arXiv subjects

Aaron Smith

Publications and source records attributed to Aaron Smith.

At least 73 records · Page 4Linked to original sources

arXiv2025

How Early Could the Milky Way's Disk Form?

We investigate early, $z > 3$, galaxy formation in a cosmological zoom-in simulation of a close, early-forming Milky Way (MW) analog extracted from TNG50 simulation and resimulated with detailed modeling of cold interstellar medium (ISM) formation, coupled with on-the-fly UV radiative transfer, turbulence-regulated star formation, and stellar feedback. In our enhanced-physics simulation, the galaxy develops a bistable ISM structure (warm, with $T \sim 10^4$ K, and cold, with $T < 100$ K) and exhibits significantly more efficient, early, and bursty star formation than in TNG. Notably, the stellar disk of this MW progenitor forms extremely early, around $z\sim6-7$, and exhibits chemo-kinematic properties consistent with the low-metallicity population of the MW stars. The disk forms rapidly, on a timescale of $\sim$0.2 Gyr which is significantly shorter than the timescale implied by the observable chemo-kinematic signatures of disk spinup, $\sim$0.7 Gyr, due to the scatter in the age--metallicity relation. The rotational support of the gas disk and the location of the galaxy on the main sequence are consistent with early disk galaxies observed by JWST and ALMA at $z\sim4-7$, suggesting that some of these galaxies could be progenitors of MW-like systems. Remarkably, the variation of the global star formation rate (SFR) before disk formation is similar to the observed SFR scatter at these early times. Our findings underscore the critical role of modeling a turbulent cold ISM and turbulence-regulated star formation and feedback in driving early SFR variability, while at the same time enabling early disk formation, without destroying it with overly efficient stellar feedback.

astro-ph.GA↗

arXiv2025

The THESAN project: environmental drivers of Local Group reionization

The timing of cosmic reionization across Local Group (LG) analogues provides insights into their early histories and surrounding large-scale structure. Using the radiation-hydrodynamic simulation THESAN-1 and its dark matter-only counterpart THESAN-DARK-1, we track the reionization histories of all haloes, including 224 LG analogues within the proximity of any of the 20 Virgo-like clusters with halo masses above 10^14 Msun at z=0 and their environments. The statistically controlled samples quantify how the reionization redshift (z_reion) correlates with halo mass, local overdensity, and present-day pair properties. Even at fixed mass, haloes in denser regions ionize earlier, and increasing the overdensity smoothing scale systematically suppresses small-scale structure, including local variations and environmental gradients in z_reion. Virgo-like clusters accelerate reionization in their surroundings out to ~5-10 cMpc, beyond which local overdensity again becomes the dominant factor. Within LG pairs, reionization timing offsets reach up to ~150 Myr and correlate with present-day halo separation, reflecting sensitivity to large-scale structure rather than mass ratio in driving asynchronous reionization. The results support an extreme inside-out picture where clustered sources rapidly ionize their immediate neighborhoods, while lower-density regions self-ionize later and voids wait for external homogenization. These links between environment and reionization timing explain the influence of protoclusters and help interpret fossil records in LG dwarfs around the Milky Way. For Milky Way analogues, we find a reionization redshift as early (late) as z_reion = 12.7^{+2.0}_{-1.7} (8.88^{+0.66}_{-0.70}) when considered on 125 ckpc (500 ckpc) scales, with LG analogues following an inside-out reionization picture.

