Ralph S. Sutherland

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 new Multi Unit Spectroscopic Explorer (MUSE) Narrow Field Mode with adaptive optics observations of the optical torus surrounding a central compact object candidate within the oxygen-rich supernova remnant 1E 0102.2–7219 located in the Small Magellanic Cloud. These data provide nearly an order-of-magnitude improvement in spatial resolution over previous MUSE Wide Field Mode observations. The improved spatial resolution resolved the previously identified torus into a cavity-like structure with a sharply defined inner edge and diffuse, outer filamentary substructure. The emission shows continuous velocity connectivity, broad intrinsic line widths, and cospatial contributions from neutral and partially ionized species, including O i , Ne i , [O i ], [O ii ], and [O iii ]. Spatially resolved line-ratio maps indicate that the emission arises from a multiphase, nonequilibrium medium rather than a single homogeneous component. Comparison with photoionization and shock models shows that no single-component model within the explored parameter space can simultaneously reproduce both the strong neutral and high-ionization diagnostics, indicating that multiple physical conditions must coexist. We favor an interpretation in which shocks propagating through density inhomogeneities in the ejecta shape the observed morphology and excitation, while also considering alternative mechanisms linked to the central source, binary evolution, or interaction with an embedded object within the remnant.

The internal dust attenuation of the H II region reduces the observed emission-line fluxes. Turbulent density fields within each H II region change the degree of the line-of-the-sight obscuration of the emission-line fluxes. In this paper, we implement the dust Monte Carlo radiative transfer in the latest M3D code, creating the emission-line maps attenuated by the internal turbulent dust obscuration with the varying Mach numbers. Due to the internal density and temperature fluctuations of H II regions the radiative transfer of hydrogen lines meet neither Case A nor Case B conditions, resulting in the global H α to H β ratio of approximately 3.02–3.03, differing from the widely used value of 2.86. This deviation from Case B is because the temperature of these H II regions is cooler than 10 000 K. We further derive the internal nebular attenuation curve from the attenuated hydrogen lines, finding that the clumpy structures within H II regions do not change the slope of the internal attenuation curve. This is because the beavy dust obscuration of dense clumps is canceled out by the high in situ production of emission-line intensities.

The distance to the Vela Junior supernova remnant (RX J0852.0–4622 or G266.2–1.2) has long remained uncertain, limiting our understanding of its physical properties. Using Very Large Telescope/Multi Unit Spectroscopic Explorer integral field spectroscopy, we uncover chemical and kinematic connections between the nebula surrounding its central compact object (CXOU J085201.4–461753) and the nearby Herbig–Haro outflow of Ve 7–27 (Wray 16–30), indicating a shared nitrogen-rich, Fe-peak-enhanced environment. This link ties stellar birth and death, with the young star Ve 7–27 embedded in material expelled by Vela Junior’s massive progenitor, and the remnant’s blast wave is expanding through the same medium. Adopting the Gaia-based distance to Ve 7–27, we revise Vela Junior’s distance to 1.41 ± 0.14 kpc. At this distance, the remnant’s physical radius is 23.3 ± 2.3 pc, and X-ray proper motions of the northwestern rim correspond to shock speeds of (2.8 ± 0.7) × 10 ³ to (5.6 ± 1.5) × 10 ³ km s ⁻¹ . These imply an age of ∼1.6–3.3 kyr and a very low ambient density, indicating that Vela Junior is expanding within a highly rarefied wind-blown cavity carved by a massive progenitor—consistent with the nondetection of strong thermal X-ray emission. This distance update also resolves long-standing inconsistencies, with major implications for its energy budget, particle acceleration efficiency, and compact object evolution.

