arXiv subjects

Blakesley Burkhart

Publications and source records attributed to Blakesley Burkhart.

At least 19 recordsLinked to original sources

arXiv2026

Radiative Feedback in Population III Protostellar Growth: HI Shielding and HII Region Trapping

We present a suite of radiation-magnetohydrodynamics simulations from the Popsicle project that follow the long-term growth ($\sim 50$ kyr) of primordial protostars while self-consistently coupling radiation, turbulence, and magnetic fields. The simulation suite is designed to quantify the relative impacts of the pathways of radiative feedback in Pop III stars -- the extreme-ultraviolet (EUV) ionization and Lyman-Werner (LW) dissociation -- by considering simulations with/without their inclusion. We find that without HI shielding, LW feedback can suppress and ultimately terminate accretion. With HI shielding, the large column densities near the protostar significantly weaken LW feedback. In the polar direction, atomic hydrogen fully shields LW radiation where H$_2$ self-shielding alone is insufficient. This leads to lower gas temperatures near the protostar and higher accretion rates, yielding larger final stellar masses than in models without shielding. The HII region remain confined, extending $\sim$100 AU beyond the sink accretion radius (75 AU), as dense gravitationally bound gas sustains high recombination rates and prevents sustained pressure-driven breakout. Turbulence and magnetic fields may also contribute to its confinement, even at high ionizing luminosities. These results demonstrate that the interplay of gas dynamics, shielding, and radiative feedback can significantly alter the growth of Pop III stars. We discuss the implications for the initial mass function of primordial stars and the influence of feedback from early stellar populations.

astro-ph.GA↗

arXiv2026

Multiscale Synthetic observations of Polarized dust emission: On the origin of Depolarization effect from Molecular clouds to Starless cores and Constraints on Dust Physics

Submillimeter observations of polarized dust emission from molecular clouds to starless cores frequently report a decrease in polarization fraction (p) with increasing dust emission intensity (I). This feature is commonly attributed to the alignment loss of dust grains or the geometrical effects of magnetic fields, yet, the detailed contributions remain unclear. To investigate the mechanism responsible for the depolarization, we use POLARIS to perform the multiscale synthetic dust polarization observations at $850μm$ from magnetically aligned dust grains by RAdiative Torques (RATs) mechanism. We adopt three collapsing cloud models with different magnetic energy levels and explore the effects of grain magnetic properties and grain growth on dust polarization. The field geometrical effect is the dominant depolarization mechanism at $N_{\rm H}<10^{21}-10^{22}\rm cm^{-2}$. We find that if grains grow beyond $>0.5μm$ and are superparamagnetic (SPM) with large iron clusters, RATs remain effective at high column densities, and grain alignment loss contributes to depolarization only at $N_{\rm H} > 10^{23}\rm cm^{-2}$. If neither of these conditions is satisfied, the alignment loss (in cases of insufficient grain growth); or the reduced grain alignment efficiency by gaseous damping (for paramagnetic grains or SPM grains with small iron cluster sizes) can become the dominant depolarization mechanism at $N_{\rm H} > 10^{22}\rm cm^{-2}$, regardless of how tangled the magnetic field lines in our simulation. Finally, we show that the depolarization mechanism and the underlying dust physical properties inside starless cores may be identified through the $p-I$ slope and the mean p at the core center.

astro-ph.GA↗

arXiv2026

Multiphase turbulence as the origin of OH+, H2O+ and H3+ column density scatter in the local ISM

Observations of the reactive ions OH+, H2O+ and H3+ in the Galactic interstellar medium reveal large sight-line-to-sight-line scatter in their column densities, commonly interpreted as evidence for substantial variations in the cosmic-ray ionization rate (CRIR). We revisit this interpretation using high-resolution three-dimensional magneto-hydrodynamic simulations of the multiphase ISM with time-dependent chemistry for H, H2, H+ and electrons, building on the fiducial model of Godard et al. (2023). We find that a single CRIR of ~2 10^{-16} s^{-1}, together with standard Galactic-scale parameters, naturally produces broad column-density distributions for all three tracers in good agreement with the observed medians and percentile widths, with no fine tuning. Reaching this match requires that the post-processing of OH+, H2O+ and H3+ retain the time-dependent H2 field generated by the turbulent flow rather than assume chemical equilibrium: turbulence drives long-lived H2 enhancements in the unstable neutral medium where OH+ and H2O+ predominantly reside, and an equilibrium treatment under-predicts their columns substantially. H3+, which receives most of its column from denser CNM gas closer to equilibrium, is much less affected. Our results caution against interpreting sight-line-to-sight-line scatter as direct evidence for large CRIR fluctuations, and motivate a shift from independent 1D equilibrium analyses toward 3D dynamical frameworks when inferring ionization conditions in the ISM.

