Zhiyuan Ji

We present a statistical census of the Na D–traced neutral interstellar medium (ISM) and outflows in 309 galaxies at 0.6 < z < 4 using JWST/NIRSpec medium-resolution spectroscopy from the SMILES, JADES, Blue Jay, and Aurora surveys. After subtracting the stellar continuum, we model Na D λλ 5890, 5896 and identify 73 Na D ISM absorptions, including 63 robust (4 in broad-line active galactic nuclei, AGN) and 10 tentative detections. Of the robust detections, 88% are found in massive galaxies ( log ( M * / M ⊙ ) > 10 ), and 12% are in lower-mass systems. At high mass, ISM absorption is seen in both star-forming and quiescent galaxies, whereas in lower-mass systems, it is observed only in star-forming galaxies. In massive quiescent galaxies, Na D detectability appears linked to star formation history: it is preferentially detected in older systems with larger 4000 Å breaks and younger, rapidly quenching galaxies with strong Balmer absorption H δ _A . We identify Na D outflows in 25 galaxies, revealing a possible dichotomy in driving mechanisms between star-forming and quiescent galaxies. In star-forming galaxies, outflow properties correlate with star formation properties, consistent with a star-formation-driven origin. In quiescent galaxies, however, outflows are not associated with residual star formation and often require more energy than such star formation can provide. Together with the high AGN fraction among outflow-detected quiescent galaxies, this suggests that AGN dominate Na D–traced neutral outflows in cosmic noon quiescent systems. We further identify four quiescent galaxies with possible AGN fossil outflows, suggesting that AGN-driven outflows can persist beyond the active accretion phase and may help maintain quiescence.

We measure the number density and field-to-field variance of massive quiescent galaxies at z ∼ 3–8 using the JWST/NIRCam pure-parallel imaging survey PANORAMIC together with archival observations, covering an area of 0.28 deg ² (∼1000 arcmin ² ) in at least six filters. We identify quiescent galaxy candidates at z ≳ 3 with M _* ≳ 10 ¹⁰ M _⊙ , comprising 101 galaxies in a gold sample of high-confidence candidates and 137 in a more inclusive silver sample. We measure their evolving comoving number density, finding (1.5 versus 3.1) × 10 ⁻⁵ Mpc ⁻³ at z = 3–4 for the gold and silver samples, respectively, and a decline by more than a factor of 20 by z ∼ 6. Comparisons with empirical models and cosmological simulations show that widely used frameworks underpredict the abundance of massive quiescent galaxies at z ≳ 4 by ≳1 dex, indicating that current implementations of early star formation, feedback, and quenching do not produce enough early quenched systems. With 34 independent sight lines, we present the first direct empirical measurement of field-to-field variance for quiescent galaxies at z > 3, finding a high cosmic variance of σ _CV ≈ 0.7 ± 0.3. This exceeds predictions from abundance-matched mock catalogs, suggesting that early quiescent galaxies are more strongly clustered, and more likely to be found near one another or in more biased regions, than expected in current galaxy-formation models. Any successful model for the emergence of early massive quiescent galaxies must reproduce both their abundance evolution and their imprint on the large-scale distribution.

Models predict that chemical enrichment and gas redistribution should begin rapidly once star formation starts, but direct constraints at the earliest epochs have been scarce. Here we show that metal-enriched gas in multiple ionic phases was already present around galaxies before the midpoint of cosmic reionization. Using JWST/NIRSpec rest-frame ultraviolet spectroscopy from SPURS, we detect blueshifted metal absorption in three galaxies at 7.2 < z < 9.3. The detected transitions span neutral, low-ionization and high-ionization species, including O I, Si II, C II, Si IV and C IV, with velocity offsets of ∣Δ v ∣ ≈ 50–250 km s ⁻¹ relative to nebular systemic redshifts. The ionic coexistence, overlapping velocity structure and equivalent-width ratios are consistent with outflowing or otherwise kinematically disturbed galaxy-associated gas, implying rapid metal enrichment. These results show that key conditions for baryon cycling were established in at least a subset of luminous galaxies within the first several hundred million years of cosmic time, well before the completion of reionization.