astro-ph.CO↗

arXiv2025

Lyman-alpha resonant-line radiative transfer in expanding media

The Lyman-alpha (LyA) line of atomic hydrogen encodes crucial information about both the intrinsic sources and surrounding environments of star-forming regions throughout the Universe. Due to the complexity of resonant scattering, analytic solutions remain scarce, with most studies focusing on idealized, static configurations. However, observations of LyA emitting galaxies consistently reveal signatures of outflows, imprinted through red-peak dominance in spectral line profiles. In this paper, we derive novel analytic solutions for resonant-line radiative transfer in moving media, specifically homologous-like cloud expansion and unbounded cosmological flows, which capture the main physics of velocity gradients. To validate these analytic solutions and identify regimes where diffusion-based assumptions hold, we introduce a robust Gridless Monte Carlo Radiative Transfer (GMCRT) method. By integrating optical depths exactly in the comoving frame, GMCRT updates photon frequencies continuously to account for Doppler shifts induced by velocity gradients. We demonstrate excellent consistency between GMCRT and our analytic solutions in regimes where diffusion approximations apply. At higher velocities or lower optical depths, discrepancies highlight the limitations of simplified formalisms. We also provide scaling relations for a point source in a cloud with a maximum-to-thermal velocity ratio beta = V_max / v_th, as modifying the standard dependence on line centre optical depth of (atau_0)^1/3 by additional factors, e.g. characteristic escape frequency scale as x_esc ~ beta^1/3, force multipliers as M_F ~ beta^-1/3, and trapping time as t_trap ~ beta^-2/3. Our work complements numerical simulations by improving physical intuition about nonstatic environments when interpreting LyA observations and guiding future subgrid prescriptions in galaxy formation models.

astro-ph.GA↗

arXiv2025

The THESAN-ZOOM project: Long-term imprints of external reionization on galaxy evolution

We investigate the impact of ionizing external ultraviolet (UV) radiation on low-mass haloes ($M_{h}<10^{10}M_\odot$) at high redshift using $1140M_\odot$ baryonic resolution zoom-in simulations of seven regions from the THESAN-ZOOM project. We compare three simulation sets that differ in the treatment of external UV radiation: one employing a uniform UV background initiated at z=10.6 in addition to radiation transport for local sources, another with the same background starting at z=5.5, and the default configuration in which the large-scale radiation field from the parent THESAN-1 simulation box acts as a boundary condition. The multi-phase interstellar medium (ISM) model, combined with its high mass resolution, allows us to resolve all star-forming haloes and capture the back-reaction of ionizing radiation on galaxy properties during the epoch of reionization. When present, external UV radiation efficiently unbinds gas in haloes with masses below $10^9M_\odot$ and suppresses subsequent star formation. As a result, in simulations with early reionization, minihaloes fail to form stars from pristine gas, leading to reduced metal enrichment of gas later accreted by more massive haloes. Consequently, haloes with masses below $10^{10}M_\odot$ at all simulated epochs (z>3) exhibit lower metallicities and altered metallicity distributions. The more accurate and realistic shielding from external UV radiation, achieved through self-consistent radiative transfer, permits the existence of a cold but low-density gas phase down to z=3. These findings highlight the importance of capturing a patchy reionization history in high-resolution simulations targeting high-redshift galaxy formation. We conclude that at minimum, a semi-numerical model that incorporates spatially inhomogeneous reionization and a non-uniform metallicity floor is necessary to accurately emulate metal enrichment in minihaloes.

astro-ph.GA↗

arXiv2025

The THESAN-ZOOM project: Population III star formation continues until the end of reionization

Population III (Pop III) stars are the first stars in the Universe, forming from pristine, metal-free gas and marking the end of the cosmic dark ages. Their formation rate is expected to sharply decline after redshift $z \approx 15$ due to metal enrichment from previous generations of stars. In this paper, we analyze 14 zoom-in simulations from the THESAN-ZOOM project, which evolves different haloes from the THESAN-1 cosmological box down to redshift $z=3$. The high mass resolution of up to $142 M_\odot$ per cell in the gas phase combined with a multiphase model of the interstellar medium (ISM), radiative transfer including Lyman-Werner radiation, dust physics, and a non-equilibrium chemistry network that tracks molecular hydrogen, allows for a realistic but still approximate description of Pop III star formation in pristine gas. Our results show that Pop III stars continue to form in low-mass haloes ranging from $10^6 M_\odot$ to $10^9 M_\odot$ until the end of reionization at around $z=5$. At this stage, photoevaporation suppresses further star formation in these minihaloes, which subsequently merge into larger central haloes. Hence, the remnants of Pop III stars primarily reside in the satellite galaxies of larger haloes at lower redshifts. While direct detection of Pop III stars remains elusive, these results hint that lingering primordial star formation could leave observable imprints or indirectly affect the properties of high-redshift galaxies. Explicit Pop III feedback and specialized initial mass function modelling within the THESAN-ZOOM framework would further help interpreting emerging constraints from the James Webb Space Telescope.

