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Partial dust obscuration in active galactic nuclei (AGNs) has been proposed as a potential explanation for some cases of AGN variability. The dust–gas mixture present in AGN tori is accelerated by radiation pressure, leading to the launching of an AGN wind. Dust under these conditions has been shown to be unstable to a generic class of fast-growing resonant drag instabilities (RDIs). In this work, we present the first numerical simulations of radiation-driven outflows that explicitly include dust dynamics in conditions resembling AGN winds. We investigate the implications of RDIs on the torus morphology, AGN variability, and the ability of radiation to effectively launch a wind. We find that the RDIs rapidly develop, reaching saturation at times much shorter than the global time-scales of the outflows, resulting in the formation of filamentary structure on box-size scales with strong dust clumping and super-Alfvénic velocity dispersions. The instabilities lead to fluctuations in dust opacity and gas column density of 10–20  per cent when integrated along mock observed lines of sight to the quasar accretion disc. These fluctuations occur over year to decade time-scales and exhibit a red-noise power spectrum commonly observed for AGNs. Additionally, we find that the radiation effectively couples with the dust–gas mixture, launching highly supersonic winds that entrain 70–90  per cent of the gas, with a factor of ≲3 photon momentum loss relative to the predicted multiple-scattering momentum loading rate. Therefore, our findings suggest that RDIs play an important role in driving the clumpy nature of AGN tori and generating AGN variability consistent with observations.

Negative feedback from accreting supermassive black holes is considered crucial in suppressing star formation and quenching massive galaxies. However, several models and observations suggest that black hole feedback may have a positive effect, triggering star formation by compressing interstellar medium gas to higher densities. We investigate the dual role of black hole feedback using cosmological hydrodynamic simulations from the Feedback In Realistic Environment (FIRE) project, incorporating a novel implementation of hyper-refined accretion-disc winds. Focusing on a massive, star-forming galaxy at z ∼ 2 ($M_{\rm halo} \sim 10^{12.5}\, {\rm M}_{\odot }$), we demonstrate that strong quasar winds with a kinetic power of ∼1046 erg s−1, persisting for over 20 Myr, drive the formation of a central gas cavity and significantly reduce the surface density of star formation across the galaxy’s disc. The suppression of star formation primarily occurs by limiting the availability of gas for star formation rather than by evacuating the pre-existing star-forming gas reservoir (preventive feedback dominates over ejective feedback). Despite the overall negative impact of quasar winds, we identify several potential indicators of local positive feedback, including (1) the spatial anticorrelation between wind-dominated regions and star-forming clumps, (2) higher local star formation efficiency in compressed gas at the edge of the cavity, and (3) increased contribution of outflowing material to local star formation. Moreover, stars formed under the influence of quasar winds tend to be located at larger radial distances. Our findings suggest that both positive and negative AGN feedback can coexist within galaxies, although the local positive triggering of star formation has a minor influence on global galaxy growth.

Without active galactic nucleus (AGN) feedback, simulated massive, star-forming galaxies become too compact relative to observed galaxies at z ≲ 2. In this paper, we perform high-resolution re-simulations of a massive ($M_{\star }\sim 10^{11}\, \rm {{\rm M}_{\odot }}$) galaxy at z ∼ 2.3, drawn from the Feedback in Realistic Environments (FIRE) project. In the simulation without AGN feedback, the galaxy experiences a rapid starburst and shrinking of its half-mass radius. We experiment with driving mechanical AGN winds, using a state-of-the-art hyper-Lagrangian refinement technique to increase particle resolution. These winds reduce the gas surface density in the inner regions of the galaxy, suppressing the compact starburst and maintaining an approximately constant half-mass radius. Using radiative transfer, we study the impact of AGN feedback on the magnitude and extent of the multiwavelength continuum emission. When AGN winds are included, the suppression of the compact, dusty starburst results in lowered flux at FIR wavelengths (due to decreased star formation) but increased flux at optical-to-near-IR wavelengths (due to decreased dust attenuation, in spite of the lowered star formation rate), relative to the case without AGN winds. The FIR half-light radius decreases from ∼1 to $\sim 0.1\, \rm {kpc}$ in $\lesssim 40\, \rm {Myr}$ when AGN winds are not included, but increases to $\sim 2\, \rm {kpc}$ when they are. Interestingly, the half-light radius at optical-NIR wavelengths remains approximately constant over $35\, \rm {Myr}$, for simulations with and without AGN winds. In the case without winds, this occurs despite the rapid compaction, and is due to heavy dust obscuration in the inner regions of the galaxy. This work highlights the importance of forward-modelling when comparing simulated and observed galaxy populations.

