 in all galaxies and [Ne V] 14.3 μm in 42% of a broader sample of local EEMPGs. The multi-wavelength diagnostic diagrams reveal that our EEMPGs are located in areas that normally are occupied by active galactic nuclei (AGNs) or between the star-formation and AGN sequences. Evaluating the energetics required to produce these lines, we find that radiative shocks and ULXs are not sufficient. Instead, our analysis suggests that a combination of sources with star-formation and a small contribution from an IMBH provide enough high-energy photon budget to explain the emission lines of ions with high-ionization potentials (> 54 eV). Our EEMPG sample, covering various properties, is ideal for studying the first galaxies. Their flux ratios of low- and high-ionization lines suggest stellar radiation as the likely ionization source, with a small contribution from an IMBH (4-8%). Pure shocks fail to explain the observed emission line ratios and ionizing photon budget.
ER -