Bethan L. James

We present a spatially resolved, multiphase study of chemical enrichment around young star clusters (YSCs) in the nearby grand-design spiral M83 by combining far-ultraviolet (UV) absorption-line spectroscopy from the Hubble Space Telescope Cosmic Origins Spectrograph with cospatial optical spectroscopy from the Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) and the Large Binocular Telescope (LBT) Multi-Object Double Spectrograph (MODS). Our sample includes 18 YSCs spanning spectroscopic ages of ∼1–6 Myr and galactocentric radii out to R / R ₂₅ = 0.56. Neutral (H i ) abundances were derived from UV absorption-line spectroscopy and compared with ionized (H ii ) abundances from reddening-corrected optical emission lines. Because auroral lines are not detected in all regions, we develop and apply an empirical multizone electron-temperature ( T _e ) calibration based on strong-line diagnostics to estimate T _e and derive reliable nebular abundances. We measure oxygen (O), sulphur (S), nitrogen (N), and iron (Fe) abundances, tracing enrichment from distinct nucleosynthetic channels. The α -elements (O and S) exhibit similar behaviour, consistent with enrichment by core-collapse supernovae, whereas Fe shows weaker variations, reflecting its delayed production by Type Ia supernovae. Nitrogen displays the largest phase offset (ionized–neutral), with enhancements of up to ΔN/H ∼ 1.5 dex and ΔN/O > 1.5 dex in the ionized gas relative to the neutral phase, indicating localized enrichment by massive stars and inefficient mixing between gas phases on Myr timescales. While the ionized gas exhibits signatures of feedback-regulated chemical enrichment and large-scale abundance gradients, corresponding trends are weak or absent in the neutral gas, consistent with metals remaining largely confined to the immediate star-forming environment during the earliest stages of cluster evolution in a massive grand-design spiral.

Star-forming galaxies drive massive outflows that play an important role in galaxy evolution by regulating feedback and influencing the dynamics of surrounding media. Measuring galactic outflow rates is essential for quantifying feedback efficiency and the amount of mass, momentum, and energy deposited into the circumgalactic medium (CGM). In this paper, we examine 17 galactic outflows from the COS Legacy Archive Spectroscopy Survey (CLASSY) survey with radiative transfer modeling of UV absorption lines presented in M. Huberty et al. to study their spatial distributions and kinematic properties. We study the Si ii , Si iii , and Si iv ionization states that trace the cool and warm phases of the outflows and find that Si ii– traced winds generally behave differently than the warmer Si iii– and Si iv– traced winds. We derive the mass, momentum, and energy loading factors, which we find scale inversely proportional to stellar mass. We find that our measurements of the mass and momentum loading factors are in agreement with the hydrodynamic FIRE-2 simulations. We model the velocity profiles of the winds, with profiles reaching a maximum velocity of 620 km s ⁻¹ on average, in agreement with hydrodynamic simulations from the Cholla Galactic OutfLow Simulations (CGOLS). We also investigate the relationship between outflow properties and the age of the stellar population from spectral energy distribution (SED) fitting. We find that outflows associated with young (<5 Myr) star-forming regions are more likely to have a column density dominated by cooler gas and have mass outflow rates that decrease with radius.

We present the spectroscopic analysis of far-ultraviolet (FUV) observations taken with the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) targeting young massive clusters in the nearby, metal-poor, blue compact dwarf galaxy NGC 5253. We characterize the stellar populations observed across seven COS pointings and report on their inferred physical parameters: age, metallicity, mass, and reddening values. Comparisons between our spectroscopic ages and those inferred using photometric methods show that the former are preferentially younger. We also investigate the impact of these young massive clusters on their surrounding interstellar medium. Using Very Large Telescope/MUSE optical observations and matching the size of the COS aperture, we measure outflow velocities of the ionized gas along the line of sight of the COS pointing with values ranging from ∼125 to 300 km s ⁻¹ . We report on strong statistically significant correlations between the outflow velocities and stellar ages, masses, and total mechanical luminosity primarily driven by supernovae (SNe) as derived from our full-spectrum fitting analysis. Although theoretical models predict a delayed injection of mechanical energy and momentum in low-metallicity environments (5 Myr ages, with no apparent delay. One possible explanation is that the decrease and suppression of mechanical feedback due to SN explosions might be dominant at even lower metallicities than those observed in NGC 5253.

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.

The Epoch of Reionization (EoR) provides critical insights into the role of early galaxies in shaping the ionization state of the Universe. However, because of the opacity of the intergalactic medium, it is often not possible to make direct measurements of the ionizing photon escape fraction ( f esc LyC ) of high-redshift ( z ≳ 4) galaxies. To explore the agreement and systematics of common indirect approaches, we applied six empirically calibrated diagnostics to predict f esc LyC for the 45 nearby star-forming galaxies from the COS Legacy Spectroscopic SurveY. These methods—based on ultraviolet (UV) absorption lines, the UV continuum slope, Ly α kinematics, a multivariate model, radiation-hydrodynamic simulations, and nebular emission-line ratios—enable us to explore systematic differences between predictions and assess how galactic properties influence inferred LyC escape. Despite significant variations in method predictions, there is broad consistency in the resulting weak and strong LyC leaker classifications, with approximately half exhibiting predicted escape fractions >1%. We find evidence for two different pathways of LyC escape in nearby star-forming galaxies: (1) an early escape model driven by very young stellar populations, and (2) a delayed escape model that is consistent with supernova-driven outflows and time-dependent interstellar medium clearing. The early escape model is favored among galaxies with a single, intense burst of recent star formation. In contrast, the delayed escape model is common among galaxies with more extended starburst histories. To interpret ionizing photon escape during the EoR, it will be necessary to recognize and understand this diversity in LyC escape mechanisms.

Recent JWST observations have uncovered a population of compact, high-redshift ( z > 6) galaxies exhibiting extreme nebular conditions and enhanced nitrogen abundances that challenge standard chemical evolution paradigms. We present a joint UV and optical abundance analysis using a new suite of Cloudy photoionization models covering a wide density range ( n _e = 10 ² –10 ⁹ cm ⁻³ ), combined with Hubble Space Telescope and JWST spectroscopy for a sample of star-forming galaxies across 0.0 ≲ z ≲ 10.6. We find that assuming uniform, low-density conditions ( n _e ∼ 10 ² cm ⁻³ ) in high-density environments ( n _e ∼ 10 ⁵ cm ⁻³ ) can bias nebular diagnostics by overestimating T _e (up to 1800 K), overpredicting log U (by >1 dex), and underestimating O/H (up to 0.67 dex), while only modestly inflating N/O. Therefore, robust abundance determinations at high z require a multiphase density model. Using this model, we recalculate O/H and N/O abundances for our sample and present the first log U diagnostics and ionization correction factors for high-ionization UV N lines. We find that the UV tracers systematically overestimate N/O by ∼0.3–0.4 dex relative to the optical benchmark. We find that N/O increases with redshift, correlating with both n _e and star formation rate surface density (Σ _SFR ), suggesting that N/O is temporarily enhanced in compact, high-pressure environments. However, the n _e evolution with z is more gradual than the (1 + z ) ³ scaling of virial halo densities, suggesting that n _e evolution is shaped by both cosmological structure growth and baryonic processes. These trends point to prompt N/O enrichment potentially driven by very massive stars, with key implications for interpreting UV emission and determining reliable chemical abundances from JWST observations 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.