astro-ph.GA↗

arXiv2026

Learning the Universe with the 2nd Generation of CAMELS: Varying 35 parameters of the IllustrisTNG model in (50Mpc/h)^3 boxes

We present a new set of 1,192 cosmological simulations as part of the CAMELS project, in which a space of 35 cosmological, astrophysical, and numerical parameters is explored around the fiducial IllustrisTNG model. The volume of each of these simulations is (50Mpc/h)^3, eight times larger than that of previous CAMELS simulations. This provides lower sample variance as well as access to more massive halos and more diverse environments. We focus this work on exploring the advantages these differences provide for parameter inference powered by neural networks. We generate training sets based on the matter power spectra, projected maps of the volumes, graphs representing galaxy spatial distributions, and thermodynamical properties of massive halos. We employ multilayer perceptrons, convolutional neural networks, graph neural networks, and Gaussian processes, respectively, to extract information on the simulation parameters from these inputs while comparing systematically to analogous results from our previous generation of (25Mpc/h)^3 simulations. We generally find that the new, larger volumes produce tighter marginal constraints on the parameters, to degrees that vary between the different inputs. The improvements, however, scale more weakly than with the square root of the increase in the amount of data (i.e., physical volume). We interpret this as originating either from information loss due to mode coupling or from complex degeneracies in parameter space. We also discuss the effects on statistics of the intergalactic medium temperature from four new parameters that are varied in these simulations, which control the amplitude and timing of the ionizing background radiation. We publicly release the simulation outputs and ancillary data at https://camels.readthedocs.io.

astro-ph.CO↗

arXiv2026

Implications of the nanoHertz Gravitational-Wave Background for Galactic Feedback and Massive Black Hole Growth

We investigate how pulsar timing array (PTA) measurements of the nanoHertz gravitational-wave background (GWB) can constrain models for the growth history of supermassive black holes (SMBHs) and how active galactic nucleus (AGN) and stellar feedback models can affect GWB predictions. Feedback regulates supermassive black hole (SMBH) growth, altering the black hole mass function (BHMF). Using BHMFs drawn from multiple cosmological simulation suites including IllustrisTNG, MillenniumTNG, Simba, and CAMELS, and combining these with a quasar-based SMBH binary population framework, we predict the resulting GWB amplitude under a range of different stellar and AGN feedback prescriptions. We find that the choice of both stellar and AGN feedback models alters the high-mass end of the BHMF and changes the predicted GWB amplitude by up to a factor of 2 for the fiducial simulations and a factor 10 for extreme feedback variations in CAMELS. Models with inefficient or absent AGN feedback produce abundant SMBHs and yield GWB amplitudes consistent with PTA data, yet fail in producing realistic galaxies. Fiducial models of AGN and stellar feedback suppress SMBH growth too much and under-predict the expected signal, an effect which could possibly be mitigated by more realistic black hole seeding and growth prescriptions. The mismatch between the GWB amplitudes predicted by cosmological simulations and that inferred by PTA measurements suggests that SMBH growth is more efficient or occurs earlier than captured by current models. This demonstrates that PTA measurements provide a powerful new probe of not only the SMBH population but also feedback physics.

astro-ph.GA↗

arXiv2026

Exploring the impact of AGN feedback model variations on the Lyman-$α$ Forest Flux Power Spectrum