astro-ph.GA↗

arXiv2025

Variance estimation after matching or re-weighting

This paper develops a variance estimation framework for matching estimators that enables valid population inference for treatment effects. We provide theoretical analysis of a variance estimator that addresses key limitations in the existing literature. While Abadie and Imbens (2006) proposed a foundational variance estimator requiring matching for both treatment and control groups, this approach is computationally prohibitive and rarely used in practice. Our method provides a computationally feasible alternative that only requires matching treated units to controls while maintaining theoretical validity for population inference. We make three main contributions. First, we establish consistency and asymptotic normality for our variance estimator, proving its validity for average treatment effect on the treated (ATT) estimation in settings with small treated samples. Second, we develop a generalized theoretical framework with novel regularity conditions that significantly expand the class of matching procedures for which valid inference is available, including radius matching, M-nearest neighbor matching, and propensity score matching. Third, we demonstrate that our approach extends naturally to other causal inference estimators such as stable balancing weighting methods. Through simulation studies across different data generating processes, we show that our estimator maintains proper coverage rates while the state-of-the-art bootstrap method can exhibit substantial undercoverage (dropping from 95% to as low as 61%), particularly in settings with extensive control unit reuse. Our framework provides researchers with both theoretical guarantees and practical tools for conducting valid population inference across a wide range of causal inference applications. An R package implementing our method is available at https://github.com/jche/scmatch2.

stat.ME↗

arXiv2025

Cosmic Ly$α$ Emission from Diffuse Gas

The Ly$α$ emission has emerged as a powerful tool for probing diffuse gas within the large-scale structure of the universe. In this paper, we investigate cosmic Ly$α$ emission by post-processing cosmological simulations from \texttt{IllustrisTNG} and \texttt{THESAN} project. Specifically, we calculate the Ly$α$ emission from galaxies, circum-galactic medium (CGM) and inter-galactic medium (IGM) across various redshifts. Our results show that IGM alone is significantly under the current observational upper limits. Meanwhile, CGM overshoots the observed galaxy contribution at $z \lesssim 0.5$ indicating that either the escape fraction for the inner CGM is less than unity or the current photoionization equilibrium treatment with an approximate self-shielding prescription is less accurate. The galaxy component also overshoots at low redshift, indicating that the escape fraction has strong evolution caused by an evolving halo mass function and dust growth distribution, that agrees with observationally inferred escape fractions. Furthermore, our findings suggest that the Ly$α$ emission from diffuse gas (CGM+IGM) peaked at $z \sim 4$ and diminishes toward lower redshift. The Ly$α$ emission from diffuse gas mainly originates through the collisional excitation of hot plasma. By comparing models with observation, our predicted Ly$α$ emission from diffuse gas remains $\sim 6$ times fainter than the observed cosmic Ly$α$ emission at $z=1-3$. However, future large telescopes may hold great promise to detect Ly$α$ emission from diffuse gas toward $z>3$.

astro-ph.CO↗

arXiv2025

Sparse Bayesian multidimensional scaling(s)

