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We analyze the rest-frame near-UV and optical nebular spectra of three z > 7 galaxies from the Early Release Observations taken with the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST). These three high-z galaxies show the detection of several strong emission nebular lines, including the temperature-sensitive [O iii] λ4363 line, allowing us to directly determine the nebular conditions and abundances for O/H, C/O, and Ne/O. We derive O/H abundances and ionization parameters that are generally consistent with other recent analyses. We analyze the mass–metallicity relationship (i.e., slope) and its redshift evolution by comparing between the three z > 7 galaxies and local star-forming galaxies. We also detect the C iii] λλ1907, 1909 emission in a z > 8 galaxy from which we determine the most distant C/O abundance to date. This valuable detection of log(C/O) = −0.83 ± 0.38 provides the first test of C/O redshift evolution out to high redshift. For neon, we use the high-ionization [Ne iii] λ3869 line to measure the first Ne/O abundances at z > 7, finding no evolution in this α-element ratio. We explore the tentative detection of [Fe ii] and [Fe iii] lines in a z > 8 galaxy, which would indicate a rapid buildup of metals. Importantly, we demonstrate that properly flux-calibrated and higher-S/N spectra are crucial to robustly determine the abundance pattern in z > 7 galaxies with NIRSpec/JWST.

The dispersion in chemical abundances provides a very strong constraint on the processes that drive the chemical enrichment of galaxies. Due to its proximity, the spiral galaxy M33 has been the focus of numerous chemical abundance surveys to study the chemical enrichment and dispersion in abundances over large spatial scales. The CHemical Abundances Of Spirals project has observed ∼100 H ii regions in M33 with the Large Binocular Telescope (LBT), producing the largest homogeneous sample of electron temperatures (T e ) and direct abundances in this galaxy. Our LBT observations produce a robust oxygen abundance gradient of −0.037 ± 0.007 dex kpc−1 and indicate a relatively small (0.043 ± 0.015 dex) intrinsic dispersion in oxygen abundance relative to this gradient. The dispersions in N/H and N/O are similarly small, and the abundances of Ne, S, Cl, and Ar relative to O are consistent with the solar ratio as expected for α-process or α-process-dependent elements. Taken together, the ISM in M33 is chemically well-mixed and homogeneously enriched from inside out, with no evidence of significant abundance variations at a given radius in the galaxy. Our results are compared to those of the numerous studies in the literature, and we discuss possible contaminating sources that can inflate abundance dispersion measurements. Importantly, if abundances are derived from a single T e measurement and T e –T e relationships are relied on for inferring the temperature in the unmeasured ionization zone, this can lead to systematic biases that increase the measured dispersion up to 0.11 dex.

Observations of low-ionization state metal lines provide crucial insights into the interstellar medium (ISM) of galaxies, yet, disentangling the physical processes responsible for the emerging line profiles is difficult. This work investigates how mock spectra generated using a single galaxy in a radiation-hydrodynamical simulation can help us interpret observations of a real galaxy. We create 22,500 C ii and Si ii spectra from the virtual galaxy at different times and through multiple lines of sight and compare them with the 45 observations of low-redshift star-forming galaxies from the COS Legacy Spectroscopic SurveY (classy). We find that the mock profiles provide accurate replicates of the observations of 38 galaxies with a broad range of stellar masses (106–109 M ⊙) and metallicities (0.02–0.55 Z ⊙). Additionally, we highlight that aperture losses explain the weakness of the fluorescent emission in several classy spectra and must be accounted for when comparing simulations to observations. Overall, we show that the evolution of a single simulated galaxy can produce a large diversity of spectra whose properties are representative of galaxies of comparable or smaller masses. Building upon these results, we explore the origin of the continuum, residual flux, and fluorescent emission in the simulation. We find that these different spectral features all emerge from distinct regions in the galaxy’s ISM, and their characteristics can vary as a function of the viewing angle. While these outcomes challenge simplified interpretations of down-the-barrel spectra, our results indicate that high-resolution simulations provide an optimal framework to interpret these observations.

