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Abstract Ultraluminous infrared galaxies are powered by a combination of rapid star formation and active galactic nucleus (AGN) emission, but their relative importance is not always observationally clear. We study the galactic continuum spectrum of a cosmologically simulated ∼4 × 10¹⁰M_⊙stellar mass starburst galaxy at redshiftz ∼ 4.4 that refines down to resolve beyond the dust sublimation boundary of its super-Eddington-accreting ∼10⁷M_⊙supermassive black hole. We find that this system resembles the rare class of hot dust-obscured galaxy (Hot DOG), with a roughly flat (inνF_ν) IR emission spectrum that sharply drops off at wavelengths ≲5μm. Our system also matches with the observational properties of many Hot DOGs, including undergoing multiple galaxy mergers and being the most massive galaxy within a dense cosmological environment. The distinctive Hot DOG spectral shape in our system is caused by AGN-heated mid-IR warm dust, predominately starburst-heated far-IR cold dust, and a steep near- to mid-IR cutoff caused by strong absorption in the dense ISM of the galactic nucleus, rather than the dust torus itself. This system is lower luminosity (L_IR ∼ 2 × 10¹²L_⊙) than those detected by the WISE survey at similar redshifts, but will be a prime target for future far-IR surveys such as PRIMA. Our results show that Hot DOGs can naturally result as a transitional phase during rapid AGN accretion, but before significant AGN-driven outflows clear optically thin paths. 

Many astrophysical simulations involve extreme dynamic range of timescales around ‘special points’ in the domain (e.g. black holes, stars, planets, disks, galaxies, shocks, mixing interfaces), where processes on small scales couple strongly to those on large scales. Adaptive resolution, multi-physics, and hybrid numerical methods have enabled tremendous progress on the spatial, physics, and numerical challenges involved. But often the limiter for following the long timescales of global evolution is the extremely short numerical timestep required in some subdomains (which leads to their dominating simulation costs). Recently several approaches have been developed for tackling this in problems where the short timescale solution is sampled and then projected as an effective subgrid model over longer timescales (e.g. ‘zooming in and out’). We generalize these to a family of models where time evolution is modulated by a variable but continuous in space-and-time dilation/stretch factor . This extends previous well-studied approaches (including reduced-speed-of-light and binary orbital dynamics methods), and ensures that the system comes to correct local steady-state solutions, and derive criteria that the dilation factor/timesteps/resolution must obey to ensure good behavior. We present a variety of generalizations to different physics or coupling scales. Compared to previous approaches, this method makes it possible to avoid imprinting arbitrary scales where there is no clear scale-separation, and couples well to Lagrangian or Eulerian methods. It is flexible and easily-implemented and we demonstrate its validity (and limitations) in test problems. We discuss the relationship between these methods and physical time dilation in GRMHD. We demonstrate how this can be used to obtain effective speedup factors exceeding $\gtrsim {10}^4$ in multiphysics simulations. 

We present a novel and powerful constraint on the physics of supermassive black hole (BH) accretion disks. We show that in the outer disk (radii $R\gtrsim 0.01$ pc or $\gtrsim 1000R_G$ ), models supported by thermal or radiation pressure predict disk masses which are much larger than the BH mass and increase with radius - i.e. rapidly-rising, extremely non-Keplerian rotation curves. More generally, we show that any observational upper limit to the deviation from Keplerian potentials at these radii directly constrains the physical form of the pressure in disks. We then show that existing maser and broad line region (BLR) kinematic observations immediately rule out the classic thermal-pressure-dominated Shakura Sunyaev-like $\alpha$ -disk model, and indeed rule out any thermal or radiation (or cosmic-ray) pressure-dominated disk, as the required temperatures and luminosities of the gas at large radii would exceed those observed by orders of magnitude. We show that models where the pressure comes entirely from turbulence (without thermal, radiation, or magnetic sources) could in principle be viable but would require turbulent Toomre $Q\gtrsim 100$ , far larger than predicted by self gravitating/gravito-turbulent models. However, recently proposed models of magnetic pressure-dominated disks agree with all of the observational constraints. These magnetically-dominated models also appear to agree better with constraints on maser magnetic fields, compared to the other possibilities. Observations appear to strongly favor the hypothesis that the outer regions of BH accretion disks are in the ‘hyper-magnetized’ state. 

