Peixin Zhu

The growth of galaxies and their central supermassive black holes is closely connected, yet the net effect of active galactic nucleus (AGN) feedback on host-galaxy star formation remains uncertain. AGN may enhance, suppress, or have little measurable impact on star formation, but distinguishing among these outcomes requires separating star formation and AGN photoionization from shock excitation, which is expected in AGN-driven outflows but has been difficult to isolate. Here we apply a recently developed theoretical three-dimensional diagnostic diagram, designed to separate star formation, AGN, and shock excitation, to Very Large Telescope/MUSE integral field unit observations of nine nearby ( z < 0.026) Type 2 Seyfert galaxies. We find a common excitation pattern across the sample: star-forming rings or arcs at projected radii of r ∼ 0.8–6 kpc, AGN-photoionized bicones extending to kiloparsec scales, central fast-shock-dominated regions that often extend perpendicular to the AGN bicone, and pure-shock-dominated regions surrounding the central fast shocks and appearing locally within the star-forming rings. Deep Chandra X-ray morphology independently supports this decomposition. The circumnuclear star-forming rings are consistent with bar-driven resonances, although positive AGN feedback may also contribute. The central fast shocks are broadly consistent with AGN jet–interstellar medium (ISM) interactions, while AGN wind–ISM interactions may also play an important role in galaxies with low-power jets. These results establish central shocks as a common feature of Seyfert galaxies and demonstrate the importance of accounting for shock excitation in AGN feedback studies.

Small-scale multiphase structure plays a central role in galactic-wind evolution, yet the parsec-scale morphology and excitation of the warm ionized gas remain poorly constrained. We present deep Hubble Space Telescope (HST) narrow-band imaging of the southern wind of Messier 82 (M82) in H α , [O iii ], [S ii ], and [N ii ], designed to resolve the warm ionized phase on parsec scales. The H α emission is detected to ≈2.1 kpc above the disk, while the fainter emission lines are detected over smaller radial extents, with [O iii ] reaching ≈1.5 kpc. We develop a filament-finding pipeline for the H α image and construct a quantitative catalogue of the filamentary structures. The wind forms a highly connected network of strands and knots, dominated by compact filaments with typical projected widths of ≃5.3 pc and lengths of ≃9.5 pc. Both the projected covering fraction and the line-flux contribution of the filamentary component decline with height, showing that the outer wind becomes increasingly dominated by diffuse emission. Optical line-ratio diagnostics indicate that the warm ionized gas occupies an intermediate excitation regime: photoionization by the central starburst can energetically power the observed H α luminosity, while the systematic separation between filamentary and diffuse emission, together with the evolution of the line ratios with vertical distance, suggests an increasing contribution from shocks or other similar heating in the diffuse outer wind. These results show that separating filamentary and diffuse emission in high-resolution imaging provides a powerful way to connect the morphology, excitation, and multiphase structure of galactic winds.

We present evidence that the Hector Galaxy Survey galaxy C901005481609968 ( z _cl = 0.0553), which exhibits spatially extended LINER-like emission in optical integral-field spectroscopy (IFS), hosts a low-luminosity active galactic nucleus (LLAGN) that contributes substantially to its ionization budget. Although the galaxy is not selected as an AGN by mid-infrared AGN color criteria, archival Chandra data reveal a compact nuclear X-ray source with log L X ≈ 41.46 erg s ⁻¹ , supporting the presence of an LLAGN. Spatially resolved emission-line diagnostics show LINER-like line ratios across most spaxels with S/N ≥ 3, while spatially resolved τ maps ( τ ≡ Q _pAGB / Q _req ) indicate a widespread photon deficit ( log τ < 0 over most of the mapped region), even under the most optimistic pAGB normalizations, the nuclear region remains at τ < 1. Line-ratio–kinematic tests find no evidence for shock-dominated excitation as the primary driver of the extended emission, although a localized or subdominant shock contribution cannot be ruled out with the present data. We use this galaxy as a pilot case because the combination of Hector IFS and an independent nuclear X-ray constraint provides a stringent validation of the spatially resolved photon-budget framework. Our results indicate that evolved stellar populations alone cannot account for the observed emission, that an additional nuclear ionizing source is required at least in the inner region, and that a weak LLAGN likely contributes to the ionizing budget, particularly in the inner region. Our results demonstrate that extended LINER-like emission can conceal a substantial LLAGN contribution even when traditional optical and infrared AGN indicators are weak and that spatially resolved photon-budget tests combined with X-ray constraints can effectively reveal such hidden activity.

