arXiv subjects

Yunwei Deng

Publications and source records attributed to Yunwei Deng.

7 recordsLinked to original sources

arXiv2026

RIGEL: Ultra-faint dwarf galaxy diversity shaped by inhomogeneous cosmic reionization

Ultra-faint dwarf galaxies (UFDs) are among the smallest and oldest galaxies in the Universe and are widely regarded as relics of cosmic reionization. To investigate how reionization quenches star formation and shapes the diversity of UFDs, we present a suite of eight cosmological zoom-in simulations of isolated UFDs with present-day halo masses of $\sim10^9\,{\rm M}_\odot$. The simulations are performed with the radiation-magnetohydrodynamic galaxy formation framework Realistic ISM modeling in Galaxy Evolution and Lifecycles (RIGEL), coupled to realistic large-scale radiation fields extracted from the THESAN reionization simulation. Despite residing in similar $z=0$ halos, the simulated galaxies span nearly two orders of magnitude in stellar mass and broadly reproduce the observed luminosities, sizes, metallicities, and stellar kinematics of Local Group UFDs. We find that reionization quenches star formation through a two-stage process. The arrival of the ionization front rapidly photoionizes the diffuse circumgalactic and intergalactic gas, suppressing further gas accretion onto the galaxy. Star formation nevertheless continues for several hundred Myr using the surviving self-shielded gas reservoir and ceases only after this gas is consumed or dispersed. Within 500 Myr after reionization, less than 40% of the initial gas mass remains in the halo, with photoevaporation constituting the dominant gas-loss channel. We further show that the halo mass at the time of reionization is a key parameter governing the subsequent evolution of UFDs. Galaxies residing in more massive halos at reionization retain gas for longer periods and undergo more extended chemical enrichment. Consequently, the halo mass at reionization strongly correlates with the final stellar mass, stellar age spread, and chemical evolution of the galaxy.

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arXiv2026

Individual Star Sampling in Star Formation Simulations: A Semi-Deterministic Model

In modern simulations that include star formation, it is common to use a universal and invariant initial mass function (IMF) to represent star populations or sample individual stars. However, stellar masses are determined by local and environmental processes that operate over a wide dynamical range and remain unresolved in simulations. We introduce a semi-deterministic (SDT) scheme for sampling individual stars from star-forming gas in numerical simulations. We represent unresolved molecular cores and protostellar disks with reservoir particles (RsvPs) and employ an on-the-fly friends-of-friends algorithm to identify star clusters. The instantaneous IMF for newly formed stars is then derived from the current cluster mass. We test the performance of this method in simulations of isolated molecular clouds and a major merger between two dwarf galaxies. Compared to existing IMF sampling methods, our SDT scheme naturally reproduces the observed $m_{\star,\text{max}}$-$M_\text{ecl}$ relation and yields numbers of massive stars consistent with optimal sampling theory. It also exhibits the smallest run-to-run variation among simulations with different random seeds. The regulated star formation results in a small ($\sim0.15$ Myr) but coherent time delay in the emergence of massive stars, reduces the large scatter arising from Poisson noise, and produces initial mass segregation within the clusters. On galactic scales, the SDT method predicts a steeper high-mass IMF slope at low star formation rates (SFRs), with the slope negatively correlated with the SFR. As the specific abundance of massive stars declines, our model naturally explains the systematically low H$\alpha$-based SFRs, where the H$\alpha$ indicator for Milky Way-like galaxies underestimate the intrinsic SFR owing to the reduced population of massive stars.