Little red dots (LRDs) appear extremely compact at rest-frame optical wavelengths, yet many show extended rest-frame UV morphology revealing more complex internal structure. We present a combined analysis of the Very Large Telescope/MUSE rest-frame UV integral-field spectroscopy and continuum-subtracted [O iii ], H β , and H α +[N ii ] emission-line maps from JWST/NIRCam imaging at subkiloparsec resolution for LRD-204851 at z = 5.482 in GOODS-S. We find that LRD-204851 hosts a remarkably thin, bipolar, elongated structure passing through the optical continuum centroid and extending several kiloparsecs on either side, traced by both the UV continuum and the rest-frame optical emission lines, with a bright [O iii ] clump-like structure ∼2 kpc to the southeast of the centroid. The MUSE observations reveal a double-peaked Ly α profile, with a broad and bright near-systemic red peak and a relatively faint peak blueshifted by ∼430 km s ⁻¹ , accompanied by a tentative N v λ 1238 detection at similar velocity. In narrowband imaging extracted from the MUSE integral-unit-field cube, both the blue Ly α peak and the tentative N v emission lean toward this same southeastern direction. Independently, radiative-transfer modeling of the integrated Ly α profile favors a biconical low-column-density cavity in a dense, slowly expanding neutral envelope, in support of the bipolar geometry traced by the line maps. Together, these results suggest that the elongated emission of LRD-204851 is connected to radiation and/or gas flow from its central engine through a low-column-density channel with a small opening angle that may trace either a slow outflow or a quasi-static ionization cone. LRD-204851 is one of the first LRDs where the central engine’s impact on its host galaxy is potentially directly observable on kiloparsec scales.

We report the discovery of a galaxy protocluster candidate (dubbed MACS0416-OD-z8p5) at a spectroscopic redshift of z ∼ 8.47, dating back to ∼550 Myr after the Big Bang. The observations are part of the JWST Cycle 3 Treasury Program Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (or SAPPHIRES) using NIRCam grism observations. Using wide-field slitless spectroscopy (WFSS) obtained in the MACS0416 parallel field, we robustly confirm nine galaxies at z _spec ∼ 8.47 via emission-line detections of [O iii ] 5008 Å (with >5 σ ) and tentatively confirm one additional galaxy (at ∼3 σ ). This discovery represents the highest-redshift spectroscopically confirmed galaxy overdensity known to date and is ∼6–8 times more dense than the average number density of galaxies at the same redshift. Furthermore, a galaxy hosting a low-mass active galactic nucleus (a “little red dot”) is found as a member, suggesting that black hole growth may already be underway in dense environments at early cosmic times. This finding of a massive dark matter halo hosting a galaxy overdensity at z ∼ 8.5 is surprising given that our survey covered only a small, random field ( 16.5 arcmin 2 ) as part of a pure-parallel observation. Comparison with cosmological simulations shows that the likelihood of finding such a large-scale structure is <5% under the current galaxy formation scenario and within the observed survey volume. Our results demonstrate the power of WFSS observations for building complete samples of line-emitting galaxies and suggest an important role for overdensities in enhancing galaxy formation by funneling large-scale gas supplies into small cosmological volumes.

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.

Our understanding of the physical mechanisms and environments conducive to the escape of Lyman continuum (LyC) radiation within the first 2 Gyr of cosmic history remains limited. Here, we present a detailed analysis of JWST NIRSpec medium-resolution IFU observations of two LyC-leaker candidates, LACES-94460 and LACES-104037 at z = 3.1, selected from deep Hubble Space Telescope/WFC3 F336W imaging and supported by ground-based spectroscopy. We first rule out LACES-94460 as a genuine LyC leaker, demonstrating that its apparent F336W signal originates from a nearby low-redshift interloper at z = 1.6, unambiguously identified through IFU spectroscopy. In contrast, for LACES-104037, we spectroscopically confirm bona fide LyC emission arising from a tidal-tail structure during the early stage of a galaxy merger, dubbed LACES104037-LyC. LACES104037-LyC exhibits extremely low rest-frame optical emission-line equivalent widths together with an exceptionally strong LyC flux. Within a picket-fence model framework, we reproduce its observed spectral and photometric properties with a young stellar population of age ∼5 Myr and an LyC escape fraction of f_esc ∼ 99%. Our identification and detailed modeling of LACES104037-LyC provide one of the first compelling observational demonstrations for merger-driven LyC escape, indicating that galaxy mergers may represent an important and previously underappreciated contributor to the ionizing photon budget relevant for cosmic reionization. Furthermore, our analysis highlights the critical role of subkiloparsec resolution spectroscopy in securely identifying LyC leakers, removing contamination from closely projected low-redshift interlopers, and pinpointing the physical regions responsible for LyC leakage.

