Danielle A. Berg

Oxygen is forged by stars and redistributed through galaxies by feedback-driven outflows, leaving a record of star formation and the baryon cycle imprinted on its present-day abundance and distribution. The Galaxies Losing Oxygen via Winds project quantifies the production, distribution, and retention of oxygen in 37 low-mass, low–metallicity, gas-rich galaxies in the nearby Universe ( D < 6 Mpc) spanning a critical stellar mass range (10 ^6.5 < M _* < 10 ^9.5 ) where feedback is expected to strongly shape galaxy evolution. We incorporate literature results to extend the analysis over 10 ⁵ < M _* < 10 ^11.5 . We couple star formation histories derived from resolved stars with measurements of interstellar medium metallicity and gas mass, and compare the oxygen budgets with empirical constraints in the circumgalactic medium (CGM). We find low-mass galaxies retain between 4% and 25% (mean 11%) of their oxygen produced by stellar nucleosynthesis, and nearly all retained oxygen resides in the gas. Galaxies exhibit increased scatter to higher oxygen retention fractions when residing in denser environments (Θ ₅ > 0.5) and within 1 Mpc of a massive galaxy ( M _* > 10 ⁹ M _⊙ ), indicating the environment likely affects the amount of oxygen retained, recycled, or accreted to galaxies. Contrary to expectations, oxygen retention does not correlate with position on the mass–metallicity relation. Although ionized oxygen is detected in the CGM, it remains unclear whether most missing oxygen resides there or has been expelled entirely. Hydrodynamical simulations, despite successfully reproducing the mass–metallicity relation, predict much higher retention fractions than measured in low-mass galaxies. Simple modeling indicates that wind mass-loading and outflow metallicity govern oxygen retention, and that low-mass galaxies accrete less baryonic material relative to the cosmic baryon fraction.

We present a spatially resolved analysis of stellar populations in the brightest z > 6 galaxy known to date (AB mag 23), the strongly lensed MACS0308−zD1 (dubbed the “Cosmic Spear”) at z _spec = 6.2. New JWST NIRCam imaging and high-resolution NIRSpec integral field unit (IFU) spectroscopy span the rest-frame ultraviolet to optical. The NIRCam imaging reveals bright star-forming clumps and a tail consisting of three distinct, extremely compact star clusters that are multiply imaged by gravitational lensing. The star clusters have delensed effective radii of R _eff ≲ 8 pc, stellar masses of M _* ∼ 10 ⁶ –10 ⁷ M _⊙ , and high stellar mass surface densities of Σ _* ≳ 2 × 10 ⁴ M _⊙ pc ⁻² . While their stellar populations are very young (∼6–11 Myr), their dynamical ages exceed unity, consistent with the clusters being gravitationally bound systems. Placing the star clusters in the size versus stellar mass density plane, we find they occupy a region similar to other high-redshift star clusters within galaxies observed recently with JWST, being significantly more massive and denser than local star clusters. Spatially resolved analysis of the brightest clump reveals a compact, intensely star-forming core. The ionizing photon production efficiency ( ξ _ion ) is slightly suppressed in this central region, potentially indicating a locally elevated Lyman continuum escape fraction facilitated by feedback-driven channels.

Gas-rich, star-forming, low-mass galaxies in the nearby Universe are powerful laboratories for studying baryonic physics in detail, including: stellar mass assembly, stellar feedback, chemical enrichment, and the interplay of the interstellar medium with star formation. Investigating these disparate yet interconnected processes requires data obtained by myriad observatories. Here, we present a comprehensive atlas of uniformly processed data on 37 low-mass galaxies within 6 Mpc. The atlas includes archival data on (i) the H i from the Very Large Array observatory; (ii) resolved stars from Hubble Space Telescope optical imaging; (iii) Spitzer Space Telescope 3.6 μ m imaging; and (iv) optical imaging from ground-based telescopes. We also compile measurements of (i) tip-of-the-red-giant-branch (TRGB) distances to the galaxies; (ii) direct method gas-phase oxygen abundances and nitrogen-to-oxygen abundance ratios; (iii) constraints on the local environment around each galaxy; and (iv) other measurements from the literature. We supplement the data with new observations where needed to complete the measurements for all galaxies in the sample. From these data, we find good agreement between stellar masses measured from color–magnitude diagrams and those estimated from 3.6 μ m imaging by assuming a mass-to-light ratio. We also provide the first mapping of the H i profiles as a function of structural parameters. These data sets and measurements are the foundation for the Galaxies Losing Oxygen via Winds (GLOW) project, whose main aim is to characterize the star formation−chemical enrichment cycle of low-mass galaxies by measuring the production, distribution, and retention of oxygen on a galaxy-by-galaxy basis.

