Nimisha Kumari

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.

Little red dots (LRDs) are compact, red sources discovered by JWST at high redshift ( z ≳ 4), marked by distinctive “V-shaped” spectral energy distributions (SEDs) and often interpreted as rapidly accreting active galactic nuclei (AGNs). Their true nature remains unclear though, and their evolutionary connection to their lower-redshift counterparts is still poorly constrained. Thus, we present WISEA J123635.56+621424.2 (here dubbed the Saguaro), a z = 2.0145 galaxy in GOODS-North, as a possible analog of high-redshift LRDs and a potential missing link in their evolutionary path toward lower-redshift systems. It features a compact LRD-like nucleus surrounded by a face-on spiral host. Its connections to LRDs include the following: (1) its nuclear spectrum shows a clear “V-shaped” SED, and (2) when redshifted to z = 7, surface brightness dimming makes the host undetectable, thus mimicking an LRD. This suggests that high-redshift LRDs may be embedded in extended hosts. To test this, we stack rest-frame UV images of 99 photometrically selected LRDs, revealing faint, diffuse emission. Stacking in redshift bins reveals mild radial growth, consistent with the expected galaxy size evolution. A simple analytic model confirms that surface brightness dimming alone can explain their compact appearance. Lastly, we show that the Saguaro is not unique by describing similar objects from the literature at z ≲ 3.5. Taken together, our results support a scenario in which LRDs may not be a distinct population, but could be the visible nuclei of galaxies undergoing a short-lived, (perhaps) AGN-dominated evolutionary phase, with their compact, red appearance driven largely by observational biases.

JWST has discovered an early period of galaxy formation that was more vigorous than expected, which has challenged our understanding of the early Universe. In this work, we present the longest spectroscopic integration ever acquired by JWST/MIRI ( t _obs ≈ 51 hr). This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at z = 14.18. Most notably, we detect [O III ] λλ 4959, 5007 at ≈14 σ and H α at ≈4 σ with these ultradeep observations. These lines reveal that JADES-GS-z14-0 has low dust attenuation with a recent star formation rate of SFR ≈ 8 ± 2 M _⊙ yr ⁻¹ , star formation rate surface density of Σ _SFR ≈ 20 ± 5 M _⊙ yr ⁻¹ kpc ⁻² , and ionizing photon production efficiency of ξ _ion ≈ 10 ^25.3±0.1 Hz erg ⁻¹ . Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of log 10 ( O / H ) + 12 ≈ 7.5 ± 0.2 (≈6% Z _⊙ ), carbon-to-oxygen ratio of [C/O] ≈ −0.4 ± 0.2, ionization parameter of log 10 ( U ) ≳ − 2.4 , and density of n _H ≈ 690 ± 200 cm ⁻³ . Using detailed photoionization modeling, we instead derive log 10 ( O / H ) + 12 ≈ 8 . 5 − 0.4 + 0.4 (≈60% Z _⊙ ), log 10 ( U ) ≈ − 1 . 4 − 0.4 + 0.3 , and n H ≈ 54 0 − 320 + 520 cm − 3 . The inferred properties of JADES-GS-z14-0 are similar to those measured for similarly luminous galaxies at z > 10 with previous MIRI/Spectroscopy, such as GHZ2/GLASSz12, GN-z11, and MACS0647-JD1. These results suggest extreme ionization conditions and rapid metal enrichment less than 300 Myr after the Big Bang. Existing simulations are unable to reproduce the empirical and inferred properties of JADES-GS-z14-0. This work demonstrates an important step toward understanding the formation of the first stars and heavy elements in the Universe. Future work will focus on the detection of the rest-frame optical continuum and its interpretation for the stellar population properties of JADES-GS-z14-0.