The distribution of gas-phase metals within galaxies encodes the impact of stellar feedback on galactic evolution. At high redshift, when galaxies are rapidly assembling, feedback-driven outflows and turbulence can strongly reshape radial metallicity gradients. In this work, we use the FIRE-2, SPICE, Thesan Box, and Thesan Zoom cosmological simulations—spanning a range of stellar feedback, from bursty (time-variable) to smooth (steady)—to investigate how these feedback modes shape gas-phase metallicity gradients at 3 < z ≲ 11. Across all models, we find that galaxies with bursty feedback (FIRE-2, SPICE Bursty, and Thesan Zoom) develop systematically flatter (by factors of ∼2–10) metallicity gradients than those with smooth feedback (SPICE Smooth and Thesan Box), particularly at stellar masses M _⋆ > 10 ⁹ M _⊙ . These results demonstrate that bursty stellar feedback provides sufficient turbulence to prevent strong negative gradients from forming, while smooth stellar feedback does not generically allow for the efficient radial redistribution of metals, thereby keeping gradients steep. Finally, we compare with recent observations, finding that the majority—but, notably, not all—of the observed gradients may favor a bursty stellar feedback scenario. In all, these results highlight the utility of high-resolution observations of gas-phase metallicity at high redshift as a key discriminator of these qualitatively different feedback types.

We present the first spatially resolved at ∼20 pc scale application of active galactic nuclei (AGN)-specific metallicity diagnostics for the nearby Compton-thick Seyfert 2 galaxy Mrk 573 ( z = 0.017). We use Hubble Space Telescope narrowband imaging, MUSE integral-field spectroscopy, and apply AGN strong-line metallicity diagnostics based on [O III ], [S II ], H β , H α , and [N II ] emission lines. We construct maps of 12 + log ( O / H ) for two different metallicity calibrations and two different N/O–O/H scaling relations out to ∼1 kpc and down to ∼20 pc scales. Our analysis reveals metallicity enhancement in AGN-dominated regions, with oxygen abundances reaching up to a few times solar. The metallicity shows a patchy spatial distribution, varying on ∼100 pc scales, which appears to trace the high Seyfert/ low-ionization nuclear emission-line region index value regions and the Very Large Array 6 cm jet/radio lobe emission. These spatial correspondences and the lack of evidence for star formation in the bicone region suggest that the enrichment originates from metals transported from the nuclear AGN regions by winds, outflows, or jets.

We report the discovery of the first transient relativistic iron K α line in an active galactic nucleus (AGN), J1047+5907. The line was detected 21.5 days (rest frame) after an X-ray coronal flare was observed in 2008, and it exhibits significant broadening consistent with relativistic reflection from the accretion disk in the vicinity of the central supermassive black hole (SMBH). The line has a width of ∼300 eV, corresponding to a Keplerian velocity of 14,000 km s ⁻¹ , at a distance of 5–41 light-days from the SMBH, strongly implying that the observed coronal flare triggered the emergence of the line. This event provides rare direct evidence of the response of the accretion disk to impulsive coronal illumination and offers a new method to probe the SMBH and disk physics. The relativistic modeling favors a broadened line produced by distant reflection from an accretion disk around a rapidly spinning black hole viewed at an intermediate inclination, consistent with other observations. Systematic monitoring of type 1 AGN following strong X-ray flares may open a new observational window into the innermost regions of AGN, enabling constraints on the physics of SMBH and its accretion disk at different radii that are otherwise challenging to access.