Bayesian multidimensional scaling (BMDS) is a probabilistic dimension reduction tool that allows one to model and visualize data consisting of dissimilarities between pairs of objects. Although BMDS has proven useful within, e.g., Bayesian phylogenetic inference, its likelihood and gradient calculations require a burdensome order of $N^2$ floating-point operations, where $N$ is the number of data points. Thus, BMDS becomes impractical as $N$ grows large. We propose and compare two sparse versions of BMDS (sBMDS) that apply log-likelihood and gradient computations to subsets of the observed dissimilarity matrix data. Landmark sBMDS (L-sBMDS) extracts columns, while banded sBMDS (B-sBMDS) extracts diagonals of the data. These sparse variants let one specify a time complexity between $N^2$ and $N$. Under simplified settings, we prove posterior consistency for subsampled distance matrices. Through simulations, we examine the accuracy and computational efficiency across all models using both the Metropolis-Hastings and Hamiltonian Monte Carlo algorithms. We observe approximately 3-fold, 10-fold and 40-fold speedups with negligible loss of accuracy, when applying the sBMDS likelihoods and gradients to 500, 1,000 and 5,000 data points with 50 bands (landmarks); these speedups only increase with the size of data considered. Finally, we apply the sBMDS variants to: 1) the phylogeographic modeling of multiple influenza subtypes to better understand how these strains spread through global air transportation networks and 2) the clustering of ArXiv manuscripts based on low-dimensional representations of article abstracts. In the first application, sBMDS contributes to holistic uncertainty quantification within a larger Bayesian hierarchical model. In the second, sBMDS provides uncertainty quantification for a downstream modeling task.

stat.ME↗

arXiv2025

The THESAN-ZOOM project: Star formation efficiency from giant molecular clouds to galactic scale in high-redshift starbursts

Star formation in galaxies is inherently complex, involving the interplay of physical processes over a hierarchy of spatial scales. In this work, we investigate the connection between global (galaxy-scale) and local (cloud-scale) star formation efficiencies (SFEs) at high redshifts ($z\gtrsim 3$), using the state-of-the-art cosmological zoom-in simulation suite THESAN-ZOOM. We find that the galaxy-scale average SFE, $\langle ε^{\rm gal}_{\rm ff} \rangle$, scales with $M_{\rm halo}^{1/3}\,(1+z)^{1/2} \sim V_{\rm vir}$, consistent with expectations from feedback-regulated models. On cloud scales, we identify giant molecular clouds (GMCs) in a broad sample of high-redshift starbursts spanning a wide range of halo masses and redshifts. Star formation in these systems is predominantly hosted by filamentary GMCs embedded in a dense and highly turbulent interstellar medium (ISM). GMCs exhibit remarkably universal properties, including mass function, size, turbulence, and surface density, regardless of the environment in which they are identified. The global gas depletion time (and the Kennicutt-Schmidt relation) is determined by the GMC mass fraction in the ISM, while the cloud-scale SFE shows little variation. In particular, we find a nearly constant gas surface density of $Σ_{\rm GMC} \approx 70\,{\rm M}_{\odot}\,{\rm pc}^{-2}$ across different host galaxies. Nevertheless, we identify two regimes where phases with high SFE can arise. First, stars may form efficiently in the shock fronts generated by feedback from a preceding starburst. Second, the increasing background dark matter surface density with redshift may contribute to the gravitational potential of clouds at $z \gtrsim 8$ and confine them in high-SFE phases over extended periods.

astro-ph.GA↗

arXiv2025

A New Framework for ISM Emission Line Models: Connecting Multi-Scale Simulations Across Cosmological Volumes

The JWST and ALMA have detected emission lines from the ionized interstellar medium (ISM) in some of the first galaxies at $z \gtrsim 6$. These measurements present an opportunity to better understand galaxy assembly histories and may allow important tests of state-of-the-art galaxy formation simulations. It is challenging, however, to model these lines in their proper cosmological context. In order to meet this challenge, we introduce a novel sub-grid line emission modeling framework. The framework uses the high-$z$ zoom-in simulation suite from the Feedback in Realistic Environments (FIRE) collaboration. The line emission signals from HII regions within each simulated FIRE galaxy are modeled using the semi-analytic HIILines code. A machine learning approach is then used to determine the conditional probability distribution for the line luminosity to stellar-mass ratio from the HII regions around each simulated stellar particle. This conditional probability distribution can then be applied to predict the line luminosities around stellar particles in lower resolution, yet larger volume cosmological simulations. As an example, we apply this approach to the IllustrisTNG simulations at $z=6$. The resulting predictions for the [OII], [OIII], and Balmer line luminosities as a function of star-formation rate (SFR) agree well with current observations. Our predictions differ, however, from related work in the literature which lack detailed sub-grid ISM models. This highlights the importance of our multi-scale simulation modeling framework. Finally, we provide forecasts for future line luminosity function measurements from the JWST and quantify the cosmic variance in such surveys.