Recent theoretical studies predict that the circumgalactic medium (CGM) around low-redshift, ∼L* galaxies could have substantial non-thermal pressure support in the form of cosmic rays. However, these predictions are sensitive to the specific model of cosmic ray transport employed, which is theoretically and observationally underconstrained. In this work, we propose a novel observational constraint for calculating the lower limit of the radially averaged, effective cosmic ray transport rate, ${\kappa _{\rm eff}^{\rm min}}$. Under a wide range of assumptions (so long as cosmic rays do not lose a significant fraction of their energy in the galactic disc, regardless of whether the cosmic ray pressure is important or not in the CGM), we demonstrate a well-defined relationship between ${\kappa _{\rm eff}^{\rm min}}$ and three observable galaxy properties: the total hydrogen column density, the average star formation rate, and the gas circular velocity. We use a suite of Feedback in Realistic Environments 2 galaxy simulations with a variety of cosmic ray transport physics to demonstrate that our analytical model of ${\kappa _{\rm eff}^{\rm min}}$ is a robust lower limit of the true cosmic ray transport rate. We then apply our new model to calculate ${\kappa _{\rm eff}^{\rm min}}$ for galaxies in the COS-Halos sample, and confirm this already reveals strong evidence for an effective transport rate that rises rapidly away from the interstellar medium to values ${\kappa _{\rm eff}^{\rm min}}\gtrsim 10^{30\!-\!31}\, {\rm cm}^2\, {\rm s}^{-1}$ (corresponding to anisotropic streaming velocities of $v^{\rm stream}_{\rm eff} \gtrsim 1000\, {\rm km}\, {\rm s}^{-1}$) in the diffuse CGM, at impact parameters larger than 50–100 kpc. We discuss how future observations can provide qualitatively new constraints in our understanding of cosmic rays in the CGM and intergalactic medium.

Most observed stars are part of a multiple star system, but the formation of such systems and the role of environment and various physical processes is still poorly understood. We present a suite of radiation-magnetohydrodynamic simulations of star-forming molecular clouds from the STARFORGE project that include stellar feedback with varied initial surface density, magnetic fields, level of turbulence, metallicity, interstellar radiation field, simulation geometry and turbulent driving. In our fiducial cloud, the raw simulation data reproduces the observed multiplicity fractions for Solar-type and higher mass stars, similar to previous works. However, after correcting for observational incompleteness the simulation underpredicts these values. The discrepancy is likely due to the lack of disc fragmentation, as the simulation only resolves multiples that form either through capture or core fragmentation. The raw mass distribution of companions is consistent with randomly drawing from the initial mass function for the companions of $\gt 1\, \mathrm{M}_{\rm \odot }$ stars. However, accounting for observational incompleteness produces a flatter distribution similar to observations. We show that stellar multiplicity changes as the cloud evolves and anticorrelates with stellar density. This relationship also explains most multiplicity variations between runs, i.e. variations in the initial conditions that increase stellar density (increased surface density, reduced turbulence) also act to decrease multiplicity. While other parameters, such as metallicity, interstellar radiation, and geometry significantly affect the star formation history or the IMF, varying them produces no clear trend in stellar multiplicity properties.

Models for cosmic ray (CR) dynamics fundamentally depend on the rate of CR scattering from magnetic fluctuations. In the ISM, for CRs with energies ∼MeV-TeV, these fluctuations are usually attributed either to ‘extrinsic turbulence’ (ET) – a cascade from larger scales – or ‘self-confinement’ (SC) – self-generated fluctuations from CR streaming. Using simple analytic arguments and detailed ‘live’ numerical CR transport calculations in galaxy simulations, we show that both of these, in standard form, cannot explain even basic qualitative features of observed CR spectra. For ET, any spectrum that obeys critical balance or features realistic anisotropy, or any spectrum that accounts for finite damping below the dissipation scale, predicts qualitatively incorrect spectral shapes and scalings of B/C and other species. Even if somehow one ignored both anisotropy and damping, observationally required scattering rates disagree with ET predictions by orders of magnitude. For SC, the dependence of driving on CR energy density means that it is nearly impossible to recover observed CR spectral shapes and scalings, and again there is an orders-of-magnitude normalization problem. But more severely, SC solutions with super-Alfvénic streaming are unstable. In live simulations, they revert to either arbitrarily rapid CR escape with zero secondary production, or to bottleneck solutions with far-too-strong CR confinement and secondary production. Resolving these fundamental issues without discarding basic plasma processes requires invoking different drivers for scattering fluctuations. These must act on a broad range of scales with a power spectrum obeying several specific (but plausible) constraints.

In dusty cool-star outflow or ejection events around asymptotic giant branch (AGB) or R Coronae Borealis or RCB-like stars, dust is accelerated by radiation from the star and coupled to the gas via collisional drag forces. It has recently been shown that such dust-gas mixtures are unstable to a super-class of instabilities called the resonant drag instabilities (RDIs), which promote dust clustering. We therefore consider idealized simulations of the RDIs operating on a spectrum of dust grain sizes subject to radiative acceleration (allowing for different grain optical properties), coupled to the gas with a realistic drag law, including or excluding the effects of magnetic fields and charged grains, and calculate for the first time how the RDIs could contribute to observed variability. We show that the RDIs naturally produce significant variations (spatially and temporally) ($\sim 10\!-\!20{{\ \rm per\ cent}}$ 1 σ-level) in the extinction, corresponding to $\sim 0.1\!-\!1\,$mag level in the stellar types above, on time-scales of order months to a year. The fluctuations are surprisingly robust to the assumed size of the source as they are dominated by large-scale modes, which also means their spatial structure could be resolved in some nearby systems. We also quantify how this produces variations in the line-of-sight grain size-distribution. All of these variations are similar to those observed, suggesting that the RDIs may play a key role driving observed spatial and temporal variability in dust extinction within dusty outflow/ejection events around cool stars. We further propose that the measured variations in grain sizes could directly be used to identify the presence of the RDIs in close by systems with observations.

