Xinfeng Xu

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.

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 a sample of 227 broad-line active galactic nuclei (BLAGNs), incorporating 148 newly identified sources, spanning a redshift interval from z = 0.8 to 7.2. We analyze spectroscopic data from the NIRSpec instrument on board the James Webb Space Telescope, using the G140H, G140M, G235H, G235M, G395H, and G395M gratings to survey N ∼ 80,000 galaxies for BLAGNs. Through emission-line fitting, using a sum of Gaussian models for H α , H β , [N ii ] λλ 6548, 6584, and [O iii ] λλ 4959, 5007, we separate active galactic nucleus (AGN) broad-line components from narrow-line emission. We find the detection rate of BLAGNs to be relatively consistent across our redshift range. Compared to typical low- z AGNs ( z ≲ 1), high- z BLAGNs are systematically fainter and less massive, yet they accrete more efficiently, with most showing Eddington ratios between 0.1 and 1.0. This confirms rapid black hole growth during the early cosmic epochs. The detection of faint, low-mass BLAGNs at high redshift also helps bridge the observational gap between local supermassive black holes and remote luminous quasars, providing a more complete view of black hole–galaxy coevolution across cosmic time. We also identify 48 known little red dots (LRDs) in our BLAGN sample. For these LRDs, we find a systematic preference for exponential over Gaussian broad-line profiles, which are consistent with electron scattering rather than virial broadening. This implies that standard virial black hole masses for LRDs may be overestimated by up to 2 orders of magnitude.

We investigate galaxies in the Galaxies at All Redshifts Deciphered and Explained with the NIRSpec microshutter array (MSA; GARDEN) survey that exhibit auroral emission lines, enabling spatially resolved measurements of electron temperature and direct oxygen abundances. Two galaxies in this survey have spectra suitable for this analysis: CANDELS 8005 at z = 3.794 and CANDELS 7986 at z = 4.702. For both galaxies, we measure auroral and key nebular emission-line fluxes across their full extent, allowing direct-method oxygen abundance determinations in individual spatial pixels (spaxels). These observations demonstrate the viability of deep JWST/NIRSpec MSA spectroscopy for spatially resolved chemical analyses at high redshift, aided by weak nebular continua and low interstellar extinction. We derive global direct abundances of log(O/H)+12 = 8.008 − 0.027 + 0.025 for CANDELS 8005 and 7.89 − 0.028 + 0.027 for CANDELS 7986. Emission-line diagnostics indicate neither galaxy hosts an active galactic nucleus. A first-order kinematic analysis suggests a potential merger in CANDELS 8005. The direct abundances are consistent with strong-line estimates based on our data and recent high-redshift calibrations. We build emission line, radial velocity, strong-line abundance indices, electron temperature, and direct abundance maps for both galaxies, thanks to the excellent spatial resolution. From the direct abundance maps, we measure linear radial metallicity gradients of − 0.11 1 − 0.025 + 0.026 dex kpc ⁻¹ for CANDELS 8005 (statistically significant) and −0.0928 ± 0.0880 dex kpc ⁻¹ for CANDELS 7986, where the large uncertainties limit the significance of the result. These results provide a rare direct measurement of a radial metallicity gradient at z > 0 from direct-method abundances, offering key observational support for inside-out galaxy growth with feedback-regulated chemical enrichment.

Galactic winds shape galaxy evolution; however, the outflowing gas is complex: it consists of multiple ionization phases, and its properties vary spatially. Therefore, methods that combine high-fidelity observations with state-of-the-art galactic-wind models are limited. Here we investigate methods for fitting the column density profiles derived from high-quality outflow observations with the multiphase, multiscale wind model from D. B. Fielding & G. L. Bryan. We identify three key outflow parameters: the initial hot-phase mass-loading factor ( η _M,hot,0 ), the initial cool-phase mass-loading factor ( η _M,cool,0 ), and the initial cool-cloud mass ( M _cl,0 ). We obtain good fits (reduced χ ² < 1.5) for most galaxies, with tight constraints on η _M,cool,0 and moderate constraints on the other two parameters. We find the inferred η _M,cool,0 and η _M,hot,0 are mostly of order unity, with significant scatter. The constraints on η _M,hot,0 suggest that the interaction between the cool and hot phases allow us to indirectly constrain the properties of the hot wind from cool-outflow observations. The model also predicts various radial trends. First, for all galaxies, the cool-phase outflow velocity increases rapidly between 1 and 2 r ₅₀ , then reaches a plateau, where r ₅₀ is the half-light radius of the galaxy. Second, most galaxies exhibit increasing η _M,cool and decreasing η _M,hot with radius, with a few showing the reverse trends. These results should be interpreted as effective, model-conditional constraints but are consistent with other recent multiphase simulations and observations. This highlights that the velocity–radius mapping encoded in UV absorption profiles enables recovery of outflow spatial structures from spatially integrated spectra. Our method paves the way for future broad parameter studies and guides updates of outflow simulations in future work.

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.

One of the current challenges in galaxy evolution studies is to establish the mechanisms that govern the escape of ionizing radiation from galaxies. Here, we investigate the connection between Lyman continuum (LyC) escape and the conditions of the circumgalactic medium (CGM), as probed by Ly α halos (LAHs) in emission. We use Ly α and UV continuum imaging data from the Ly α and Continuum Origins Survey (LaCOS), targeting 42 nearby ( z ≃0.3), star-forming galaxies with LyC observations (escape fractions of f esc LyC ≃ 0.01 − 0.49 ). LaCOS galaxies show extended Ly α emission ubiquitously, with LyC emitters (LCEs) having more compact Ly α morphologies than non-LCEs, and Ly α spatial offsets that do not exceed the extent of the UV continuum. We model the diffuse LAHs using a combined Sérsic plus exponential 2D profile, and find that the characteristic scale length of the Ly α halo is 10 times larger than the UV, on average. We unveil a significant anticorrelation between f esc LyC and the Ly α halo fraction (HF; or contribution of the halo to the total Ly α luminosity), which we propose as a new LyC indicator. Our observations show that halo scale lengths and HFs both scale positively with the optical depth of the neutral gas in the interstellar medium, revealing a picture in which Ly α and LyC photons in LCEs either emerge directly from the central starbursts or escape isotropically, and in the case of Ly α , minimize the number of scattering interactions in a less-extended CGM.

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.