arXiv subjects

Zipeng Hu

Publications and source records attributed to Zipeng Hu.

9 recordsLinked to original sources

arXiv2026

Accurate Extragalactic Magnetic Fields from Faraday Rotation with Optimal Dispersion Measure Estimators

Faraday rotation measures (RMs) are one of our few observational tools for measuring magnetic field strengths in extragalactic systems, but converting an RM to a magnetic field estimate requires knowledge of the electron column density -- the dispersion measure (DM) -- to the RM source. Because DMs are difficult to measure for most extragalactic radio galaxies, observers have adopted a range of strategies to estimate them from more easily measured quantities, but the accuracy of these approaches is poorly known. To address this, we carry out simulated observations of high-resolution magnetohydrodynamic simulations of a range of galactic environments to explore the performance of various possible DM estimators. We obtain the best results using an estimator $\mathrm{DM} \propto \mathrm{EM}^{\alpha} \ N_\mathrm{Hi}^{\beta}$, where EM is the emission measure and $N_\mathrm{Hi}$ is the atomic hydrogen column density, with exponents $\alpha \approx 0.2-0.4$ and $\beta \approx 0-0.1$ depending on galactic environment (e.g., galaxy centres versus outskirts, and dwarfs versus spirals). We show that the variation of these exponents with environment can be understood in terms of simple physical arguments. Based on our tests, we provide recommended best practices for extracting galactic magnetic fields from RM data as a function of galactic environment and of proxy data availability, and show that using these methods one can obtain field measurements that are accurate to a few tenths of a dex. This work therefore represents an important step toward making use of RM data from next-generation surveys with the SKA.

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arXiv2026

Ripples of Stellar Enrichment (RoSE) - simulating element production and mixing in a Milky Way-mass galactic disc star-by-star

We present the Ripples of Stellar Enrichment (RoSE) simulations, which follow an isolated Milky Way-mass disc galaxy with star-by-star feedback and nucleosynthesis from five physical enrichment channels -- Wolf-Rayet stars, type II supernovae, type Ia supernovae, asymptotic giant branch stars, and neutron star mergers. We use these simulations to test how elements' diverse nucleosynthetic origins imprint spatial, temporal, and inter-element abundance correlations in gas and newly formed stars. We find that nucleosynthetic source composition is the primary organising principle of elemental structure: elements sharing a dominant production channel exhibit similar spatial and temporal statistics and their abundances are strongly correlated with one another, while mixed-source pairs are much more weakly correlated. We show that a simple linear regression model based only on how element pairs differ in their nucleosynthetic origin is able to predict, with high fidelity, how strongly their abundances correlate, in both interstellar medium gas and coeval stars. Comparison with Milky Way stellar abundance data shows encouraging qualitative agreement, with differences between simulations and observations comparable to the scatter between independent observational datasets. Together, these results show that the covariance of galactic abundances is largely predictable from the mixture of nucleosynthetic sources supplying each element: same-source elements remain tightly correlated in gas and young stars, whereas differing source mixtures produce systematically weaker correlations.

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arXiv2025

A pan-galaxy study of synthetic giant molecular filaments: a turbulence-dominated life cycle

Recent surveys of the Galactic plane have revealed dozens of giant molecular filaments (GMFs), with lengths ranging from tens to hundreds of parsecs, yet their origins and life cycles remain debated. In this work, we analyze over 700 GMFs identified from synthetic CO emission maps of a high-resolution magnetohydrodynamic simulation of a Milky Way-like galaxy, whose lengths range from $\sim 10$ pc to $\sim 300$ pc. We find that turbulent shock from galactic shear and stellar feedback are the primary drivers of GMF formation. Magnetized turbulence dominates their internal dynamics, supporting the filaments against global collapse while simultaneously inducing fragmentation into dense clumps. This fragmentation follows the turbulence-driven sausage instability model, rather than pure Jeans instability, and triggers efficient star formation along the filaments. Cloud-cloud collisions are frequent, affecting more than $70\%$ of GMFs, and often disrupt or reshape their morphology. The typical filamentary lifetime is $t_{\text{fil}} \sim 14$ Myr, comparable to the crossing time of giant molecular clouds (GMCs). The molecular gas half-life is $\sim 7$ Myr, similar to that of GMCs, indicating that GMFs are transient but dynamically important structures.