astro-ph.GA↗

arXiv2025

Evaluating Mass Outflow Rate Estimators in FIRE-2 Simulations: Towards a Self-Consistent Framework for Spectral Line Based Predictions

$\require{mediawiki-texvc}$Galactic outflows shape galaxy evolution, but their mass, energy, and momentum transfer remain uncertain. High-resolution spectroscopy can help, but systematic discrepancies hinder model interpretation. In this study, we evaluate the performance of semi-analytical line transfer (SALT) and empirical partial covering models (PCMs) to recover the properties of outflows in the FIRE-2 simulation suite from synthetic Si II lines (1190 $Å$, 1193 $Å$, 1260 $Å$, 1304 $Å$, 1527 $Å$). When applicable, we assess each model's ability to recover mass, energy, and momentum outflow rates, as well as radial density and velocity profiles, column densities, and flow geometries. We find that the PCM underestimates column densities by 1.3 dex on average in the range $15 < \log N\ [\text{cm}^{-2}] < 17$ with dispersion 1.3 dex. We attribute this bias to instrumental smoothing. Since the PCM underestimates column densities, it also underestimates flow rates, though its predictions are independent of radius, with a dispersion of 0.55 dex. We detect no bias in the SALT estimates of the column density with dispersion 1.3 dex. When the velocity and density field obey power laws, SALT can constrain the mass, momentum, and energy outflow rates to 0.36 (0.63), 0.56 (0.56), and 0.97 (0.80) dex at $0.15(0.30)R_{\text{vir}}$, respectively. However, certain profiles in FIRE-2 fall outside the SALT framework, where the model breaks down. We find that SALT effectively tracks the flow geometry, capturing the temporal evolution of the photon escape fraction that is out of phase with the star formation rate, fully consistent with hydrodynamic simulations. We advocate for integral field unit spectroscopy to better constrain flow properties.

astro-ph.GA↗

arXiv2025

A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that displays singular properties: among the largest Hydrogen Balmer breaks reported at any redshift, broad multi-peaked H$β$ emission, and Balmer line absorption in multiple transitions. We model this source as a "black hole star" (BH*) where the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free "atmosphere" around a supermassive black hole. This source may provide evidence of an early black hole embedded in dense gas -- a theoretical configuration proposed to rapidly grow black holes via super-Eddington accretion. Radiation from the BH* appears to dominate almost all observed light, leaving limited room for contribution from its host galaxy. We demonstrate that the recently discovered "Little Red Dots" (LRDs) with perplexing spectral energy distributions can be explained as BH*s embedded in relatively brighter host galaxies. This source provides evidence that black hole masses in the LRDs may be over-estimated by orders of magnitude -- the BH* is effectively dust-free contrary to the steep dust corrections applied while modeling LRDs, and the physics that gives rise to the complex line shapes and luminosities may deviate from assumptions underlying standard scaling relations.

astro-ph.GA↗

arXiv2025

JWST ASPIRE: How Did Galaxies Complete Reionization? Evidence for Excess IGM Transmission around ${\rm [O\,{\scriptstyle III}]}$ Emitters during Reionization