Recent JWST observations have revealed a population of high-redshift galaxies ( z ≳ 5) exhibiting unexpectedly bright ultraviolet (UV) nitrogen emission lines. The strong N iii ] and N iv ] features imply nitrogen-to-oxygen abundance ratios (N/O) as high as − 0.8 ≲ log ( N / O ) ≲ 0.4 in these low-metallicity galaxies ( 12 + log ( O / H ) ≲ 8.2 ), compared to the local value of log ( N / O ) ≈ − 1.5 . If confirmed, this level of nitrogen enrichment challenges existing models of nucleosynthesis and galaxy evolution. However, the presence of active galactic nuclei (AGNs) can affect spectral diagnostics, and previous studies often excluded AGN contamination using photoionization models based on local N/O ratios. In this work, we compare nitrogen-enhanced AGN and H ii region models to observed spectra of eight high-redshift galaxies to constrain their nitrogen abundance, excitation source, gas-phase metallicity, ionization parameter, and gas pressure simultaneously. We find seven galaxies (GHZ9, GS 3073, GN-z9p4, CEERS-1019, GHZ2, GN-z11, and GS-z9-0) are best described by nitrogen-enhanced AGN models, while RXCJ2248-ID is best reproduced by the nitrogen-enhanced H ii model. The presence of AGN does not significantly impact (≲0.1 dex) the derived N/O ratio. We also find that equivalent width—based diagrams are the most robust UV diagnostic diagrams to distinguish AGNs and star-forming galaxies for situations where the nitrogen abundance varies. All nitrogen-enhanced galaxies have moderate-to-high gas pressure ( 7.0 ≤ log ( P / k ) ≤ 9.8 ) and a high-ionization parameter ( log ( U ) ≳ − 2.0 ), indicating a dense and compact environment. We suggest that super star clusters containing Wolf–Rayet stars and massive stars are the most likely contributors to the elevated nitrogen abundance in these galaxies.

The detections of bright UV nitrogen emission lines in some high-redshift galaxies suggest unexpectedly high nitrogen-to-oxygen ratios ( log ( N / O ) ≳ − 1.0 ) compared to local values ( log ( N / O ) ≳ − 1.5 ) at similar metallicities ( 12 + log ( O / H ) ≲ 8.0 ). Although the presence of these “N-enhanced” galaxies indicates signatures of atypical chemical enrichment processes in the early Universe, the prevalence of nitrogen enhancement in high- z galaxies is unclear. So far, only ∼10 z > 5 galaxies have nitrogen abundance measurements, and they all suggest elevated N/O ratios. Do all high-redshift galaxies exhibit elevated N/O ratios, or are we simply missing “N-normal” galaxies whose nitrogen abundances follow the local N/O scaling relation? To tackle these questions, we calculate the detection limits of UV N iii ] or N iv ] lines in current JWST surveys CEERS and JADES, and compare them to predictions from both “N-enhanced” and “N-normal” active galactic nuclei (AGN) narrow-line region and H ii region photoionization models. We find that CEERS can only detect galaxies with significant nitrogen enhancement ( log ( N / O ) ≳ − 0.4 ), while JADES can only detect galaxies with moderately elevated N/O ratios compared to local values ( log ( N / O ) ≳ − 1.0 ). Even for the deepest exposure in JADES, UV nitrogen lines produced by “N-normal” galaxies at z > 5 are too faint and thus not detectable, making their nitrogen abundance unmeasurable. Our results suggest that the existing sample of galaxies with measurable nitrogen abundances at z ≳ 5 is incomplete and biased toward galaxies with significantly elevated N/O ratios. Deep ( t exp ∼ 40 – 500 hr) spectroscopic surveys will be crucial for building a complete sample to study nitrogen enrichment mechanisms in the early Universe.