We study the effects of varying different Active Galactic Nuclei (AGN) feedback parameters on the Lyman-$α$ (Ly$α$) forest 1D transmitted flux power spectrum (P1D). We use the Cosmological and Astrophysics with Machine Learning Simulations (CAMELS) suite to explore variations on the Simba simulation AGN feedback model. The parameters explored include AGN momentum flux, AGN jet speed, supermassive black hole (SMBH) radiative efficiency, jet velocity threshold, and minimum SMBH mass needed to produce jet feedback. Although all parameters affect the P1D, this work explores the radiative efficiency, jet velocity threshold, and minimum SMBH mass in this context for the first time and finds the following results: Primarily, the most massive SMBHs impact the Ly$α$ forest through the jet feedback mode. While heating AGN jets to the virial temperature at injection aids in the removal of neutral hydrogen from the Ly$α$ forest, this heating also inhibits further jet feedback. Similar behaviors are seen when varying the SMBH radiative efficiency, with higher values resulting in a suppression of SMBH growth and thus a later reduction in AGN feedback and lower values directly reducing the impact of AGN feedback on the Ly$α$ forest P1D. These results imply that increasing the AGN feedback strength in the Simba simulation model suppresses the Ly$α$ forest P1D, but only if the feedback does not impact the number of massive jet producing BHs. Future studies of AGN feedback models will require careful exploration of the unique aspects of the specific subgrid model, and how they interact with one another, for a complete understanding of the potential astrophysical impacts of SMBH feedback.

astro-ph.GA↗

arXiv2026

A cosmological framework for stellar collisions at high redshift in proto-globular clusters, nuclear star clusters, and Little Red Dots

Observations and cosmological simulations indicate that the early Universe hosted numerous compact, high-density stellar systems, where close encounters and physical collisions between stars were likely common. We develop a bottom-up framework for stellar dynamics in such environments, spanning systems with and without intermediate- and supermassive black holes, and covering regimes where stellar collisions may or may not dominate the evolution. This radially-resolved analytic model connects dense star clusters in their cosmological context to observable outcomes mediated by stellar collisions. Initial conditions and environmental properties are drawn from high-resolution cosmological simulations, enabling exploration across a broad region of parameter space. The analytic predictions are validated against Monte Carlo simulations, demonstrating good agreement across key regimes. We find that stellar collisions are ubiquitous in many high-redshift environments, with runaway sequences naturally leading to the formation of very massive stars at early times. Finally, we show that high rates of destructive collisions can rapidly build up extremely dense gaseous environments around massive black holes, potentially providing an analogue to the observed population of Little Red Dots.

astro-ph.GA↗

arXiv2025

A Tentative Detection of Molecular Hydrogen (H$_2$) Emission Lines at Cosmic Dawn

We present a theoretical framework for interpreting far-ultraviolet (FUV) fluorescent emission from molecular hydrogen (H$_2$) in high-redshift galaxies, motivated by the unique capabilities of the James Webb Space Telescope (JWST) to probe the rest frame FUV at cosmic dawn. Using the Meudon photodissociation region (PDR) code, we model the H$_2$ fluorescence spectrum under extreme interstellar medium (ISM) conditions in terms of high pressure ($10^{11}~\mathrm{K~ cm^{-3}}$), high radiation field ($10^6$ $G_0$) combined with low metallicity ($Z = 0.1~Z_\odot$) and high cosmic ionization rate ($ζ= 10^{-14}~\mathrm{s}^{-1}$), characteristic of early galaxies. As a case study, we apply this framework to stacked NIRSpec spectra from the JWST Advanced Deep Extragalactic Survey (JADES) for galaxies at redshifts $z\geq7$. The stacked spectrum exhibits emission features consistent in profile and wavelength with the predicted H$_2$ fluorescence lines, including a blue shift suggestive of an outflow of molecular gas. Although individual features remain below robust detection thresholds, this demonstration illustrates the feasibility of using FUV fluorescence modeling to guide and interpret JWST spectroscopy of the molecular ISM at high redshift. Our framework provides a foundation for future searches for molecular hydrogen emission and the study of galactic feedback processes in the early universe.