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arXiv2025

RIGEL: Feedback-regulated cloud-scale star formation efficiency in a simulated dwarf galaxy merger

Major mergers of galaxies are likely to trigger bursty star formation activities. The accumulation of dense gas and the boost of star formation efficiency (SFE) are considered to be the two main drivers of starbursts. However, it remains unclear how each process operates on the scale of individual star-forming clouds. Here, we present a high-resolution (2 Msun) RHD simulation of a gas-rich dwarf galaxy merger using the RIGEL model to investigate how mergers affect the properties of the structure of dense star-forming gas and the cloud-scale SFE. We tracked the evolution of sub-virial dense clouds in the simulation by mapping them across successive snapshots taken at intervals of 0.2 Myr. We find that the merger triggers a 130 fold increase in the SFR and shortens the galaxy-wide gas-depletion time by two orders of magnitude compared to those in two isolated galaxies. However, the depletion time of individual clouds and their lifetime distribution remained unchanged over the simulation period. The cloud life cycles and cloud-scale SFE are determined by the local stellar feedback rather than such environmental factors as tidal fields regardless of the merger process, and the integrated SFE ($\epsilon_{\rm int}$) of clouds in complex environments remains well-described by an $\epsilon_{\rm int}-\Sigma_{\rm tot}$ relation found in idealized isolated-cloud experiments. During the peak of the starburst, the media SFE was lower by only 0.17-0.33 dex compared to the value when the galaxies were not interacting. The merger boosts the SFR through the accumulation and compression of dense gas fueling star formation. Strong tidal torques assemble $>10^5$ Msun clouds, which seed massive star clusters. The average separation between star-forming clouds decreases during the merger, which in turn decreases the cloud--cluster spatial de-correlation from >1 kpc to 0.1 kpc depicted in tuning fork diagrams.

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arXiv2025

The Entangling of Supernova Feedback Impacts with Coarsening Simulation Resolution

It is often understood that supernova (SN) feedback in galaxies is responsible for regulating star formation (SF) and generating gaseous outflows. However, a detailed look at their small-scale effects on the interstellar medium (ISM) in simulations shows that these processes proceed in distinct and separate channels. We demonstrate this finding in two independent simulations of isolated dwarf galaxies with very high ($m_{\rm gas}$ $\sim \msun$) numerical resolution, {\small LYRA} and {\small RIGEL}. Focusing on the immediate environment surrounding SNe, our findings suggest that the macroscopic effect of a given SN on the galaxy is best predicted by its local density. Outflows are driven by SNe in diffuse regions expanding to their cooling radii on large ($\sim$ kpc) scales, while dense SF regions are disrupted in a localized ($\sim$ pc) manner. However, these separate feedback channels are only distinguishable at very high resolutions capable of following mass scales $\lesssim 10^2 \,\msun$. When averaging on coarser scales, ISM densities are greatly mis-estimated, and variations between different SF and SNe-affected regions are severely washed out. It therefore cannot be self-consistently determined, from coarse-resolution information \textit{alone}, (1) whether a SN tends to contribute to outflows or direct SF suppression, and (2) the rate of SF in a given region. In particular, commonly used parameters in coarse-resolution (subgrid) models, such as the SN cooling radius and SF density threshold, may require more detailed treatments informed by high-resolution studies.

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arXiv2024

RIGEL: Simulating dwarf galaxies at solar mass resolution with radiative transfer and feedback from individual massive stars

We introduce the RIGEL model, a novel framework to self-consistently model the effects of stellar feedback in the multiphase ISM of dwarf galaxies with radiative transfer (RT) on a star-by-star basis. The RIGEL model integrates detailed implementations of feedback from individual massive stars into the RHD code, AREPO-RT. It forms individual massive stars from the resolved multiphase ISM by sampling the IMF and tracks their evolution individually. The lifetimes, photon production rates, mass-loss rates, and wind velocities of these stars are determined by their initial masses and metallicities based on a library that incorporates a variety of stellar models. The RT equations are solved in seven spectral bins accounting for the IR to HeII ionizing bands, using an M1 RT scheme. The thermochemistry model tracks the non-equilibrium H, He chemistry and the equilibrium abundance of CI, CII, OI, OII, and CO to capture the thermodynamics of all ISM phases. We evaluated the performance of the RIGEL model using $1\,{\rm M}_\odot$ resolution simulations of isolated dwarf galaxies. We found that the SFR and ISRF show strong positive correlations to the metallicity of the galaxy. Photoionization and photoheating can reduce the SFR by an order of magnitude by removing the available cold-dense gas fuel for star formation. The ISRF also changes the thermal structure of the ISM. Radiative feedback occurs immediately after the birth of massive stars and rapidly disperses the molecular clouds within 1 Myr. As a consequence, radiative feedback reduces the age spread of star clusters to less than 2 Myr, prohibits the formation of massive star clusters, and shapes the cluster initial mass function to a steep power-law form with a slope of $\sim-2$. The mass-loading factor of the fiducial galaxy has a median of $\sim50$, while turning off radiative feedback reduces this factor by an order of magnitude.