The spatial correlation between galaxies and the Ly$α$ forest provides insights into how galaxies reionized the Universe. Here, we present initial results on the spatial cross-correlation between [OIII] emitters and Ly$α$ forest at 5.4 6.5 QSO fields. Using data from five QSO fields, we find $2σ$ evidence for excess Ly$α$ forest transmission at ~20-40 cMpc around [OIII] emitters at z=5.86, indicating that [OIII] emitters reside within a highly ionized IGM. At smaller scales, the Ly$α$ forest is preferentially absorbed, suggesting gas overdensities around [OIII] emitters. Comparing with models including THESAN simulations, we interpret the observed cross-correlation as evidence for significant large-scale fluctuations of the IGM and the late end of reionization at z<6, characterized by ionized bubbles over 50 cMpc around [OIII] emitters. The required UV background necessitates an unseen population of faint galaxies around the [OIII] emitters. Furthermore, we find that the number of observed [OIII] emitters near individual transmission spikes is insufficient to sustain reionization in their surroundings, even assuming all [OIII] emitters harbour AGN with 100 % LyC escape fractions. Despite broad agreement, a careful analysis of ASPIRE and THESAN, using the observed host halo mass from the clustering of [OIII] emitters, suggests that the simulations underpredict the observed excess IGM transmission around [OIII] emitters, challenging our model of reionization. Potential solutions include larger ionized bubbles at z<6, more enhanced large-scale UV background or temperature fluctuations of the IGM, and possibly a patchy early onset of reionization at z>10. Current observational errors are dominated by cosmic variance, meaning future analyses of more QSO fields from JWST will improve the results.

astro-ph.GA↗

arXiv2025

The THESAN-ZOOM project: Star-formation efficiencies in high-redshift galaxies

Recent JWST observations hint at unexpectedly intense cosmic star-formation in the early Universe, often attributed to enhanced star-formation efficiencies (SFEs). Here, we analyze the SFE in THESAN-ZOOM, a novel zoom-in radiation-hydrodynamic simulation campaign of high-redshift ($z \gtrsim 3$) galaxies employing a state-of-the-art galaxy formation model resolving the multiphase interstellar medium (ISM). The halo-scale SFE ($ε^{\ast}_{\rm halo}$) - the fraction of baryons accreted by a halo that are converted to stars - follows a double power-law dependence on halo mass, with a mild redshift evolution above $M_{\rm halo} \gtrsim 10^{9.5}\,{\rm M}_{\odot}$. The power-law slope is roughly $1/3$ at large halo masses, consistent with expectations when gas outflows are momentum-driven. At lower masses, the slope is roughly $2/3$ and is more aligned with the energy-driven outflow scenario. $ε^{\ast}_{\rm halo}$ is a factor of $2-3$ larger than commonly assumed in empirical galaxy-formation models at $M_{\rm halo} \lesssim 10^{11}\,{\rm M}_{\odot}$. On galactic (kpc) scales, the Kennicutt-Schmidt (KS) relation of neutral gas is universal in THESAN-ZOOM, following $Σ_{\rm SFR} \propto Σ_{\rm gas}^2$, indicative of a turbulent energy balance in the ISM maintained by stellar feedback. The rise of $ε^{\ast}_{\rm halo}$ with halo mass can be traced primarily to increasing gas surface densities in massive galaxies, while the underlying KS relation and neutral, star-forming gas fraction remain unchanged. Although the increase in $ε^{\ast}_{\rm halo}$ with redshift is relatively modest, it is sufficient to explain the large observed number density of UV-bright galaxies at $z \gtrsim 12$. However, reproducing the brightest sources at $M_{\rm UV} \lesssim -21$ may require extrapolating the SFE beyond the halo mass range directly covered by THESAN-ZOOM.