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arXiv2025

Fundamental limits to orbit reconstruction due to non-conservation of stellar actions in a Milky Way-like simulation

The conservation of stellar actions is a fundamental assumption in orbit reconstruction studies in the Milky Way. However, the disc is highly dynamic, with time-dependent, non-axisymmetric features like transient spiral arms and giant molecular clouds (GMCs) driving local fluctuations in the gravitational potential on top of the near-axisymmetric background. Using high-resolution magnetohydrodynamic simulations that incorporate gas dynamics and star formation, we quantify the rate at which these effects drive non-conservation of the actions of young stars from Myr to Gyr timescales. We find that action evolution is well described as a logarithmic random walk, with vertical action evolving more rapidly than radial action; the diffusion rate associated with this random walk is weakly dependent on the stellar birth environment and scales approximately linearly with the galactic orbital frequency at a star's position. The diffusion rates we measure imply a fundamental limit of $\sim 100$ Myr as the timescale over which stellar orbits can be reliably reconstructed using methods that assume action conservation. By comparing diffusion rates for younger stars to those measured for an older and more vertically-extended control population, we conclude that radial action evolution is driven primarily by transient spiral arms, while vertical action evolution is driven by gravitational scattering off gaseous structures. Our results have significant implications for galactic archaeology and disc dynamics studies, necessitating a closer look at the timescales over which actions are assumed to be conserved in the disc.

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arXiv2024

Understanding the Mechanisms Behind the Distribution of Galactic Metals

The evolution and distribution of metals within galaxies are critical for understanding galactic evolution and star formation processes, but the mechanisms responsible for shaping this distribution remain uncertain. In this study, we carry out high-resolution simulations of an isolated Milky Way-like galaxy, including a star-by-star treatment of both feedback and element injection. We include seven key isotopes of observational and physical interest, and which are distributed across different nucleosynthetic channels--primarily AGB stars (N, Ba, Ce), supernovae (O, Mg, S), and Wolf-Rayet stars (C) show measurably different correlation statistics in space and time and their fluctuations. This difference arises from the distinct ejection mechanisms associated with each nucleosynthetic process. The large-scale properties ensure that different elements, despite having different nucleosynthetic origins, are highly correlated with one another (>0.85 for all, >0.99 for same orgins), and their spatial correlations vary together in time. However small-scale variations naturally break elements into distinct nucleosynthetic familiars, with elements originating from the same channels correlating better with each other than with elements from different origins. Our findings suggest both challenges and opportunities for ongoing efforts to use chemical measurements of gas and stars to unravel the history and physics of galaxy assembly.

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arXiv2024

On the limitations of H alpha luminosity as a star formation tracer in spatially resolved observations

This study examines the limitations of H$\alpha$ luminosity as a tracer of star formation rates (SFR) in spatially resolved observations. We carry out high-resolution simulations of a Milky Way-like galaxy including both supernova and photoionization feedback, and from these we generate synthetic H$\alpha$ emission maps that we compare to maps of the true distribution of young stellar objects (YSOs) on scales from whole-galaxy to individual molecular clouds ($\lesssim 100$ pc). Our results reveal significant spatial mismatches between H$\alpha$ and true YSO maps on sub-100 pc scales, primarily due to ionizing photon leakage, with a secondary contribution from young stars drifting away from their parent molecular clouds. On small scales these effect contribute significantly to the observed anti-correlation between gas and star formation, such that there is noticeably less anti-correlation if we replace an H$\alpha$-based star formation map with a YSO-based one; this in turn implies that previous studies have underestimated the time it takes for young stars to disperse their parent molecular clouds. However, these effects are limited in dense regions with hydrogen columns $N_\mathrm{H} > 3 \times 10^{21}$ cm$^{-2}$, where the H$\alpha$- and YSO-based SFR maps show better agreement. Based on this finding we propose a calibration model that can precisely measure the SFR of large molecular clouds (mean radius > 100 pc) with a combination of H$\alpha$ and CO observations, which provides a foundation for future study of star formation processes in extragalactic molecular clouds.

