Alessandra Aloisi

Large surveys of galaxies in the local and high-redshift Universe have, traditionally, relied on the intensity of rest-optical emission lines from metal ions in the interstellar medium (ISM) to indirectly estimate the O/H abundance in the gas. However, these optical strong line diagnostics are also sensitive to the electron gas temperature ( T _e ), resulting in large systematic uncertainties that inherently limit their utility as metallicity tracers, especially in dust-obscured and metal-rich environments. To this end, we provide the first empirical calibration of Ne ₂₃ , a novel abundance diagnostic using the mid-infrared (MIR) T _e -insensitive [Ne ii ] λ 12.81 μ m and [Ne iii ] λ 15.56 μ m fine-structure lines. We present new JWST/MIRI medium-resolution spectroscopy (MRS) observations of 10 H ii regions with optical measurements of T _e and O/H from the CHAOS project, and we analyze MIRI observations of eight low-metallicity galaxies with similarly high-fidelity direct O/H. We measure Ne ₂₃ from 1D MIR spectra extracted from apertures matched to the ground-based spectroscopy used to obtain O/H, a method that is unfeasible from MIR spectra acquired on prior space-based observatories. From these nebulae, Ne ₂₃ is strongly correlated with O/H over 1.5 dex in 12+log(O/H). We calibrate the O/H–Ne ₂₃ relation from the empirical data, finding a scatter of just 0.06 dex in O/H at fixed Ne ₂₃ . The O/H–Ne ₂₃ relation presented here provides a means to reliably estimate 12+log(O/H) from JWST/MIRI MRS observations of ionized nebulae out to z ≈ 0.8, enabling new chemical abundance surveys of highly attenuated regions and in the metal-rich ISM.

We present a first-of-its-kind analysis of the metal content across two interstellar medium (ISM) phases in a sample of 31 local star-forming galaxies from the COS Legacy Archive Spectroscopic SurveY, selected as analogs of high- z systems. Using cospatial UV absorption and optical emission-line spectroscopy, we compare abundances of N, O, S, and Fe in the low-ionization (neutral) and high-ionization (ionized) gas, providing a multiphase view of enrichment shortly after the current starburst and over longer timescales when ejecta from previous episodes have cooled and mixed. We find that O and S, produced predominantly in short-lived massive stars, are well mixed between the two phases, with scatter reflecting local inhomogeneities. Fe, predominantly produced by Type Ia supernovae on ∼1 Gyr timescales, is higher in the neutral gas, reflecting either delayed mixing of older Fe-enriched material or preferential depletion of Fe from the ionized phase through dust formation in core-collapse supernova ejecta. N exhibits the largest phase offset, with N/H _ion systematically ∼0.7 dex higher than N/H _neu , and the magnitude of this offset correlates with stellar mass, metallicity, star formation rate, and most strongly with the ISM outflow velocity. N/O ratios in the ionized phase rise rapidly within 3–6 Myr relative to the neutral gas, consistent with N enrichment dominated by Wolf–Rayet stars rather than intermediate-mass asymptotic giant branch stars on longer timescales. These results demonstrate that localized stellar feedback, outflows, and phase-dependent mixing collectively regulate the chemical evolution of star-forming galaxies, providing key insight into the extreme N/O abundances recently observed in galaxies at cosmic dawn.

Polycyclic aromatic hydrocarbons (PAHs) are key diagnostics of the physical conditions in the interstellar medium and are widely used to trace star formation in the mid-infrared (mid-IR). The relative strengths of mid-IR PAH features (e.g., 6.2, 7.7, and 11.3 μ m) are sensitive to both the size and ionization state of the molecules and can be strongly influenced by the local radiation field. However, at low metallicities ( Z < 0.2 Z _⊙ ), detecting PAHs remains notoriously difficult, likely reflecting a combination of suppressed formation and enhanced destruction mechanisms. We present new JWST/MIRI Medium Resolution Spectroscopy (MRS) observations of the metal-poor ( Z ∼ 0.1 Z _⊙ ) dwarf galaxy CGCG 007-025. We confirm the tentative PAH detection previously reported from Spitzer data and, for the first time, identify a compact (∼50 pc) PAH-emitting region nearly cospatial with the newly detected [Ne v ] (Ionization potential (I.P.) ∼ 97 eV) emission and the galaxy’s most metal-poor, strongly star-forming region. The PAH _{11.3 μ m} and PAH _{12.7 μ m} features are detected, while no emission is found from other typically brighter features, suggesting a PAH population dominated by large, neutral molecules resilient to hard ionizing fields. When compared with models, mid-IR line ratios involving [Ne iii ], [O iv ], and [Ne v ] can only be reproduced by a combination of star formation and AGN ionization, with the latter contributing 4%–8%. The [O iv ] and [Ne v ] luminosities exceed what massive stars or shocks can produce, highlighting a puzzling scenario in line with recent JWST observations of similar galaxies. This work provides a crucial reference for studying the physical conditions of the dust and star formation in low-metallicity starburst regions, environments typical of the early Universe.