Our understanding of the physical properties of star-forming galaxies during the Epoch of Reionization (EoR; at z > 6) suffers from degeneracies among the apparent properties of the stars, nebular gas, and dust. These degeneracies are most prominent with photometry, which has insufficient (1) spectral resolution and (2) rest-frame spectral coverage. We explore ways to break these degeneracies with a sample of N = 22 high-redshift star-forming galaxies at 7 < z _phot ≤ 9, using some of the deepest existing imaging from JWST/NIRCam and JWST/MIRI with JADES. Key to this study is the imaging from JWST/MIRI at 7.7 μ m, which provides coverage of the rest-frame I band at the observed redshifts. We infer stellar population properties and rest-frame colors using a variety of filter sets and star formation history (SFH) assumptions to explore the impact of these choices. Evaluating these quantities both with and without the 7.7 μ m data point shows that dense spectral coverage with JWST/NIRCam (eight or more filters, including at least one medium-band at λ _obs ≈ 4–5 μ m) can compensate for lacking the rest-frame I -band coverage for the vast majority (≈80%) of our sample. Furthermore, these galaxy properties are most consistently determined by assuming the delayed-tau SFH, which provides the smallest offsets and scatters around these offsets when including JWST/MIRI. Within extragalactic surveys like JADES and CEERS, our findings suggest that robust characterization of the stellar population properties and rest-frame colors for typical high-redshift star-forming galaxies (i.e., galaxies with M _UV ≈ −20 and β _UV ≈ −2) is possible with JWST/NIRCam alone at z ≈ 8.

We report a galaxy overdensity candidate at z ≈ 10.5 in the JWST Advanced Deep Extragalactic Survey. This overdensity contains 18 galaxies with consistent photometric redshifts within 8 comoving Mpc in projection. The galaxy number density is 4 times higher than the field expectation, accounting for one-third of comparably bright galaxies and nearly 50% of the total star formation rate (SFR) at 10 < z _phot < 12 in the GOODS-S field. Galaxies in the overdensity more frequently have close companions or substructure, with one-third showing such features within 1 kpc at consistent photometric redshifts, implying enhanced interactions. Most galaxies have stellar masses of 0.6–3 × 10 ⁸ M _⊙ , half-light radii of ∼200 pc, and SFRs of ∼5 M _⊙ yr ⁻¹ . Their stellar masses and SFRs are slightly higher than those of field galaxies, but remain broadly consistent with typical high-redshift scaling relations. Two compact objects show possible Balmer breaks, suggestive of evolved stellar populations or little red dots. We find tentative evidence for a spatially varying Ly α transmission inferred photometrically, consistent with an emerging ionized bubble. This overdensity provides a rare opportunity for probing the environmental impact on galaxy evolution and the onset of cosmic reionization within the first 500 Myr.

Observational campaigns with JWST have revealed a higher-than-expected abundance of UV-bright galaxies at z ≳ 10, with various proposed theoretical explanations. A powerful complementary constraint to break degeneracies between different models is galaxy clustering. In this paper, we combine PANORAMIC pure parallel and legacy imaging along 34 independent sight lines to measure the cosmic variance ( σ _CV ) in the number counts of Lyman break galaxies at z ∼ 10, which is directly related to their clustering strength. We find σ CV = 0.9 6 − 0.18 + 0.20 , 1.4 6 − 0.44 + 0.54 , and 1.7 1 − 0.59 + 0.72 per NIRCam pointing ( ∼ 9.7 arcmin 2 , ≲1.5 pMpc at z ∼ 10) for galaxies with M _UV < −19.5, −20, and −20.5. Comparing to galaxies in the fiducial UniverseMachine , we find that σ _CV is consistent with our measurements, but that the number densities are a factor ≳5 lower. We implement simple models in the UniverseMachine that represent different physical mechanisms to enhance the number density of UV-bright galaxies. All models decrease σ _CV by placing galaxies at fixed M _UV in lower-mass halos, but to varying degrees. Combined constraints on σ _CV and the UV luminosity function thus tentatively disfavor models that globally increase the star formation efficiency (SFE) or the scatter in the M _UV – M _halo relation, while models that decrease the mass-to-light ratio, or assume a power-law scaling of the SFE with M _halo agree better with the data. We show that with sufficient additional independent sight lines, robust discrimination between models is possible, paving the way for powerful constraints on the physics of early galaxy evolution through NIRCam pure parallel imaging.