astro-ph.GA↗

arXiv2025

Observing compact Pop III star clusters and the presence of cosmic streaming

The formation of the Universe's first luminous stellar structures depends on the unique conditions at "Cosmic Dawn," which are set by the underlying cosmological model and early baryonic physics. Observations suggest that high-$z$ star clusters reached stellar surface densities above $10^5 M_\odot$ pc$^{-2}$, suggesting scenarios where models predict that the ability of stellar feedback to counter gravitational collapse is severely limited. We investigate the first star clusters in a suite of AREPO simulations, which explore the capacity for $Λ$CDM halos to maximally form high-density systems without feedback. We include the effects of the supersonic baryon-dark matter streaming velocity, an effect that impacts gas density and distribution in early minihalos. We show that early star clusters can reach high densities even in regions of strong supersonic streaming, provided feedback is weak. We analyze the interplay of the stream velocity and the dynamical processes of structure formation, finding that JWST has the opportunity to detect the brightest, most massive objects in our computational box. The detection of individual $z\geq12$ Pop III star clusters below $10^7M_\odot$ is challenging, although lensing could reveal these objects in rare configurations, especially if a top-heavy IMF is present. We find that accounting for baryonic clusters separately from dark matter halos complicates predictions for the faint-end of the high-$z$ UVLF, with competing effects from the stream velocity and low-mass clusters outside of halos. Finally, we explore clustering of star clusters as a promising probe of the stream velocity in these systems.

astro-ph.GA↗

arXiv2025

Protostellar Jets in Star Cluster Formation and Evolution: I. Implementation and Initial Results

Stars form in clusters from the gravitational collapse of giant molecular clouds, which is opposed by a variety of physical processes, including stellar feedback. The interplay between these processes determines the star formation rate of the clouds. To study how feedback controls star formation, we use a numerical framework that is optimized to simulate star cluster formation and evolution. This framework, called Torch, combines the magnetohydrodynamical code FLASH with N-body and stellar evolution codes in the Astrophysical Multipurpose Software Environment (AMUSE). Torch includes stellar feedback from ionizing and non-ionizing radiation, stellar winds, and supernovae, but, until now, did not include protostellar jets. We present our implementation of protostellar jet feedback within the Torch framework and describe its free parameters. We then demonstrate our new module by comparing cluster formation simulations with and without jets. We find that the inclusion of protostellar jets slows star formation, even in clouds of up to M $= 2 \times 10^4$ M$_{\odot}$. We also find that the star formation rate of our lower mass clouds (M $= 5 \times 10^3$ M$_{\odot}$) is strongly affected by both the inclusion of protostellar jets and the chosen jet parameters, including the jet lifetime and injection velocity. We follow the energy budget for each simulation and find that the inclusion of jets systematically increases the kinetic energy of the gas at early times. The implementation of protostellar jet feedback in Torch opens new areas of investigation regarding the role of feedback in star cluster formation and evolution.

astro-ph.GA↗

arXiv2025

Joint Analysis of HI Absorption Zeeman Measurements and the Morphology of Filamentary HI Emission

We present a joint analysis of HI absorption Zeeman measurements and the morphology of filamentary HI emission to investigate the three-dimensional structure of the magnetic field in the diffuse neutral interstellar medium (ISM). Our analysis is based on the Arecibo Millennium Survey and new data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) toward radio sources 3C 75, 3C 207, and 3C 409. Toward 3C 409, we make a 4$σ$ Zeeman detection and infer $B_{LOS}$ = 9.1 +/- 1.9$μ$G, in agreement with Arecibo results. We quantify the dispersion of HI filaments at the locations and velocities of Zeeman components using GALFA-HI narrow-channel emission maps. Focusing on a subsample of 42 spectrally distinct components, we find a weak but statistically significant positive correlation (Spearman r = 0.3, $p = 0.01$) between $|B_{LOS}|$ and the circular variance of HI filament orientation angles. To examine its origin, we characterize the environments probed by HI absorption using dust emission, 3D dust maps, OH absorption, and CO emission. We find evidence that existing HI absorption Zeeman measurements trace magnetic fields that are coherent on parsec scales, probe primarily local gas ($100$-$500$ pc, often at distances consistent with the Local Bubble wall), and exhibit systematic differences in the magnitude of $B_{LOS}$. We attribute the correlation between Zeeman measurements and filamentary HI morphology to large-scale variations in magnetic field strength and/or inclination angle across different Galactic environments, which could arise due to the Local Bubble geometry or enhanced total field strength in denser regions.