We present the analysis of the stellar and interstellar medium (ISM) properties of MARTA-4327, a star-forming galaxy at z = 2.223 observed by means of deep JWST/NIRSpec spectroscopy in both medium- and high-resolution gratings as part of the “Measuring Abundances at high Redshift with the Te Approach” (MARTA) program. We report one of the highest-redshift detections of the Wolf–Rayet (WR) blue and red bumps in a non-lensed system. The broad He II λ 4686 feature is consistent with a young (∼5−6 Myr) burst dominated by WN stars, although both stellar population synthesis models and empirical templates struggle to reproduce nitrogen stellar features at ≈4640 Å. Based on the relative strength of the available optical stellar features, we disfavor the presence of very massive stars (VMS) in this system. Elemental abundance ratios such as Ne/O, N/O, and Ar/O align with observations of local star-forming galaxies (including WR galaxies), suggesting that any impact of the WR population on the chemical enrichment of the ISM is strongly localized. However, the gas-phase Fe/O ratio appears enhanced compared to local galaxies of similar metallicity, which we interpret as evidence for reduced Fe depletion onto dust grains, possibly linked to localized destruction in WR-driven wind environments. In addition, we detect a broad (∼445 km s ⁻¹ ) and blueshifted (∼70 km s ⁻¹ ) H α component, suggesting the presence of an ionized outflow with a mass loading factor η ∼ 0.2. Finally, we report the robust detection of O I λ 8446 emission (among the firsts at high redshift), which we interpret as originating from Ly β fluorescence and/or collisional excitation in dense clumps. Overall, MARTA-4327 represents a unique system for studying the role of massive stars in shaping the ISM properties of galaxies at Cosmic Noon.

The abundance discrepancy problem refers to the systematic differences observed between chemical abundances derived from collisionally excited lines (CELs) and recombination lines (RLs) of heavy ions. It remains a major unsolved problem in the study of ionized nebulae and is quantified by the abundance discrepancy factor (ADF). In this work, we present a deep integral field spectroscopic data set of the entire Lagoon Nebula (M8), obtained by the Sloan Digital Sky Survey V Local Volume Mapper project, at a spatial resolution of 0.21 pc spaxel ⁻¹ . This unique data set allows us, for the first time, to investigate spatially resolved maps of oxygen RL intensities (O ii V1), together with maps of H i RLs, heavy-ion CELs, and dust attenuation across a whole H ii region. We map the electron temperature using CELs and RLs of O ²⁺ and CELs of N ⁺ , and we map the electron density using CELs of S ⁺ . We derive CEL-based ionic and elemental oxygen abundances and, for the first time, a spatially resolved map of the RL-based O ²⁺ abundance in an H ii region. These measurements enable construction of the first spatially resolved ADF(O ²⁺ ) map of an H ii region and yield a global mean ADF of ∼0.47 ± 0.02 dex. Focusing on the central region of M8, where ionization is dominated by the O-type star Her 36, we find radial variations in the ADF, ranging between ∼0.35 and 0.50 dex. Our findings provide novel constraints on the spatial behavior and origin of the abundance discrepancy in the H ii regions.

We present an overview of the James Webb Space Telescope (JWST) Advanced Deep Extragalactic Survey (JADES), an ambitious program of infrared imaging and spectroscopy in the GOODS-S and GOODS-N deep fields, designed to study galaxy evolution from high redshift to cosmic noon. JADES uses about 770 hr of Cycle 1 guaranteed time largely from the Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec) instrument teams. In GOODS-S, in and around the Hubble Ultra Deep Field and Chandra Deep Field South, JADES produces a deep imaging region of ∼42 arcmin ² with over 100 hr of exposure time spread over nine NIRCam filters, including two medium-band filters. This is extended at medium depth in GOODS-S and GOODS-N with NIRCam imaging of ∼167 arcmin ² , averaging 25 hr of exposure over 8–10 filters. In both fields, we conduct extensive NIRSpec multiobject spectroscopy, including two deep pointings of 55 hr exposure time, 14 medium pointings of ∼12 hr, and 15 shallower pointings of ∼4 hr, targeting over 5000 Hubble Space Telescope– and JWST-detected faint sources with five low-, medium-, and high-resolution dispersers covering 0.6–5.3 μ m. Finally, JADES extends redward via coordinated parallels with the JWST Mid-Infrared Instrument, featuring ∼10 arcmin ² with 43 hr of exposure at 7.7 μ m and thrice that area with 1.4–6.8 hr of exposure at 12.8 and 15 μ m. For nearly 30 yr, the GOODS-S and GOODS-N fields have been developed as the premier deep fields on the sky; JADES is now providing a compelling start on JWST's legacy in these fields.