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Siang Peng Oh

Publications and source records attributed to Siang Peng Oh.

9 recordsLinked to original sources
arXiv2026

Particle Acceleration in Magnetized Shear-Driven Turbulence

Shear flows, ubiquitous in space and astrophysical plasmas, can accelerate particles through turbulence excited by the Kelvin-Helmholtz instability. We present the first numerical study of particle acceleration in sustained, subsonic, non-relativistic, and magnetized turbulence driven purely by velocity shear, including full particle backreaction. Using two-dimensional MHD-PIC simulations with an initially uniform flow-aligned magnetic field and external stirring force, we demonstrate that, in this subsonic regime, sustained particle acceleration requires continuously driven turbulence, whereas freely decaying turbulence rapidly depletes its energy reservoirs and halts the acceleration. The acceleration mechanism operates through the systematic distortion of gyro-orbits by turbulent electric fields: acceleration phases extend the particle trajectory along the electric force, increasing the energy gain, while deceleration phases shorten the trajectory, reducing the energy loss. This asymmetry produces net energy gain despite stochastic fluctuations, with the mean energy change scaling quadratically with shear velocity, characteristic of second-order Fermi acceleration. Initially monoenergetic particles develop substantial non-thermal tails after the turbulence onset. The energization of particles that repeatedly cross shear layers follows geometric Brownian motion, yielding a log-normal distribution. High-energy particles exhibit pitch-angle anisotropy, becoming preferentially perpendicular to the flow-aligned magnetic field as their gyroradii exceed the turbulent layer width. These results establish shear-driven turbulence as a viable particle acceleration mechanism, providing a general model for particle energization in shear flows.

astro-ph.HE↗
arXiv2025

Lévy Flights and Leaky Boxes: Anomalous Diffusion of Cosmic Rays

In classical diffusion, particle step-sizes have a Gaussian distribution. However, in superdiffusion, they have power-law tails, with transport dominated by rare, long Lévy flights. Similarly, if the time interval between scattering events has power-law tails, subdiffusion occurs. Both forms of anomalous diffusion are seen in cosmic ray (CR) particle tracking simulations in turbulent magnetic fields. They also likely occur if CRs are scattered by discrete intermittent structures. Anomalous diffusion mimics a scale-dependent diffusion coefficient, with potentially wide-ranging consequences. However, the finite size of galaxies implies an upper bound on step-sizes before CRs escape. This truncation results in eventual convergence to Gaussian statistics by the central limit theorem. Using Monte-Carlo simulations, we show that this occurs in both standard finite-thickness halo models, or when CR diffusion transitions to advection or streaming-dominated regimes. While optically thick intermittent structures produce power-law trapping times and thus subdiffusion, gaussianization also eventually occurs on timescales longer than the maximum trapping time. Anomalous diffusion is a transient, and converges to standard diffusion on the (usually short) timescale of particle escape, either from confining structures (subdiffusion), or the system as a whole (superdiffusion). Thus, standard assumptions of classical diffusion are physically justified in most applications, despite growing simulation evidence for anomalous diffusion. However, if escape times are long, this is no longer true. For instance, anomalous diffusion in the CGM or ICM would change CR pressure profiles. Finally, we show the standard diagnostic for anomalous diffusion, $\langle d^2 \rangle \propto t^α$ with $α\neq 1$, is not justified for truncated Lévy flights, and propose an alternative robust measure.

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arXiv1999

Observational Signatures of the First Luminous Objects

The next generation of astronomical instruments will be able to detect dense pockets of ionized gas created by the first luminous objects. The integrated free-free emission from ionized halos creates a spectral distortion of the Cosmic Microwave Background, greater than the distortion due to the reionized intergalactic medium. Its detection is well within the capabilities of the planned DIMES satellite. Ionized halos may also be imaged directly in free-free emission by the Square Kilometer Array. Bright halos will be detected as discrete sources, while residual unresolved sources can still be detected statistically from temperature fluctuations in maps. Balmer line emission from ionized halos is detectable by the Next Generation Space Telescope and can be used to obtain redshift information. Unlike Ly-alpha, the H-alpha line does not suffer from resonant scattering by neutral hydrogen, and is still observable when the IGM is not fully reionized. In addition, it is less susceptible to dust extinction. Finally, the kinetic Sunyaev-Zeldovich effect may be used to detect ionized gas at high redshift. Used in concert, these observations will probe ionizing emissivity, gas clumping, and star formation in the early universe.