The tip of the red giant branch (TRGB) is a standardizable candle, identifiable as the discontinuity at the bright extreme of the red giant branch (RGB) stars in color–magnitude diagram space. The TRGB-based distance method has been used to measure distances to galaxies out to D ≤ 20 Mpc with the Hubble Space Telescope F814W filter, and is an important rung in the distance ladder to measure the Hubble constant, H 0. In the infrared (IR), the TRGB apparent magnitude ranges from 1–2 mag brighter than in the optical. Now with the James Webb Space Telescope (JWST), the feasible distance range of the TRGB method can reach ∼50 Mpc. However, the IR TRGB luminosity depends to varying degrees on stellar metallicity/age. Here we standardize the TRGB luminosity using stellar colors as a proxy for metallicity/age to derive color-based corrections for the JWST Near-Infrared Camera short-wavelength filters F090W, F115W, and F150W, and the long-wavelength filters F277W, F356W, and F444W. We provide recommended filters for distance measurements depending on the requisite precision. For science requiring high precision (≤1% in distance), we recommend measuring the TRGB in F090W versus F090W − F150W or F115W versus F115W − F277W with the caveat that even with JWST, long integration times will be necessary at farther distances. If lower precision (>1.5% in distance) can be tolerated, or if shorter integration times are desirable, we recommend measuring the TRGB in either F115W or F150W. We do not recommend F444W for precision TRGB measurements due to its lower angular resolution.

The tip of the red giant branch (TRGB) based distance method in the I band is one of the most efficient and precise techniques for measuring distances to nearby galaxies (D ≲ 15 Mpc). The TRGB in the near-infrared (NIR) is 1–2 mag brighter relative to the I band, and has the potential to expand the range over which distance measurements to nearby galaxies are feasible. Using Hubble Space Telescope (HST) imaging of 12 fields in eight nearby galaxies, we determine color-based corrections and zero-points of the TRGB in the Wide Field Camera 3 IR (WFC3/IR) F110W and F160W filters. First, we measure TRGB distances in the I band equivalent Advanced Camera System (ACS) F814W filter from resolved stellar populations with the HST. The TRGB in the ACS F814W filter is used for our distance anchor and to place the WFC3/IR magnitudes on an absolute scale. We then determine the color dependence (a proxy for metallicity/age) and zero-point of the NIR TRGB from photometry of WFC3/IR fields that overlap with the ACS fields. The new calibration is accurate to ∼1% in distance relative to the F814W TRGB. Validating the accuracy of the calibrations, we find that the distance modulus for each field using the NIR TRGB calibration agrees with the distance modulus of the same field as determined from the F814W TRGB. This is a JWST preparatory program, and the work done here will directly inform our approach to calibrating the TRGB in JWST NIRCam and NIRISS photometric filters.

We provide one of the most comprehensive metallicity studies at z ∼ 4 by analyzing the UV/optical Hubble Space Telescope photometry and rest-frame Very Large Telescope (VLT)/FORS2 UV and VLT/XSHOOTER optical spectra of J0332−3557, a gravitationally lensed galaxy magnified by a factor of 20. With a 5σ detection of the auroral O iii] λ1666 line, we are able to derive a direct gas metallicity estimate for our target. We find Z gas =12+log(O/H)=8.26±0.06 , which is compatible with an increase of both the gas fraction and the outflow metal loading factor from z ∼ 0 to z ∼ 4. J0332−3557 is the most metal-rich individual galaxy at z ∼ 4 for which the C/O ratio has been measured. We derive a low log(C/O) = −1.02 ± 0.2, which suggests that J0332−3557 is in the early stages of interstellar medium carbon enrichment driven mostly by massive stars. The low C/O abundance also indicates that J0332−3557 is characterized by a low star formation efficiency, higher yields of oxygen, and longer burst duration. We find that EWC III]1906,9 is as low as ∼3 Å, and the main drivers of the low EWC III]1906,9 are the higher gas metallicity and the low C/O abundance. J0332−3557 is characterized by one diffuse and two more compact regions ∼1 kpc in size. We find that the carbon emission mostly originates in the compact knots. Our study on J0332−3557 serves as an anchor for studies investigating the evolution of metallicity and C/O abundance across different redshifts.