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arXiv2023

The sub-critical illusion: synthetic Zeeman effect observations from galactic zoom-in simulations

Mass-to-flux ratios measured via the Zeeman effect suggest the existence of a transition from a magnetically sub-critical state in HI clouds to a super-critical state in molecular clouds. However, due to projection, chemical, and excitation effects, Zeeman measurements are subject to a number of biases, and may not reflect the true relations between gravitational and magnetic energies. In this paper, we carry out simulations of the formation of magnetised molecular clouds, zooming in from an entire galaxy to sub-pc scales, which we post-process to produce synthetic HI and OH Zeeman measurements. The mass-to-flux ratios we recover from the simulated observations show a transition in magnetic criticality that closely matches observations, but we find that the gravitational-magnetic energy ratios on corresponding scales are mostly super-critical, even in the HI regime. We conclude that HI clouds in the process of assembling to form molecular clouds are already super-critical even before H_2 forms, and that the apparent transition from sub- to super-criticality between HI and H_2 is primarily an illusion created by chemical and excitation biases affecting the Zeeman measurements.

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arXiv2021

High-precision star formation efficiency measurements in nearby clouds

On average molecular clouds convert only a small fraction epsilon_ff of their mass into stars per free-fall time, but differing star formation theories make contrasting claims for how this low mean efficiency is achieved. To test these theories, we need precise measurements of both the mean value and the scatter of epsilon_ff, but high-precision measurements have been difficult because they require determining cloud volume densities, from which we can calculate free-fall times. Until recently, most density estimates assume clouds as uniform spheres, while their real structures are often filamentary and highly non-uniform, yielding systematic errors in epsilon_ff estimates and smearing real cloud-to-cloud variations. We recently developed a theoretical model to reduce this error by using column density distributions in clouds to produce more accurate volume density estimates. In this letter, we apply this model to recent observations of 12 nearby molecular clouds. Compared to earlier analyses, our method reduces the typical dispersion of epsilon_ff within individual clouds from 0.35 dex to 0.31 dex, and decreases the median value of epsilon_ff over all clouds from ~ 0.02 to ~ 0.01. However, we find no significant change in the ~ 0.2 dex cloud-to-cloud dispersion of epsilon_ff, suggesting the measured dispersions reflect real structural differences between clouds.

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arXiv2020

Reconstructing three-dimensional densities from two-dimensional observations of molecular gas

Star formation has long been known to be an inefficient process, in the sense that only a small fraction $\epsilon_{\rm ff}$ of the mass of any given gas cloud is converted to stars per cloud free-fall time. However, developing a successful theory of star formation will require measurements of both the mean value of $\epsilon_{\rm ff}$ and its scatter from one molecular cloud to another. Because $\epsilon_{\rm ff}$ is measured relative to the free-fall time, such measurements require accurate determinations of cloud volume densities. Efforts to measure the volume density from two-dimensional projected data, however, have thus far relied on treating molecular clouds as simple uniform spheres, while their real shapes are likely filamentary and their density distributions far from uniform. The resulting uncertainty in the true volume density is likely one of the major sources of error in observational estimates of $\epsilon_{\rm ff}$. In this paper, we use a suite of simulations of turbulent, magnetized, radiative, self-gravitating star-forming clouds to examine whether it is possible to obtain more accurate volume density estimates and thereby reduce this error. We create mock observations from simulations, and show that current analysis methods relying on the spherical assumption likely yield ~ 0.26 dex underestimations and ~ 0.51 dex errors in volume density estimates, corresponding to a ~ 0.13 dex overestimation and a ~ 0.25 dex scatter in $\epsilon_{\rm ff}$, comparable to the scatter in observed cloud samples. We build a predictive model that uses information accessible in two-dimensional measurements -- most significantly the Gini coefficient of the surface density distribution -- to estimate volume density with ~ 0.3 dex less scatter. We test our method on a recent observation of the Ophiuchus cloud, and show that it successfully reduces the $\epsilon_{\rm ff}$ scatter.

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