One of the key mysteries of star formation is the origin of the stellar initial mass function (IMF). The IMF is observed to be nearly universal in the Milky Way and its satellites, and significant variations are only inferred in extreme environments, such as the cores of massive elliptical galaxies and the Central Molecular Zone. In this work, we present simulations from the STARFORGE project that are the first cloud-scale radiation-magnetohydrodynamic simulations that follow individual stars and include all relevant physical processes. The simulations include detailed gas thermodynamics, as well as stellar feedback in the form of protostellar jets, stellar radiation, winds, and supernovae. In this work, we focus on how stellar radiation, winds, and supernovae impact star-forming clouds. Radiative feedback plays a major role in quenching star formation and disrupting the cloud; however, the IMF peak is predominantly set by protostellar jet physics. We find that the effect of stellar winds is minor, and supernovae ‘occur too late’ to affect the IMF or quench star formation. We also investigate the effects of initial conditions on the IMF. We find that the IMF is insensitive to the initial turbulence, cloud mass, and cloud surface density, even though these parameters significantly shape the star formation history of the cloud, including the final star formation efficiency. Meanwhile, the characteristic stellar mass depends weakly on metallicity and the interstellar radiation field, which essentially set the average gas temperature. Finally, while turbulent driving and the level of magnetization strongly influence the star formation history, they only influence the high-mass slope of the IMF.

Radiation-dust driven outflows, where radiation pressure on dust grains accelerates gas, occur in many astrophysical environments. Almost all previous numerical studies of these systems have assumed that the dust was perfectly coupled to the gas. However, it has recently been shown that the dust in these systems is unstable to a large class of ‘resonant drag instabilities’ (RDIs) which de-couple the dust and gas dynamics and could qualitatively change the non-linear outcome of these outflows. We present the first simulations of radiation-dust driven outflows in stratified, inhomogeneous media, including explicit grain dynamics and a realistic spectrum of grain sizes and charge, magnetic fields and Lorentz forces on grains (which dramatically enhance the RDIs), Coulomb and Epstein drag forces, and explicit radiation transport allowing for different grain absorption and scattering properties. In this paper, we consider conditions resembling giant molecular clouds (GMCs), H ii regions, and distributed starbursts, where optical depths are modest (≲1), single-scattering effects dominate radiation-dust coupling, Lorentz forces dominate over drag on grains, and the fastest-growing RDIs are similar, such as magnetosonic and fast-gyro RDIs. These RDIs generically produce strong size-dependent dust clustering, growing non-linear on time-scales that are much shorter than the characteristic times of the outflow. The instabilities produce filamentary and plume-like or ‘horsehead’ nebular morphologies that are remarkably similar to observed dust structures in GMCs and H ii regions. Additionally, in some cases they strongly alter the magnetic field structure and topology relative to filaments. Despite driving strong micro-scale dust clumping which leaves some gas ‘behind,’ an order-unity fraction of the gas is always efficiently entrained by dust.

Recent strides have been made developing dust evolution models for galaxy formation simulations but these approaches vary in their assumptions and degree of complexity. Here, we introduce and compare two separate dust evolution models (labelled ‘Elemental’ and ‘Species’), based on recent approaches, incorporated into the gizmo code and coupled with fire-2 stellar feedback and interstellar medium physics. Both models account for turbulent dust diffusion, stellar production of dust, dust growth via gas-dust accretion, and dust destruction from time-resolved supernovae, thermal sputtering in hot gas, and astration. The ‘Elemental’ model tracks the evolution of generalized dust species and utilizes a simple, ‘tunable’ dust growth routine, while the ‘Species’ model tracks the evolution of specific dust species with set chemical compositions and incorporates a physically motivated, two-phase dust growth routine. We test and compare these models in an idealized Milky Way-mass galaxy and find that while both produce reasonable galaxy-integrated dust-to-metals (D/Z) ratios and predict gas-dust accretion as the main dust growth mechanism, a chemically motivated model is needed to reproduce the observed scaling relation between individual element depletions and D/Z with column density and local gas density. We also find the inclusion of theoretical metallic iron and O-bearing dust species are needed in the case of specific dust species in order to match observations of O and Fe depletions, and the integration of a sub-resolution dense molecular gas/CO scheme is needed to both match observed C depletions and ensure carbonaceous dust is not overproduced in dense environments.