The excitation sources in galaxies are frequently mixed owing to active galactic nucleus (AGN) and stellar feedback, including star formation, AGN, and shock excitation. Disentangling the star formation, AGN, and shocks in galaxy integral field unit (IFU) spectra at optical wavelengths is crucial to expanding the galaxy sample for AGN and stellar feedback studies, given the lack of multiwavelength observations for most of the galaxies that are observed in optical wavelengths. Previous methods to address this issue either have a limited application range or are highly uncertain in separating AGN from shock excitation. Here we propose a theoretical three-dimensional (3D) diagram. This theoretical 3D diagram overcomes the limitations of previous methods and can simultaneously separate star formation, AGN, and shocks in active galaxies. Along with the separation, the new theoretical 3D diagram also constrains the gas metallicity, ionization parameter, and gas pressure within the galaxy. By applying the Very Large Telescope/MUSE IFU data and the Wide Field Spectrograph IFU data for NGC 5728 on the theoretical 3D diagram, we find a star-forming ring surrounding the galaxy center with a projected radius of ∼1 kpc in the sky plane, an AGN ionized bicone extended up to ∼2 kpc from the nuclear center, and a fast-shock-dominated disk region at the base of the AGN outflow, which is likely associated with a nuclear accretion disk or the result of jet–interstellar medium interaction. The theoretical 3D diagram opens a new window in which to study the interplay among star formation, AGN, and shocks in active galaxies.

The excitation sources in galaxies are frequently mixed due to AGN and stellar feedback, including star formation, active galactic nuclei (AGNs), and shock excitation. Disentangling the star formation, AGN, and shocks in galaxy integral-field spectra (IFU) at optical wavelengths is crucial to expanding the galaxy sample for AGN and stellar feedback studies, given the lack of multiwavelength observations for most of the galaxies that are observed in optical wavelengths. Previous methods to address this issue either have a limited application range or are highly uncertain in separating AGN from shock excitation (D'Agostino et al. 2019; Johnston et al. 2023). Here, we propose a theoretical three-dimensional (3D) diagram. This theoretical 3D diagram overcomes the limitations of previous methods and can simultaneously separate star formation, AGNs, and shocks in active galaxies. Along with the separation, the new theoretical 3D diagram also constrains the gas metallicity, ionization parameter, and gas pressure within the galaxy. By applying the Very Large Telescope (VLT)/MUSE IFU data and the Wide Field Spectrograph IFU data for NGC5728 on the theoretical 3D diagram, we find a star-forming ring surrounding the galaxy center with a projected radius of $\sim$1 kpc in the sky plane, an AGN ionized-bicone extended up to $\sim$2 kpc from the nuclear center, and a fast shock dominated disk region at the base of the AGN outflow, which is likely associated with a nuclear accretion disk or a result of jet-ISM interaction. The theoretical 3D diagram opens a new window to study the interplay among star formation, AGN, and shocks in active galaxies.

Context. The origin of nebular He II emission in both local and high-redshift galaxies remains an unsolved problem. Various theories have been proposed to explain it, including He II-ionizing photons produced by high mass X-ray binaries, ultra-luminous X-ray sources, or “stripped” He stars produced by binary interaction or evolution of rapidly rotating (v/v_crit ≫ 0.4) single massive stars, shock ionization, and hidden active galactic nuclei. All of these theories have shortcomings, however, leaving the cause of nebular He II emission unclear. Aims. We investigate the hypothesis that the photons responsible for driving nebular He II emission are produced by the evolution of single massive stars and/or Wolf-Rayet (WR) stars whose winds are on the verge of becoming optically thin due to clumping, thus allowing significant escape of hard ionizing photons. We combined models of stellar evolution with population synthesis and nebular models to identify the most favorable scenarios for producing nebular He II via this channel. Methods. We used the Modules for Experiments in Stellar Astrophysics (MESA) code to compute evolutionary tracks for stars with initial masses of 10 − 150 M_⊙ and a range of initial metallicities and rotation rates. We then combined these tracks with a range of custom treatments of stellar atmospheres, which were intended to capture the effects of clumping, in the population synthesis code Stochastically Lighting Up Galaxies (SLUG) in order to produce the total ionizing photon budgets and spectra. We used these spectra as inputs to CLOUDY calculations of nebular emission at a range of nebular densities and metallicities. Results. We find that if WR winds are clumpy enough to become close to optically thin, stellar populations with a wide range of metallicities and rotation rates can produce He II ionizing photons at rates sufficient to explain the observed nebular I(He II)/I(Hβ) ratio ∼0.004 − 0.07 found in He II-emitting galaxies. Metal-poor rapidly rotating stellar populations ([Fe/H]= − 2.0, v/v_crit = 0.4) also reach these levels of He II production, even for partially clumpy winds. These scenarios also yield He II, Hβ, and “blue bump” line equivalent widths comparable to those observed in He II emitters. Only for homogeneous non-clumpy winds did we fail to find combinations of metallicity and stellar rotation rate that yield I(He II)/I(Hβ) values as high as those observed in He II emitters. Conclusions. Contrary to previous findings, we conclude that single WR stars can be a strong source for nebular He II emission if their winds are sufficiently clumpy. This scenario also reproduces a range of other properties found in He II emitters, suggesting that hard photons escaping through clumpy WR winds are a strong candidate to explain nebular He II-emission.