We present the first star formation history (SFH) and age–metallicity relation (AMR) derived from resolved stellar populations imaged with the JWST NIRCam instrument. The target is the Local Group star-forming galaxy WLM at 970 kpc. The depth of the color–magnitude diagram (CMD) reaches below the oldest main sequence turnoff with a signal-to-noise ratio = 10 at M F090W = + 4.6 mag. This is the deepest CMD for any galaxy that is not a satellite of the Milky Way. We use Hubble Space Telescope (HST) optical imaging that overlaps with the NIRCam observations to directly evaluate the SFHs derived based on data from the two great observatories. The JWST and HST-based SFHs are in excellent agreement. We use the metallicity distribution function measured from stellar spectra to confirm the trends in the AMRs based on the JWST data. Together, these results confirm the efficacy of recovering an SFH and AMR with the NIRCam F090W−F150W filter combination, and validate the sensitivity and accuracy of stellar evolution libraries in the near-infrared relative to the optical for SFH recovery work. From the JWST data, WLM shows an early onset to star formation, followed by an extended pause post-reionization before star formation reignites, which is qualitatively similar to what has been observed in the isolated galaxies Leo A and Aquarius. Quantitatively, 15% of the stellar mass formed in the first Gyr, while only 10% formed over the next ∼5 Gyr. The stellar mass then rapidly doubled in ∼2.5 Gyr, followed by constant star formation over the last ∼5 Gyr.

We use deep Spitzer mid-infrared spectroscopic maps of radial strips across three nearby galaxies with well-studied metallicity gradients (M101, NGC 628, and NGC 2403) to explore the physical origins of the observed deficit of polycyclic aromatic hydrocarbons (PAHs) at subsolar metallicity (i.e., the PAH–metallicity relation or PZR). These maps allow us to trace the evolution of all PAH features from 5–18 μm as metallicity decreases continuously from solar (Z ⊙) to 0.2 Z ⊙. The total PAH-to-dust luminosity ratio remains relatively constant until reaching a threshold of ∼ 2/3 Z ⊙, below which it declines smoothly but rapidly. The PZR has been attributed to preferential destruction of the smallest grains in the hard radiation environments found at low metallicity. In this scenario, a decrease in emission from the shortest-wavelength PAH features is expected. In contrast, we find a steep decline in long-wavelength power below Z ⊙, especially in the 17 μm feature, with the shorter-wavelength PAH bands carrying an increasingly large fraction of power at low metallicity. We use newly developed grain models to reproduce the observed PZR trends, including these variations in fractional PAH feature strengths. The model that best reproduces the data employs an evolving grain size distribution that shifts to smaller sizes as metallicity declines. We interpret this as a result of inhibited grain growth at low metallicity, suggesting continuous replenishment in the interstellar medium is the dominant process shaping the PAH grain population in galaxies.

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 (ne = 102–109 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 (ne ∼ 102 cm−3) in high-density environments (ne ∼ 105 cm−3) can bias nebular diagnostics by overestimating Te (up to 1800 K), overpredicting logU (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 logU 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 ne and star formation rate surface density (ΣSFR), suggesting that N/O is temporarily enhanced in compact, high-pressure environments. However, the ne evolution with z is more gradual than the (1 + z)3 scaling of virial halo densities, suggesting that ne 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.

Far-ultraviolet (FUV; ∼1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before; however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N1500 Å ≳ 5/resel), high-resolution (R ∼ 15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < log M ⋆(M ⊙) < 10.1), star formation rate (−2.0 < log SFR (M ⊙ yr−1) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O32 < 38.0), reddening (0.02 < E(B − V) < 0.67), and nebular density (10 < n e (cm−3) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with the z ∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to z ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.