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Active galactic nuclei (AGNs) are a promising source of the binary black hole mergers observed in gravitational waves with LIGO-Virgo-Kagra (LVK). Constraining the AGN channel allows us to limit AGN parameter space (disk density, size, average lifetime) and nuclear star cluster (NSC) parameter space. Constraints on AGNs and NSCs have implications for Λ cold dark matter models of AGN feedback and models of AGN-driven supermassive black hole merger and growth. Here we present several qualitative studies of the AGN channel using new public, open-source, fast, reproducible code McFACTS (https://github.com/mcfacts/mcfacts): Monte Carlo for AGN channel Testing and Simulation. We demonstrate several important features for testing the AGN channel, including: (i) growth to large-mass intermediate-mass black hole is helped by the presence of migration traps or swamps, (ii) flat BH initial mass functions highlight hierarchical merger features in the mass spectrum, (iii) the (q, χeff) anticorrelation is a strong test of the bias to prograde mergers in the AGN channel, (iv) spheroid encounters can drive a fraction of mergers with high in-plane spin components (χp), (v) a high rate of extreme mass ratio inspirals are driven by an initial population of embedded retrograde BH, and (vi) both LVK and LISA are powerful probes of models of AGN disks and their embedded populations.

The active galactic nucleus (AGN) channel for the formation of binary black hole (BBH) mergers has been previously studied as a potential formation channel for the merging compact binaries observed by the LIGO–Virgo–KAGRA (LVK) scientific collaboration. The first two papers in this series explored the McFACTS code for the evolution of black hole orbits in AGN accretion disks for individual galaxy models and described the characteristics of predicted BBH populations in realizations of those models (such as the correlation between mass ratio and aligned spin). In this work, we explore the impact of the properties of AGN host galaxies and assume an AGN lifetime and cosmological model for the density of AGN in a universe like our own. By sampling from an inferred population of AGN, we marginalize over galaxy mass to predict a population of BBH mergers observable by modern ground-based gravitational-wave observatories. We find that for reasonable assumptions, AGN disk environments may account for massive BBH mergers such as GW190521 and GW190929_012149. We find that the majority of observable BBH mergers from our simulation are expected to originate in galaxies with a supermassive black hole between 107 M⊙ and 109.4 M⊙. We also find that if hierarchical mergers from AGN disks account for a substantial part of the LVK population, our current models require an AGN lifetime of 0.5–2.5 Myr.

We use the Monte Carlo For AGN Channel Testing and Simulation (McFACTS; https://www.github.com/mcfacts/mcfacts) code to study the effect of active galactic nucleus (AGN) disk and nuclear star cluster (NSC) model parameters on predicted mass distributions for LIGO–Virgo–KAGRA compact binaries forming in AGN disks. The assumptions we vary include the black hole (BH) initial mass function (IMF), disk model, disk size, disk lifetime, NSC model, and the prograde-to-retrograde fraction of newly formed BH binaries. Broadly, we find that dense, moderately short-lived AGN disks are preferred for producing a (q, χeff) anticorrelation like those identified from existing gravitational-wave observations. Additionally, a BH IMF (IMF ∝ M−2) is preferred over a more top-heavy IMF (M−1). To produce results consistent with observations, the preferred fraction of prograde to retrograde is >90%.

The outer regions of AGN disks have temperatures similar to those of circumstellar disks, permitting dust condensation. Therefore, planet formation and growth could be active in these dust tori through similar mechanisms. We aim at quantifying the parameter space for the occurrence of streaming instability, and its outcomes in terms of the masses of the objects formed, their total number, and their continued growth via pebble accretion. We use a recently proposed disk model with strong magnetization to keep the disk gravitationally stable. We find that the dust grain sizes required for streaming instability are easily attained through coagulation; the dust filaments it produces can contain solar masses, collapsing into tens of millions of “planetesimals” ranging from Earth to super-Jupiter masses. These planets are usually born in the 3D Bondi regime of pebble accretion, and have mass-doubling times from 103 to 107 yr, though 3D Hill and geometric accretion are also realized. Gas accretion occurs concurrently, and crossover mass can be attained while still in the planetary mass range. As a result, vigorous accretion can occur, leading to objects with stellar masses—defining a core accretion channel for star formation. The pebble isolation mass is beyond the hydrogen-burning limit, so accretion is limited by stellar feedback instead of gap carving. We also predict a population of exotic objects directly formed above the hydrogen-burning limit, yet of pure dust. Our model suggests that AGN dust tori host the largest populations of planets in the Universe.

Supernova (SN) shocks that originate from stars on orbits embedded in dense active galactic nucleus (AGN) accretion disks evolve differently from those that occur in the interstellar medium. We aim to assess how shocks evolve in this dense stratified medium and understand where SNe are muffled and have their kinetic energy absorbed by an AGN disk versus escaping. We use Sirko–Goodman (SG) and Thompson–Quataert–Murray (TQM) AGN disk models for midplane radial profiles, generated with the pAGN code; we compare the disk pressure to the energy of a standard core-collapse SN (1051 erg) to find radii where shock breakout can occur. For verification, we evolve three-dimensional hydrodynamic shearing box simulations of stratified Gaussian disks constructed from the midplane values that are injected with energy and mass from SNe placed at multiple radii and vertical locations, using the Athena code. We find SN shocks in SG disks around black holes with mass MBH = 106 M⊙ become muffled beyond R ∼ 106 Rs, and that this muffling radius is inversely proportional to supermassive black hole mass with muffling occurring at R ∼ 102 Rs for MBH = 109 M⊙. Around TQM disks, the muffling radius occurs at R ∼ 106 Rs, independent of MBH. The largest determining factor for muffling an SN shock is the local scale height of the AGN disk. In conclusion, we developed a predictive analytic criterion to identify where AGN disks can muffle SN shocks depending on their density and vertical scale.