Gas metallicity, ionization parameter, and gas pressure can affect the observed ratios of specific strong emission lines within galaxies. While the theoretical strong lines diagnostics for gas metallicity, ionization parameters, and gas pressure in star-forming regions are well-established, theoretical diagnostics for active galactic nuclei (AGNs) narrow-line regions are still lacking. In P. Zhu et al., we presented a new AGN model that provides the best predictions for observations spanning the UV, optical, and infrared wavelengths. This paper presents a suite of theoretical diagnostics for the gas metallicity, ionization parameter, gas pressure, and the peak energy in AGN ionizing radiation field E _peak for AGN narrow-line regions spanning the UV and optical wavelengths. We investigate the model dependency on the ionization parameter, gas pressure, E _peak, and the nitrogen scaling relation and make recommendations on metallicity diagnostics that are most robust against these parameters. We test our new AGN metallicity diagnostics using optical galaxy spectra from Sloan Digital Sky Survey DR16. These tests show that the metallicities measured from different diagnostics in this paper are consistent within ∼0.3 dex. We compare consistent H ii and AGN diagnostics and demonstrate that H ii and AGN diagnostics should not be used interchangeably. With a wide wavelength coverage, we anticipate that these AGN diagnostics will enable new metallicity studies of galaxies dominated by AGN.

Investigating the impact of galaxy properties on emergent Lyα emission is crucial for reionization studies, given the sensitivity of Lyα to neutral hydrogen. This study presents an analysis of the physical characteristics of 155 star-forming galaxies, 29 with Lyα detected, and 126 with Lyα not detected with Lyα EW < 20 Å, at z = 1.9–3.5, drawn from the MOSFIRE Deep Evolution Field survey, that have overlapping observations from the Hobby–Eberly Telescope Dark Energy Experiment survey. To unravel the interstellar medium (ISM) conditions in our sample, we developed a custom nebular line modeling algorithm based on the MAPPINGS V photoionization model grid and the emcee framework. Combining nebular-based ISM properties with photometry-based global properties, constrained via Bagpipes, we explore distinctions in the stellar and gas properties between Lyα-detected and Lyα-nondetected galaxies. Our analysis reveals statistically significant differences between the two samples in terms of stellar mass and dust attenuation (A _V ) at >2σ significance, as determined via a Kolmogorov–Smirnov test. Moreover, there are weaker (≲1σ significance) indications that the ionization parameter and metallicity differ between the two samples. Our results demonstrate that the escape fraction of Lyα ( f esc Ly α ) is inversely correlated with stellar mass, star formation rate, and dust attenuation, while it is positively correlated with the ionization parameter, with significance levels exceeding 2σ. Our findings suggest that the interstellar environments of Lyα-detected galaxies, characterized by low mass, low dust, low gas-phase metallicity, and high ionization parameters, play a pivotal role in promoting the escape of Lyα radiation.