We present JWST/MIRI spectra from the Medium-resolution Spectrometer of I Zw 18, a nearby dwarf galaxy with a metallicity of ∼3% Solar. Here, we investigate warm molecular hydrogen, H ₂ , observed in spectra extracted in ∼120 pc apertures centered on eleven regions of interest. We detect seven H ₂ rotational lines, some of which are among the weakest ever measured. The H ₂ population diagrams are fit with local-thermodynamic-equilibrium models and models of photodissociation regions. We also fit the ortho-/para-H ₂ ratios (OPRs); in three of the six regions for which it was possible to fit the OPR, we find values significantly greater than 3, the maximum value for local thermodynamic equilibrium. To our knowledge, although predicted theoretically, this is the first time that OPR significantly >3 has been measured in interstellar gas. We find that an OPR tends to increase with decreasing H ₂ column density, consistent with the expected effects of self-shielding in advancing photodissociation fronts. The population diagrams are consistent with H nucleon densities of ∼10 ⁵ cm ⁻³ , and an interstellar radiation field scaling factor, G ₀ , of ∼10 ³ . This warm, dense H ₂ gas coexists with the same highly ionized gas that emits [O IV] and [Ne V]. Emission from T ≳ 50 K dust is detected, including an as-yet-unidentified dust emission feature near 14 μ m; possible identification of Al ₂ O ₃ is discussed. The continuum emission from several regions requires that a considerable fraction of the refractory elements be incorporated in dust. Despite stacking spectra in the SE where H ₂ is found, no significant emission from polycyclic aromatic hydrocarbons is detected.

We present JWST/MIRI spectra from the Medium-Resolution Spectrometer of I Zw 18, a nearby dwarf galaxy with a metallicity of ∼3% solar. Its proximity enables a detailed study of highly ionized gas that can be interpreted in the context of newly discovered high-redshift dwarf galaxies. We derive aperture spectra centered on 11 regions of interest; the spectra show very low extinction, A _V ≲ 0.1, consistent with optical determinations. The gas is highly ionized; we have detected 10 fine-structure lines, including [O iv] 25.9 μm with an ionization potential (IP) of ∼55 eV, and [Ne v] 14.3 μm with an IP of ∼97 eV. The ionization state of I Zw 18 falls at the extreme upper end of all of the line ratios we analyzed, but not coincident with galaxies containing an accreting massive black hole (active galactic nucleus). Comparison of the line ratios with state-of-the-art photoionization and shock models suggests that the high-ionization state in I Zw 18 is not due to shocks. Rather, it can be attributed to metal-poor stellar populations with a self-consistent contribution of X-ray binaries or ultra-luminous X-ray sources. It could also be partially due to a small number of hot low-metallicity Wolf−Rayet stars ionizing the gas; a small fraction (a few percent) of the ionization could come from an intermediate-mass black hole. Our spectroscopy also revealed four 14 μm continuum sources, ≳30–100 pc in diameter, three of which were not previously identified. Their properties are consistent with H ii regions ionized by young star clusters.

We present observations of the nearby extremely metal-poor galaxy I Zw 18 using the Keck Cosmic Web Imager (KCWI) and the JWST Mid-InfraRed Instrument (MIRI) Integral Field Spectrographs. From optical and mid-IR oxygen emission lines, we measured direct-method abundances for three ionic states of oxygen, including O³⁺/H⁺. In contrast to previous studies of I Zw 18 the high spatial resolution afforded by KCWI and MIRI/MRS revealed chemical inhomogeneities on 60 pc scales in the form of metal-poor pockets and metal-enriched gas. These are located outside I Zw 18’s star-forming complexes having possibly been dispersed beyond these regions via stellar feedback effects. We found that metallicities derived using a single low-ionization density tracer, and T _e([O ii]) derived from a temperature relationship commonly used in high-z galaxy studies, exhibited the largest scatter and underestimated the metallicity compared to those derived using multi-ion densities and estimated T_e([N ii]). Finally, we compared O³⁺/H⁺ abundances from a theoretical ionization correction factor (ICF) against observed values and found that the oxygen ICF underestimates the O³⁺/H⁺ abundance by a factor of 2, indicating that either additional ionizing sources are needed or standard stellar population models are unable to produce the requisite ionizing flux.