In this work, we use a radiation-hydrodynamic simulation of a single ∼10 ⁹ M _⊙ virtual galaxy to study Si ii and C ii line profiles seen in stacked Hubble Space Telescope Cosmic Origins Spectrograph spectra of 58 galaxies from the LzLCS+ sample. We compare stacks across three mass bins ( M _⋆ ≤ 10 ⁸ , 10 ⁸ –10 ⁹ , and ≥10 ⁹ M _⊙ ) and three stacking methods (mean, median, weighted averaged) to a library of 22,500 mock spectra. We investigate if the simulation can accurately mimic real gas features, reveal clear trends with galaxy properties, and provide indirect estimates of the ionizing escape fraction ( f _esc ). We find reasonable agreement between simulated and observed profiles ( χ ² < 1) across all mass regimes. Notably, extracting line properties such as EW and R _f from best-fit mock profiles provides a robust alternative to direct empirical trends, particularly in the low-signal-to-noise regime where noise frequently biases results. The simulation-based low-ionization state features, although derived from a single virtual object, exhibit clear correlations with M _⋆ , star formation rate, and f _esc , mirroring established empirical scaling relations. We find that the best-matching mock spectra predominantly originate from simulation time steps corresponding to peak UV luminosity and intense starburst phases, suggesting these active periods generate the ISM diversity observed in star-forming galaxies. Finally, simulation-based estimates ( f esc virtual ) reproduce the observed mass-dependent trends in f _esc and are in close agreement with the average f _esc of the generated stacks. This simulation-based framework establishes a relevant methodology for interpreting spectroscopic observations, including inferring f _esc and characterizing physical scaling relations, in high-redshift galaxies from the Epoch of Reionization.

We report the detection of a previously unknown object with a single emission line within the Hubble Ultra Deep Field, using a new JWST NIRSpec technique called Multiobject Coronagraphy for Slitless Spectroscopy (MOCSS) with the G140M/F100LP observing mode. The MOCSS technique involves opening almost all the shutters on the NIRSpec microshutter array, while simultaneously closing the shutters that lie on top of known low-redshift objects, facilitating a wide-field slitless spectroscopic search without contamination from bright objects. This observational technique has enabled the detection of a strong emission line in four different dither positions and in two position angles, and thus cannot be artificial. This line is detected at 1.60 μ m with a line flux of [2.5 ± 0.3] × 10 ⁻¹⁸ erg s ⁻¹ cm ⁻² and an FWHM of 13.2 ± 2.0 Å, and does not correspond to any known source with some of the deepest Hubble Space Telescope and JWST imaging in existence. Based upon the absence of imaging detection, we place a lower limit on the observed frame equivalent width (EW) of 7620 Å on this object, which strongly limits the number of possible lines that this line can be. Given the wavelength, an EW limit that is near the ceiling of expectation, and the lack of other lines in the bandpass, this line cannot be easily explained by [O ii ] at z = 3.29, [O iii ] at z = 2.20, H α at z = 1.44, and Ly α at z = 12.16. We discuss possible active galactic nucleus and transient phenomena that could explain the finding.

The Treasury of Extremely Metal-Poor O Stars (TEMPOS) is a Hubble Space Telescope survey of hot and massive O-type stars in nearby, low-metallicity galaxies (≲20% of the solar metallicity, Z _⊙ ). Understanding massive-star physics in this regime is essential to interpret observations of metal-poor galaxies, including both low-mass dwarf galaxies and chemically unevolved galaxies in the early Universe. Yet, few far-ultraviolet (FUV) spectra of O stars of sufficient quality to characterize their fundamental properties and stellar winds exist below 20% Z _⊙ , and the heterogeneous observation design and incomplete coverage of parameter space pose significant barriers to progress. To remedy this, TEMPOS obtained new Cosmic Origins Spectrograph (COS) FUV spectra of 12 very metal-poor O stars, building upon archival data to assemble a spectroscopic atlas of 29 homogeneously observed stars that efficiently samples a wide range of spectral types and luminosity classes. Here, we describe the motivation, sample selection, and observation design for TEMPOS and present the first data release of reduced and coadded COS spectra. We then present initial results on the empirical properties of FUV O-star spectra below 20% Z _⊙ , including radial velocities, equivalent widths of photospheric lines, and terminal wind velocities ( v _∞ ). We show that v _∞ correlates with host galaxy metallicity across ∼5%–50% Z _⊙ and find tentative evidence of a steeper decline in wind strength below ∼10% Z _⊙ . The combined dataset of FUV spectra and planned releases of photometry and optical spectra from the TEMPOS Treasury program will advance our understanding of both stellar astrophysics and the interstellar medium in the extremely metal-poor regime.