astro-ph.GA↗

arXiv2025

Topological Chiral Edge States in a Synthetic Dimension of Atomic Trap States

A key hallmark of quantum Hall physics is the existence of topological chiral states at the system boundary. Signatures of these edge states have been experimentally observed in cold atoms by using different approaches, including notably that of ``synthetic dimension'' in which internal states are coupled together and reinterpreted as sites along an artificial spatial dimension. However, previous atomic synthetic dimension implementations have been limited to relatively small system sizes with inflexible boundaries. In this paper, we propose instead how to use a synthetic dimension of atomic trap states to observe chiral edge states in a large quantum Hall system with a tunable edge. We present numerical simulations for relevant experimental parameters, showing how this scheme may be used to probe the properties and robustness of the edge states to defects. Our work opens the way for future experiments in topological physics with synthetic dimensions, while also providing new ways to manipulate and control highly-excited trap states.

cond-mat.quant-gas↗

arXiv2025

Two-dimensional Si spin qubit arrays with multilevel interconnects

The promise of quantum computation is contingent upon physical qubits with both low gate error rate and broad scalability. Silicon-based spins are a leading qubit platform, but demonstrations to date have not utilized fabrication processes capable of extending arrays in two dimensions while maintaining complete control of individual spins. Here, we implement an interconnect process, common in semiconductor manufacturing, with multiple back-end-of-line layers to show an extendable two-dimensional array of spins with fully controllable nearest-neighbor exchange interactions. In a device using three interconnect layers, we encode exchange-only qubits and achieve average single-qubit gate fidelities consistent with single-layer devices, including fidelities greater than 99.9%, as measured by blind randomized benchmarking. Moreover, with spin connectivity in two dimensions, we show that both linear and right-angle exchange-only qubits with high performance can be formed, enabling qubit array reconfigurability in the presence of defects. This extendable device platform demonstrates that industrial manufacturing techniques can be leveraged for scalable spin qubit technologies.

quant-ph↗

arXiv2025

Improving community detection via community association strength scores

Community detection methods play a central role in understanding complex networks by revealing highly connected subsets of entities. However, most community detection algorithms generate partitions of the nodes, thus (i) forcing every node to be part of a community and (ii) ignoring the possibility that some nodes may be part of multiple communities. In our work, we investigate three simple community association strength (CAS) scores and their usefulness as post-processing tools given some partition of the nodes. We show that these measures can be used to improve node partitions, detect outlier nodes (not part of any community), and help find nodes with multiple community memberships.

cs.SI↗

arXiv2025

Lyman-$α$ feedback prevails at Cosmic Dawn: Implications for the first galaxies, stars, and star clusters

Radiation pressure from Lyman-$α$ (Ly$α$) scattering is a potentially dominant form of early stellar feedback, capable of injecting up to $\sim 100 \, \times$ more momentum into the interstellar medium (ISM) than UV continuum radiation pressure and stellar winds. Ly$α$ feedback is particularly strong in dust-poor environments and is thus especially important during the formation of the first stars and galaxies. As upcoming galaxy formation simulations incorporate Ly$α$ feedback, it is crucial to consider processes that can limit it to avoid placing $Λ$CDM in apparent tension with recent \textit{JWST} observations indicating efficient star formation at Cosmic Dawn. We study Ly$α$ feedback using a novel analytical Ly$α$ radiative transfer solution that includes the effects of continuum absorption, gas velocity gradients, Ly$α$ destruction (e.g. by $2p \rightarrow 2s$ transitions), ISM turbulence, and atomic recoil. We verify our solution for uniform clouds using extensive Monte Carlo radiative transfer (MCRT) tests, and resolve a previous discrepancy between analytical and MCRT predictions. We then study the sensitivity of Ly$α$ feedback to the aforementioned effects. While these can dampen Ly$α$ feedback by a factor $\lesssim \textrm{few} \times 10$, we find it remains $\gtrsim 5 - 100 \, \times$ stronger than direct radiation pressure and therefore cannot be neglected. We provide an accurate fit for the Ly$α$ force multiplier $M_{\rm F}$, suitable for implementation in subgrid models for galaxy formation simulations. Our findings highlight the critical role of Ly$α$ feedback in regulating star formation at Cosmic Dawn, and underscore the necessity of incorporating it into simulations to accurately model early galaxy evolution.

astro-ph.GA↗