arXiv subjects

Avi Chen

Publications and source records attributed to Avi Chen.

11 recordsLinked to original sources

arXiv2026

Radiative Feedback in Population III Protostellar Growth: HI Shielding \& 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.

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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.

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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 $\Lambda$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.

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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.

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arXiv2025

$\Lambda$CDM star clusters at cosmic dawn: stellar densities, environment, and equilibrium

The James Webb Space Telescope (JWST) has opened a window on many new puzzles in the early Universe, including a population of high-redshift star clusters with extremely high stellar surface density, suggesting unique star formation conditions in the Universe's early evolution. We study the formation and evolution of these first star clusters and galaxies using an AREPO cosmological simulation box designed to resolve the intricate environments of the smallest halos hosting Population III star clusters at $z \geq 12$. Our approach, which prioritizes baryonic structure identification through a friends-of-friends algorithm, provides new insights into early star cluster formation and delivers predictions directly relevant to observations. We investigate the dynamical properties of these first star clusters and use numerical and analytical methods to understand the populations of virialized and non-virialized systems. Our findings indicate that high-$z$ star clusters in a feedback-free regime can achieve extreme surface densities, consistent with the systems detected by JWST. These results imply that JWST may have the opportunity to uncover stellar systems at high redshift whose dynamical state preserves evidence of the hierarchical structure formation process.

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arXiv2024

The Stellar Initial Mass Function of Early Dark Matter-free Gas Objects

Among the remarkable strides made by JWST is the discovery of the earliest star clusters found to date. These have been proposed as early progenitors of globular clusters, which are known to come from the early stages of star formation in the Universe. This is an exciting development in modern astronomy, as it offers an opportunity to connect theoretical models of globular cluster formation to actual observations of these high-redshift structures. In this work, we aim to develop observational signatures of a star cluster formation route known as supersonically induced gas objects, which are dark matter-less gas clouds in the early Universe proposed as a potential origin of some globular clusters. For the first time, we follow the star formation process of these early Universe objects using high-resolution hydrodynamical simulations, including mechanical feedback. Our results suggest that the first dark matter-less star clusters are top-heavy, meaning that they have a flatter IMF slope compared to very young low-metallicity star clusters in the local Universe, and they also have extremely high stellar mass surface densities compared to their local counterparts.

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arXiv2024

The Supersonic Project: Early Star Formation with the Streaming Velocity

At high redshifts ($z\gtrsim12$), the relative velocity between baryons and dark matter (the so-called streaming velocity) significantly affects star formation in low-mass objects. Streaming substantially reduces the abundance of low-mass gas objects while simultaneously allowing for the formation of supersonically-induced gas objects (SIGOs) and their associated star clusters outside of dark matter halos. Here, we present a study of the population-level effects of streaming on star formation within both halos and SIGOs in a set of simulations with and without streaming. Notably, we find that streaming actually enhances star formation within individual halos of all masses at redshifts between $z=12$ and $z=20$. This is demonstrated both as an increased star formation rate per object as well as an enhancement of the Kennicutt-Schmidt relation for objects with streaming. We find that our simulations are consistent with some observations at high redshift, but on a population level, they continue to under-predict star formation relative to the majority of observations. Notably, our simulations do not include feedback, and so can be taken as an upper limit on the star formation rate, exacerbating these differences. However, simulations of overdense regions (both with and without streaming) agree with observations, suggesting a strategy for extracting information about the overdensity and streaming velocity in a given survey volume in future observations.

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arXiv2024

Neutral hydrogen filaments in interstellar media: Are they physical?

The trending term "filament" is extensively used in the interstellar medium (ISM) and the star formation community, and is believed to be one of the most important objects that gauge molecular cloud and star formation. However, the physical definition of these ubiquitous, elongated, high contrast features is poorly defined and still actively debated. Despite the absence of a unified consensus, filaments are believed to be involved in many important physical processes from galaxy structure formation to the emergence of protostellar objects. Therefore, understanding how filaments form, what constrains their growth, and their general physical properties, are extremely important for theorists and observers who study the dynamics of the ISM and consequent star formations. This review serves as a collection of the community's views and develops the concept of "filaments" in the context of the ISM and star-forming clouds. Observationally, filaments are seen across the entire sky and often carry an aspect ratio of the order of hundreds. In the context of the ISM, filaments are believed to form by stretching and tearing from magnetized ISM turbulence. ISM filaments are subjected to heating and cooling phases, and are likely to be magnetically aligned. Cold clouds are formed inside ISM due to turbulence instability. This review updates the understanding of ISM filaments in the community.

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arXiv2023

The Supersonic Project: Lighting up the faint end of the JWST UV luminosity function

The James Webb Space Telescope (JWST) is capable of probing extremely early eras of our Universe when the supersonic relative motions between dark matter and baryonic overdensities modulate structure formation ($z>\sim 10$). We study low-mass galaxy formation including this "stream velocity" using high resolution AREPO hydrodynamics simulations, and present theoretical predictions of the UV luminosity function (UVLF) and galaxy stellar mass function (GSMF) down to extremely faint and low mass galaxies ($M_{UV}>\sim-15$, $10^4M_\odot<=M_*<=10^8 M_\odot)$. We show that, although the stream velocity suppresses early star formation overall, it induces a short period of rapid star formation in some larger dwarfs, leading to an enhancement in the faint-end of the UVLF at $z=12$. We demonstrate that JWST observations are close to this enhanced regime, and propose that the UVLF may constitute an important probe of the stream velocity at high redshift for JWST and future observatories.

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arXiv2022

Turbulent universal galactic Kolmogorov velocity cascade over 6 decades

We report the evidence for the existence of the universal, continuous turbulent cascade of velocity fluctuations with Kolmogorov -5/3 slope spanning 6 orders of length scales, from $10^4$ pc down to $10^{-2}$ pc. This was achieved by applying our innovative technique of separating density and velocity fluctuations to a set of spectroscopic surveys featuring various galactic spectral lines. This unified velocity cascade involves different interstellar phases from diffuse galactic media to dense self-gravitating clouds and persists despite interstellar phase transitions. However, the turbulent density fluctuations do not show this universality as the value of the spectral slope changes in different interstellar phases. This agrees with the expectation of compressible turbulence theory and demonstrates that the density is only an indirect tracer of interstellar turbulence. We report that the density fluctuations for clouds and filaments that are preferentially parallel to magnetic fields exhibit the spectral slope of -2. The universal grand turbulent velocity cascade that is established in our paper has significant implications for key galactic physical processes, including star formation, cosmic ray transport etc. We anticipate our result to be a starting point for in vitro models of multiphase interstellar turbulence studies with a significant impact for modeling of spiral galaxies.

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arXiv2021

Is the recently discovered large scale filamentary feature Cattail in the cold or unstable phase?

A recent publication (Li et al. 2021) discovered one of the largest filamentary neutral hydrogen features dubbed Cattail from high resolution FAST observations that might be a new galactic arm of our own Milky Way. However in the analysis, it was suggested that this neutral hydrogen feature is cold despite having 12km/s total linewidth. We evaluate the probability whether the Cattail is actually cold neutral media via the newly developed Velocity Decomposition Algorithm (Yuen et al. 2021a) and Force Balancing Model (Ho et al. 2021a). We discovered that even with the inclusion of the galactic shear term, the feature is still at the unstable neutral media regime. Moreover, we also discover that the Cattail is two disjoint features in caustics space, suggesting that the Cattail might have two different turbulent systems. We check the spectra of the individual system separated via VDA to confirm this argument. We do not exclude the existence of smaller scale cold media being embedded within this structure.

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