We present a new determination of the primordial helium abundance based on new, high-quality Large Binocular Telescope (LBT) observations of 54 metal-poor H ii regions. These regions have been observed and analyzed uniformly. We also describe a number of updates to our methodology, including updated helium emissivities. Enabled by the large, high-quality dataset, we examine our sample targets for potential systematic errors, which could bias their results. We perform a standard 95% confidence level (CL) χ ² cut and find that a significantly larger fraction (47/54 = 87%) of our sample qualifies than for previous datasets. We also screen for quality and reliability, flagging targets that may introduce significant systematic errors, producing a dataset of 41 targets. In a significant breakthrough for the field, that dataset includes 15 high signal-to-noise ratio (SNR) targets with low metallicity (O/H ≤ 4 × 10 ⁻⁵ ). Due to this low-metallicity dataset, for the first time, a weighted average for determining the primordial helium abundance ( y _p ) is well justified and produces a robust result. By weighted average of our 15 low-metallicity targets, we determine y _p = 0.08146 ± 0.00058, with the corresponding mass fraction Y _p = 0.2458 ± 0.0013. This result achieves an unprecedented precision of 0.5%, and it is in good agreement with the Big Bang nucleosynthesis (BBN) result, Y _p = 0.2467 ± 0.0002, based on the Planck determination of the baryon density.

Accurately determining the elemental abundances of a low-metallicity nebula strongly depends on measuring the density ( n _e ) and temperature ( T _e ) of the gas. Because these two parameters are inherently degenerate when derived solely from H and He recombination lines, we rely on the density-sensitive He I λ 10830 line to assist in resolving this issue, especially for accurate He abundances. To facilitate this, we present near-IR (NIR) LBT Utility Camera in the Infrared (LUCI) spectra of 48 low-metallicity targets and homogeneously reduce them using Pypeit as part of the LBT Y _p Project. IR spectra require special care, and we wavelength calibrate by hand using the bright OH emission lines, carefully apply proper telluric corrections, and coadd the spectra of LUCI1 and LUCI2 on a resampled grid to ensure accurate results. We use a Gaussian profile to fit the emission lines and measure the fluxes relative to Paschen-gamma (P γ ), resulting in He I λ 10830 to P γ ratios consistent with previous studies. As a result, this work significantly expands the available dataset of NIR He I λ 10830 fluxes in low-metallicity galaxies. These high-quality measurements, where we find a median flux ratio uncertainty of σ ˜ = 0.08 , reduce the overall uncertainties in helium abundance estimates for individual targets. The increased size of the high-quality sample enables searching for systematic uncertainties and improves the reliability of the helium abundance determinations used to infer the primordial helium abundance.

Empirically measuring the primordial He mass fraction, Y _p , requires a significant number of low-metallicity nebulae with direct constraints on He/H and O/H abundances. This technique requires high-fidelity measurements of the gas-phase physical conditions, namely the electron temperature ( T _e ) and density ( n _e ). To this end, we present deep rest-optical spectroscopy for a sample of 62 low-metallicity (≲20% solar O/H) galaxies acquired using the Multi-Object Double Spectrographs on the LBT as part of the LBT Y _p Project. We discuss new fitting methods that recover the intensity of up to 61 H and He recombination lines, of which, up to 26 will be used to determine gas-phase He abundances, and we examine the emission line properties of the LBT Y _p Project sample. We assess different scaling relations in the low-metallicity interstellar medium (ISM), finding that n _e [Ar iv ] measured in 31 targets is systematically larger than n _e [S ii ] or n _e [O ii ]. The larger densities are insufficient to significantly bias T _e [O iii ] or the O/H abundance. T _e [S iii ] and T _e [O iii ] are strongly correlated over a range of ∼10 ⁴ K with very low scatter, and we calibrate new T _e [S iii ]– T _e [O iii ] scaling relations for use in other low-metallicity environments. We examine different T _e measured in the low-ionization gas, finding significant scatter compared to T _e [O iii ]. The precision direct O/H derived in this analysis (median uncertainty ∼4%) are consistent with prior literature measurements, albeit with relatively large scatter. These data provide a key component necessary to empirically measure Y _p and the abundance patterns of other elements in the ISM.

Extremely low-metallicity H ii regions have been observed with the goal of determining the primordial helium abundance ( Y _p ). Y _p , combined with standard Big Bang nucleosynthesis and the half-life of the neutron, provides a direct measurement of the number of neutrino families, but Y _p must be measured very precisely to provide meaningful constraints on physics beyond the Standard Model. Here we describe a program to combine new Large Binocular Telescope (LBT) observations with a new analysis methodology to significantly improve the determination of Y _p . The LBT, with its MODS and LUCI instruments, produces spectra, which, when combined with our new analysis methodology, are capable of delivering He abundances in individual H ii regions with uncertainties of approximately 2% or less. Archival LBT/MODS spectra of standard stars over a 4 yr period enable the determination of a wavelength-dependent uncertainty in the MODS spectral response, resulting in improved relative emission line uncertainties. An optimized sample of low-metallicity galaxies has been selected with the goal of producing a determination of Y _p with a precision of ∼0.5%, sufficient to provide an independent constraint on the effective number of neutrino families of ∼3%.