We present observations of the nearby extremely metal-poor galaxy I Zw 18 using the Keck Cosmic Web Imager (KCWI) and the JWST Mid-InfraRed Instrument (MIRI) Integral Field Spectrographs (IFS). From optical and mid-IR oxygen emission lines, we measured direct-method abundances for three ionic states of oxygen, including O$^{3+}$/H$^+$. In contrast to previous studies of I Zw 18, the high spatial resolution afforded by KCWI and MIRI/MRS revealed chemical inhomogeneities on 60 pc scales in the form of metal-poor pockets and metal-enriched gas. These are located outside I Zw 18's star-forming complexes having possibly been dispersed beyond these regions via stellar feedback effects. We found that metallicities derived using a single low-ionization density tracer, and T$_{\rm e}$([O II]) derived from a temperature relationship commonly used in high-$z$ galaxy studies, exhibited the largest scatter and underestimated the metallicity compared to those derived using multi-ion densities and estimated T$_{\rm e}$([N II]). Finally, we compared O$^{3+}$/H$^+$ abundances from a theoretical ionization correction factor (ICF) against observed values and found that the oxygen ICF underestimates the O$^{3+}$/H$^+$ abundance by a factor of 2, indicating that either additional ionizing sources are needed or standard stellar population models are unable to produce the requisite ionizing flux.

We present a spatially resolved (∼3 pc pixel⁻¹) analysis of the distribution, kinematics, and excitation of warm H₂ gas in the nuclear starburst region of M83. Our JWST/MIRI integral field unit spectroscopy reveals a clumpy reservoir of warm H₂ (>200 K) with a mass of ∼2.3 × 10⁵ M _⊙ in the area covered by all four Medium Resolution Spectrometer (MRS) channels. We additionally use the [Ne ii] 12.8 μm and [Ne iii] 15.5 μm lines as tracers of the star formation rate (SFR), ionizing radiation hardness, and kinematics of the ionized interstellar medium (ISM), finding tantalizing connections to the H₂ properties and stellar populations (e.g., broad and blueshifted [Ne ii] emission coincident with a region with >5 Myr old stellar populations, low SFR, and low H₂ surface density.) Finally, qualitative comparisons to the trove of public, high-spatial-resolution multiwavelength data available on M83 shows that our MRS spectroscopy potentially traces all stages of the process of creating massive star clusters, from the embedded protocluster phase through the dispersion of ISM from stellar feedback.

Nearby blue compact dwarf galaxies (BCDs) are considered analogs to objects from the Epoch of Reionization revealed by JWST, having similarly low stellar masses, low metallicities, and high specific star formation rates. Thus, they represent ideal local laboratories for detailed multiwavelength studies of their properties and mechanisms that shape them. We report the first JWST MIRI/MRS observations of the BCD SBS 0335-052 E, analyzing MIR emission lines tracing different levels of ionization (e.g., [Ne ii], [S iv], [Ne iii], [O iv], [Ne v]) of the ionized gas. SBS 0335-052 E MIR emission is characterized by a bright point source, located in one of the youngest and most embedded stellar clusters (t ∼ 3 Myr, A _V ∼ 15), and underlying extended high-ionization emission (i.e., [O iv] and [Ne v]) from the surroundings of the older and less dusty stellar clusters (t < 20 Myr, A _V ∼ 8). From a comparison with state-of-the-art models, we can exclude shocks, X-ray binaries, and old stellar populations as the main sources of ionization. Interestingly, a 4%–8% contribution of a ∼10⁵ M _⊙intermediate massive black hole (IMBH) is needed to justify the strong [Ne v]/[Ne ii] and would be consistent with optical/UV line ratios from previous studies. However, even IMBH models cannot explain the strongest [O iv]/[Ne iii]. Also, star-forming models (regardless of including X-ray binaries) struggle to reproduce even the lower ionization line ratios (e.g., [S iv]/[Ne ii]) typically observed in BCDs. Overall, while current models suggest the need to account for an accreting IMBH in this high-z analog, limitations still exist in predicting high-ionization emission lines (I.P. > 54 eV) when modeling these low-metallicity environments, and thus other sources of ionization cannot be fully ruled out.