Abstract We present the first infrared (IR) spectral predictions from a self-consistent simulation of the formation of a quasar in a starburst galaxy, spanning the cosmological environment to scales well below the dust sublimation region. The IR emission is dominated by a torus-like dust structure composed of a highly magnetized, turbulence-supported outer accretion disk and of accreting gas tidally torn from the interstellar medium (ISM). At these early stages, the active galactic nucleus is buried and Compton thick. The near- to mid-IR escaping luminosity varies by almost an order of magnitude across sight lines, largely due to extinction from the inflowing stream of cold dust. Self-absorption within the torus suppresses silicate emission features, and further reprocessing by the ambient ISM leads to prominent silicate absorption and colder IR emission. The sublimation structure is stratified by composition and size, producing sight-line-dependent extinction curves that intrinsically vary in shape. However, after repeated scattering in the optically thick dusty medium, these curves emerge substantially grayed. We also demonstrate that bipolar outflows from the central black hole, which carves biconical cavities and reveals the central engine in later stages, can preserve IR anisotropy and silicate features. These results suggest that dusty starburst quasars can undergo a buried, IR-bright phase early in their evolution. 

Abstract We present spatially resolved parsec-scale measurements of nuclear conditions (gas density and kinetic temperature) relevant for black hole accretion rate predictions in the Seyfert 2 galaxy, NGC 1068. We inject these parameters into the prescription for a Bondi-like accretion model, then compare the resulting accretion rate prediction to the empirical accretion rate derived from hard X-ray observations. Cosmological simulations have spatial resolution ranging from ∼10 pc to approximately kiloparsec scales, and so for reasonable comparison we test these accretion rate predictions in pixel-sized radial steps out to 500 pc. Compared to warm H₂gas, CO gas is the dominant mass carrier close to the SMBH. We find that the Bondi accretion rate ( ${\dot{M}}_{\mathrm{Bondi}}$ ) of cold molecular gas alone (measured using CO) overestimates the true accretion rate by up to 14 dex in a small aperture (r≲5 pc) around the black hole, and by at least 8 dex inside large apertures (r≲500 pc). These results are the first in a series of direct tests of accretion rate prescriptions, and they suggest that using a Bondi accretion formalism to model supermassive black hole accretion in Seyfert 2 galaxies may lead to overestimated accretion rates in simulations. 

We present global magnetohydrodynamic (MHD) simulations of idealized accretion disks with a strong toroidal magnetic field using an equation of state that fixes the gas thermal scale height. The disk forms from the inflow of a rotating magnetized gas cloud with a toroidal magnetic field. We find that the system maintains a moderately strong mean azimuthal field in the midplane, with plasma- $\beta \sim 1$ , trans-Alfv'enic fluctuations, and large accretion stresses $\alpha \sim 0.1$ . The azimuthal field in the disk is continuously escaping along the vertical direction but is also replenished via a local dynamo. The inflowing gas initially forms a strongly magnetized Keplerian disk with $\beta \ll 1$ and $\alpha \gg 1$ . The disk gradually collapses from the inside out over $\sim 50-80$ orbits to form a moderately magnetized disk with $\beta \sim 1$ and $\alpha \sim 0.1$ . Radial advection of azimuthal magnetic field can maintain $\beta \lesssim 1$ exterior to the circularization radius but not inside of it. Inclusion of a net initial vertical magnetic field can lead to an even more strongly magnetized disk midplane, consistent with previous work. When the gas thermal scale is not resolved ( $\lesssim 4$ cells per thermal scale height), however, the disk remains highly magnetized with $ $. We discuss our results in the context of related shearing box simulations and other global disk simulations. The level of angular momentum transport found here is consistent with that inferred observationally in dwarf novae and X-ray transient outbursts, unlike simulations of weakly magnetized accretion disks. 

Abstract Understanding cosmic-ray (CR) diffusion in a partially ionized medium is both crucial and challenging. In this study, we investigate CR perpendicular superdiffusion and parallel transport in turbulent, partially ionized media using high-resolution 3D two-fluid simulations that treat ions and neutrals separately. We examine the influence of neutral-ion decoupling and the associated damping of turbulence on CR propagation in both transonic and supersonic conditions. Our simulations demonstrate that neutral-ion decoupling significantly damps velocity and magnetic field fluctuations at small scales, producing spectral slopes steeper than those of Kolmogorov and Burgers scaling. In supersonic turbulence, large-scale shock motion is not subject to damping and generates small-scale density enhancements. Moreover, the damping of magnetic field fluctuations substantially decreases pitch-angle scattering, which, however, only slightly affects the CR parallel mean free pathλ_∥, due to the nonresonant mirror interactions of CRs. In the direction perpendicular to the mean magnetic field, we identify two regimes of the perpendicular superdiffusion of CRs: a diffusive regime (λ_∥ < L_inj, whereL_injis the turbulence injection scale) with perpendicular separation of CR proportional tot^3/4and a ballistic regime (λ_∥ > L_inj) with perpendicular separation scaling ast^3/2. At initially large pitch angles, the effects of magnetic mirroring—naturally arising in magnetohydrodynamic turbulence—become significant, enhancing the confinement of CRs and resulting inλ_∥ < L_inj, despite the damping effect. These results imply that large-pitch-angle CRs can be well confined in the cold interstellar medium, such as molecular clouds.