astro-ph.GA↗

arXiv2025

Development of Turbulence in Post-shock Regions

Supersonic isothermal turbulence is ubiquitous in the interstellar medium. This work presents high-resolution AREPO hydrodynamical simulations of isolated shocks moving through supersonic turbulence to study the development and evolution of turbulence in the pre- and post-shock regions. We find that shocks can amplify turbulent energy in the post-shock region while inducing a preferential orientation for the vorticity. This results in the creation of anisotropic turbulence in the post-shock region. Turbulent energy and dissipation are also strongly enhanced near the shock front. By applying typical scalings from the cold neutral medium to simulations, we find that shocks moving into turbulence on the scale of superbubbles can generate compressive flows on the order of $10^{3} M_{\odot}$/Myr. Our results also show good agreement with related studies on turbulent fluctuations generated by shocks in pure fluid mechanics.

astro-ph.GA↗

arXiv2025

Magnetic fields in the Eos Cloud: dynamically important fields in the interface between atomic and molecular gas

The recently-discovered Eos molecular cloud, is a CO-dark, low-density cloud located at a distance of approximately 94 pc from the Sun which does not appear to have formed stars at any point in its history. In this paper we investigate the magnetic fields in the Eos cloud, near the interface between the atomic Cold Neutral Medium (CNM) and molecular gas, using dust emission and extinction polarimetry. A Histogram of Relative Orientation analysis shows that the magnetic field is preferentially parallel to the density structure of the cloud, while a Davis-Chandrasekhar-Fermi analysis finds magnetic field strengths of 6$\pm$3 $μ$G across the Eos cloud and 12$\pm$4 $μ$G in the somewhat denser MBM 40 sub-region. These results are consistent with a previous estimate of magnetic field strength in the Local Bubble and suggest that the fields in the Eos cloud are dynamically important compared to both gravity and turbulence. Our findings are fully consistent with the expected behavior of magnetized, non-self-gravitating gas near the CNM/molecular cloud boundary.

astro-ph.GA↗

arXiv2025

Planets and planetesimals at cosmic dawn: Vortices as planetary nurseries

Low-mass, metal-enriched stars were likely present as early as cosmic dawn. In this work, we investigate whether these stars could have hosted planets in their protoplanetary disks. If so, these would have been the first planets to form in the Universe, emerging in systems with metallicities much lower than solar. In the core accretion model, planetesimals serve as the building blocks of planets, meaning that planetesimal formation is a prerequisite for planet formation. In a non-structured disk, planetesimal formation typically requires near-solar metallicities according to our current understanding. However, mechanisms that concentrate solid material can significantly lower this metallicity threshold. Here, we explore whether vortices can facilitate the formation of the first planets and planetesimals at cosmic dawn. Vortices are prime sites for planetesimal formation due to their ability to efficiently trap and concentrate dust. We conduct simulations spanning a range of metallicities, and identify a metallicity threshold at Z >=~ 0.04 Zsun for planetesimal formation. If these planetesimals remain inside the vortex long enough to accrete the remaining trapped solids, Mercury-mass planets can form. The formation of Mars-mass planets or larger requires a metallicity of Z >=~ 0.08 Zsun. These results assume a low level of disk turbulence, with higher turbulence levels leading to higher metallicity thresholds.

astro-ph.EP↗

arXiv2025

Supersonic Turbulence in Primordial Halos: A Comparison With and Without The Stream Velocity

Turbulence plays a critical role in regulating star formation in molecular clouds and is also observed in simulations of primordial halos that host Population III (Pop III) stars. The relative velocity between baryons and dark matter at the time of recombination is thought to be a source of turbulence in the early universe. In this paper, we study how this stream velocity affects the turbulence inside primordial halos using high-resolution cosmological simulations across the redshift range of $z = 30$ to $z = 20$. We find that at a fixed redshift, the stream velocity enhances turbulence in low-mass halos ($M \lesssim 10^6 \ \mathrm{M_\odot}$) and suppresses it for high-mass halos ($M \gtrsim 10^6 \ \mathrm{M_\odot}$). The enhancement in low-mass halos likely arises from residual kinetic energy introduced by the stream velocity, while the suppression in high-mass halos likely arises from a reduction in inflowing accretion-driven turbulence. This mass-dependent modulation of turbulence suggests that the initial conditions inside primordial halos are altered in the presence of the stream velocity, potentially influencing their fragmentation and the resulting star formation.