Galaxy formation models within cosmological hydrodynamical simulations contain numerous parameters with nontrivial influences over the resulting properties of simulated cosmic structures and galaxy populations. It is computationally challenging to sample these high dimensional parameter spaces with simulations, in particular for halos in the high-mass end of the mass function. In this work, we develop a novel sampling and reduced variance regression method, CARPoolGP, which leverages built-in correlations between samples in different locations of high dimensional parameter spaces to provide an efficient way to explore parameter space and generate low-variance emulations of summary statistics. We use this method to extend the Cosmology and Astrophysics with machinE Learning Simulations to include a set of 768 zoom-in simulations of halos in the mass range of 1013–1014.5 M ⊙ h −1 that span a 28-dimensional parameter space in the IllustrisTNG model. With these simulations and the CARPoolGP emulation method, we explore parameter trends in the Compton Y–M, black hole mass–halo mass, and metallicity–mass relations, as well as thermodynamic profiles and quenched fractions of satellite galaxies. We use these emulations to provide a physical picture of the complex interplay between supernova and active galactic nuclei feedback. We then use emulations of the Y–M relation of massive halos to perform Fisher forecasts on astrophysical parameters for future Sunyaev–Zeldovich observations and find a significant improvement in forecasted constraints. We publicly release both the simulation suite and CARPoolGP software package.

The amphibian skin microbiome is an important component of anti-pathogen defense, but the impact of environmental change on the link between microbiome composition and host stress remains unclear. In this study, we used radiotelemetry and host translocation to track microbiome composition and function, pathogen infection, and host stress over time across natural movement paths for the forest-associated treefrog, Boana faber . We found a negative correlation between cortisol levels and putative microbiome function for frogs translocated to forest fragments, indicating strong integration of host stress response and anti-pathogen potential of the microbiome. Additionally, we observed a capacity for resilience (resistance to structural change and functional loss) in the amphibian skin microbiome, with maintenance of putative pathogen-inhibitory function despite major temporal shifts in microbiome composition. Although microbiome community composition did not return to baseline during the study period, the rate of microbiome change indicated that forest fragmentation had more pronounced effects on microbiome composition than translocation alone. Our findings reveal associations between stress hormones and host microbiome defenses, with implications for resilience of amphibians and their associated microbes facing accelerated tropical deforestation. Short-term tracking of treefrogs in southern Brazil, alongside microbiome and stress hormone profiling, suggests that forest fragmentation impacts microbiome diversity and composition, and that these changes might be mediated by the host stress response.

Given NASA’s plans for manned lunar and Mars missions, it is critical to assess the risk of splenic immune dysregulation by using ground-based models of simulated microgravity (SMG) and/or chronic irradiation (CIR). To address this, C57BL/6 J mice of both sexes exposed to SMG and/or CIR for 29 days and alterations in immune cell distribution, function and phenotype were assessed. SMG and/or CIR altered a greater variety of immune cells in both lymphoid and myeloid lineages in female mice than in male mice; the function of splenic CD4 + T cells, CD8 + T cells, and CD19 + B cells altered in a sex-specific manner; and the distribution of different immune cells altered based on animal sex. These findings indicate that SMG and/or CIR alter the splenic immune cell distribution, phenotype and function in a sex-specific manner, underscoring the need for tailored strategies to mitigate health risks for crew members on long-term deep-space missions.

Active galactic nuclei host a supermassive black hole, a swarm of stars and black holes in a nuclear star cluster, and most critically, an accretion disk. Some of those stars and black holes will lie on orbits aligned with the disk, while others will have orbits that cross through it. Gas drag from the disk produces torques on their orbits that will align more orbits with the disk and once their orbits are sufficiently circular will cause embedded objects to migrate. Since more massive objects migrate faster, they can sweep up lighter black holes to form binaries on their way towards migration traps, where inwards and outwards migration meet, which can drastically increase binary formation rates. Experiencing more gas effects and dynamical encounters with other objects can cause these binaries to merge quickly relative to systems without such mechanisms. Once close enough, runaway orbital shrinkage will occur until they emit a burst of energy in the form of gravitational waves as they warp space-time in the final moments before coalescing into a single object that can be detected by ground-based gravitational wave observatories. The environment imprints unique signatures on the underlying black hole population that can be used to probe the conditions of the disk and star cluster with enough events. Black holes in this binary formation channel can participate in many merger events that may result in intermediate mass black holes due to their high concentration. Since the disk aligns the orbits and spins of embedded black holes over time and increases their mass through hierarchical events, the channel naturally replicates an anti-correlation present in the observed gravitational wave events where mergers between unequal mass black holes tend to have their spins more aligned with the binary’s orbital angular momentum. In addition to the gravitational waves, the shocks from the remnant interacting with the gas disk may produce a bright electromagnetic counterpart. The stars of the nuclear star cluster experience the same orbital changes caused by the disk, and massive stars may detonate in supernova explosions. While these light curves are well-studied and documented, the dense disk causes these to differ from naked supernovae as suggested by studies of those that must expand into dense envelopes expelled by strong winds of their progenitor stars. As such, these transients could become sources of contamination in searches for merger counterparts without understanding their characteristics. Additionally, while the accretion disk is an important factor for these phenomena, the details of their structures and conditions are not well constrained. Therefore, we can use these multi-messenger transient events to learn about the disk that produces them. In this dissertation I will show work I have done to explore this dynamic environment. I used Monte Carlo simulations to study populations of black hole mergers to understand how variations to initial conditions and physical processes affect their mass ratios and effective spins. I will also show hydrodynamic simulations of embedded supernovae and their light curves produced in post-processing radiative transfer calculations.