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arXiv2023

Simulating ionization feedback from young massive stars: impact of numerical resolution

Modelling galaxy formation in hydrodynamic simulations has increasingly adopted various radiative transfer methods to account for photoionization feedback from young massive stars. However, the evolution of HII regions around stars begins in dense star-forming clouds and spans large dynamical ranges in both space and time, posing severe challenges for numerical simulations in terms of both spatial and temporal resolution that depends strongly on gas density ($\propto n^{-1}$). In this work, we perform a series of idealized HII region simulations using the moving-mesh radiation-hydrodynamic code Arepo-RT to study the effects of numerical resolution. The simulated results match the analytical solutions and the ionization feedback converges only if the Strömgren sphere is resolved by at least $10$--$100$ resolution elements and the size of each time integration step is smaller than $0.1$ times the recombination timescale. Insufficient spatial resolution leads to reduced ionization fraction but enhanced ionized gas mass and momentum feedback from the HII regions, as well as degrading the multi-phase interstellar medium into a diffuse, partially ionized, warm ($\sim8000$ K) gas. On the other hand, insufficient temporal resolution strongly suppresses the effects of ionizing feedback. This is because longer timesteps are not able to resolve the rapid variation of the thermochemistry properties of the gas cells around massive stars, especially when the photon injection and thermochemistry are performed with different cadences. Finally, we provide novel numerical implementations to overcome the above issues when strict resolution requirements are not achievable in practice.

astro-ph.GA↗

arXiv2022

Multiple gas phases in supernova remnant IC 443: mapping shocked H$_2$ with VLT/KMOS

Supernovae and their remnants provide energetic feedback to the ambient interstellar medium (ISM), which is often distributed in multiple gas phases. Among them, warm molecular hydrogen (H$_2$) often dominates the cooling of the shocked molecular ISM, which has been observed with the H$_2$ emission lines at near-infrared wavelengths. Such studies, however, were either limited in narrow filter imaging or sparsely sampled mid-infrared spectroscopic observations with relatively poor angular resolutions. Here we present near-infrared ($H$- and $K$-band) spectroscopic mosaic observations towards the A, B, C, and G regions of the supernova remnant (SNR) IC 443, with the K-band Multi-Object Spectrograph (KMOS) onboard the Very Large Telescope (VLT). We detected 20 ro-vibrational transitions of H$_2$, one H line (Br$γ$), and two [Fe II] lines, which dominate broadband images at both $H$- and $K$-band. The spatial distribution of H$_2$ lines at all regions are clumpy on scales from $\sim 0.1$ pc down to $\sim 0.008$ pc. The fitted excitation temperature of H$_2$ is between 1500 K and 2500 K, indicating warm shocked gas in these regions. The multi-gas-phase comparison shows stratified shock structures in all regions, which explains the co-existence of multiple types of shocks in the same regions. Last, we verify the candidates of young stellar objects previously identified in these regions with our spectroscopic data, and find none of them are associated with young stars. This sets challenges to the previously proposed scenario of triggered star formation by SNR shocks in IC~443.

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