astro-ph.GA↗

arXiv2025

Reconstructing galactic feedback history via the Lyman-$α$ forest with Habitable Worlds Observatory

Recent studies have focused on the low-$z$ Ly$α$ forest as a potential constraint on galactic feedback, as different AGN and stellar feedback models in hydrodynamic simulations produce varying intergalactic medium (IGM) statistics. However, existing low-$z$ Ly$α$ forest data provide insufficient observational constraints for simulations due to their low precision and the lack of observations from $z \sim 0.4$ to $1.8$. The Habitable Worlds Observatory, equipped with an ultraviolet (FUV / NUV) spectrograph, could provide transformative data for this science by increasing both absorbers in the redshift range where current data are lacking and increasing the precision of the Ly$α$ forest observational data at $z \leq 0.4$. This spectrograph should cover the wavelengths $1215-3402$ Å~and have a spectral resolution of $R =$~40,000. Distinguishing between certain active-galactic nuclei feedback models requires high precision ($\sim 5-10$\%) measurements of the Ly$α$ forest 1D transmitted flux power spectrum for $z\leq 1.8$. This requires $\sim 830$ QSO spectra with S/N~$\geq5$ - $25$ depending on redshift. This data would also enable a comprehensive study on the thermal state of the IGM across time, and address the tension between the observed and simulated Ly$α$ forest $b$-value distribution.

astro-ph.GA↗

arXiv2025

Correlations, mean-field limits, and transition to the concentrated regime in motile particle suspensions

Suspensions of swimming particles exhibit complex collective behaviors driven by hydrodynamic interactions, showing persistent large-scale flows and long-range correlations. While heavily studied, it remains unclear how such structures depend on the system size and swimmer concentration. To address these issues, we simulate very large systems of suspended swimmers across a range of system sizes and volume fractions. For this we use high-performance simulation tools that build on slender body theory and implicit resolution of steric interactions. At low volume fractions and long times, the particle simulations reveal dynamic flow structures and correlation functions that scale with the system size. These results are consistent with a mean-field limit and agree well with a corresponding kinetic theory. At higher concentrations, the system departs from mean-field behavior. Flow structures become cellular, and correlation lengths scale with the particle size. Here, translational motion is suppressed, while rotational dynamics dominate. These findings highlight the limitations of dilute mean-field models and reveal new behaviors in dense active suspensions.

cond-mat.soft↗

arXiv2025

A data-driven approach for star formation parameterization using symbolic regression

Star formation (SF) in the interstellar medium (ISM) is fundamental to understanding galaxy evolution and planet formation. However, efforts to develop closed-form analytic expressions that link SF with key influencing physical variables, such as gas density and turbulence, remain challenging. In this work, we leverage recent advancements in machine learning (ML) and use symbolic regression (SR) techniques to produce the first data-driven, ML-discovered analytic expressions for SF using the publicly available FIRE-2 simulation suites. Employing a pipeline based on training the genetic algorithm of SR from an open software package called PySR, in tandem with a custom loss function and a model selection technique which compares candidate equations to analytic approaches to describing SF, we produce symbolic representations of a predictive model for the star formation rate surface density ($Σ_\mathrm{SFR}$) averaged over both 10 Myr and 100 Myr based on eight extracted variables from FIRE-2 galaxies. The resulting model that PySR finds best describes SF, on both averaging timescales, features equations that incorporates the surface density of gas, $Σ_\mathrm{gas}$, the velocity dispersion of gas $σ_{\mathrm{gas,~z}}$ and the surface density of stars $Σ_\mathrm{*}$. Furthermore, we find that the equations found for the longer SFR timescale all converge to a scaling-relation-like equation, all of which also closely capture the intrinsic physical scatter of the data within the Kennicutt-Schmidt (KS) plane. This observed convergence to physically interpretable scaling relations at longer SFR timescales demonstrates that our method successfully identifies robust physical relationships rather than fitting to stochastic fluctuations.

astro-ph.GA↗