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arXiv1998

Clustering of Ionizing Sources

Using existing models for the evolution of clustering we show that the first sources are likely to be highly biased and thus strongly clustered. We compute intensity fluctuations for clustered sources in a uniform IGM. The strong fluctuations imply the universe was reheated and reionized in a highly inhomogeneous fashion.

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arXiv1998

An Efficient Technique to Determine the Power Spectrum from Cosmic Microwave Background Sky Maps

There is enormous potential to advance cosmology from statistical characterizations of cosmic microwave background sky maps. The angular power spectrum of the microwave anisotropy is a particularly important statistic. Existing algorithms for computing the angular power spectrum of a pixelized map typically require O(N^3) operations and O(N^2) storage, where N is the number of independent pixels in the map. The MAP and Planck satellites will produce megapixel maps of the cosmic microwave background temperature at multiple frequencies; thus, existing algorithms are not computationally feasible. In this article, we introduce an algorithm that requires O(N^2) operations and O(N^1.5) storage that can find the minimum variance power spectrum from sky map data roughly one million times faster than was previously possible. This makes feasible an analysis that was hitherto intractable.

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arXiv1997

Fast Tides in Slow Stars: the Efficiency of Eddy Viscosity

The turbulent viscosity of convection is believed to circularize the orbits of close binary stars. When the tidal period is shorter than the turnover time of the largest eddies, turbulent viscosity is believed to be suppressed. The degree of suppression is disputed. We re-examine these beliefs via direct perturbative calculations, linearizing the fluid equations on a turbulent background, and by numerical integration of a chaotic dynamical system subject to periodic forcing. We find that (i) dissipation of rapid tides is severely suppressed; and (ii) turbulent convection does not circularize binaries while the stars are on the main sequence. The latter conclusion seems secure (though weaker) even without the former.

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arXiv1996

The Stellar Dynamics of M87

We extract the shape of the stellar velocity ellipsoid as a function of radius in M87 from van der Marel's (1994) velocity dispersion data. We include the gravitational force of a central black hole with the mass quoted by Harms et al. (1994). The kinematical data are corrected for the effects of seeing and instrumental blurring using a nonparametric algorithm. We find that the stellar motions in M87 are slightly radially anisotropic throughout the main body of the galaxy. However the tangential velocity dispersion exceeds the radial velocity dispersion within the inner 1''-2'' by a statistically significant amount. A number of models for the formation of nuclear black holes predict a tangential anisotropy in the stellar motions, and our results provide evidence for such an effect in M87.

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arXiv1996

The Peculiar Velocity Function of Galaxy Clusters

The peculiar velocity function of clusters of galaxies is determined using an accurate sample of cluster velocities based on Tully-Fisher distances of Sc galaxies (Giovanelli et al 1995b). In contrast with previous results based on samples with considerably larger velocity uncertainties, the observed velocity function does not exhibit a tail of high velocity clusters. The results indicate a low probability of $\lesssim$\,5\% of finding clusters with one-dimensional velocities greater than $\sim$ 600 {\kms}. The root-mean-square one-dimensional cluster velocity is 293$\pm$28 {\kms}. The observed cluster velocity function is compared with expectations from different cosmological models. The absence of a high velocity tail in the observed function is most consistent with a low mass-density ($Ω\sim$0.3) CDM model, and is inconsistent at $\gtrsim 3 σ$ level with $Ω$= 1.0 CDM and HDM models. The root-mean-square one-dimensional cluster velocities in these models correspond, respectively, to 314, 516, and 632 {\kms} (when convolved with the observational uncertainties). Comparison with the observed RMS cluster velocity of 293$\pm$28 {\kms} further supports the low-density CDM model.

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