arXiv subjects

Tak-Pong Woo

Publications and source records attributed to Tak-Pong Woo.

9 recordsLinked to original sources

arXiv2026

Born in the Dark: The Catastrophic Collapse of Fuzzy Dark Matter Solitons as the Origin of Little Red Dots

JWST surveys have uncovered a population of compact, red sources ("Little Red Dots," LRDs) at $z \ge 5$ that exhibit broad Balmer emission yet remain X-ray faint, implying heavy obscuration with $N_H \ge 10^{24}$ cm$^{-2}$. We propose that LRDs may trace a short-lived, obscured phase associated with rapid baryonic inflow inside the deep solitonic cores of fuzzy dark matter (FDM) halos. Combining the soliton size scaling with (i) the observed compact radii ($r_e \sim 30-100$ pc) and (ii) the requirement that Compton-thick columns be achievable within a region of order the core radius, we find that particle masses $m$ few $\times 10^{-22}$ eV are plausible for soliton masses $M_s \sim 10^8 - 10^9 M_\odot$; we adopt $m_{22}=2$ as a fiducial choice. A conservative mass-budget estimate for the obscuring column, together with isothermal hydrostatic stratification, indicates that configurations reaching $N_H \ge 10^{24} - 10^{25}$ cm$^{-2}$ require densities for which radiative losses (cooling and/or diffusion) occur faster than the dynamical time, suggesting that a long-lived static hot atmosphere is unlikely (an "Opacity Crisis") and that rapid inflow or radiation-pressure-driven evolution is favored. Using $512^3$ pseudo-spectral Schrödinger-Poisson simulations of idealized soliton mergers, we illustrate that compact, high-density soliton cores can form via violent relaxation under representative scalings. We discuss observational implications and tests, and outline the need for future radiation-hydrodynamic modeling to predict demographics and detailed spectra.

astro-ph.GA↗

arXiv2025

Propagating Instability for Wave Dark Matter

In the early Universe, large-scale flows were omnipresent, and the flow collisions produced sheets and filaments. This phenomenon occurs for both particle and wave dark matter. But for the latter, these sheets and filaments are the modulations of even finer-scale, large-amplitude interference fringes. This work aims to investigate the instability of the interference fringes arising from colliding waves. Two colliding streams in classical collisionless fluid systems can produce small-scale unstable oscillations with a finite complex frequency, identified as propagating instabilities. In fact, propagating unstable oscillations have never been observed in the conventional quantum system due to its being Sturm-Liouville property. For example, quantum fluid equations with Madelung variables only exhibit either Jeans instability, a purely growing unstable mode, or stable oscillations, for which the squared frequency is real. Despite that, this work discovers that quantum interference fringes can indeed generate propagating unstable oscillations with a complex squared frequency when the gravitational feedback perturbation is included. The presence of local density nulls in the background density is shown to be the necessary condition for such an instability. We establish a phase diagram separating the propagating instability, Jeans instability, and stable oscillation regions, and is verified by computer simulations. Generally speaking, Jeans instabilities tend to occur for long-wave density perturbations as expected; propagating instabilities on the other hand tend to occur for short density waves with wavelengths comparable to the fringe size, i.e., near the center of the Bloch zone; lastly, both instabilities diminish for very low density fringes. The propagating unstable fluctuation may possibly collapse into halos of small sizes, potentially seeding the formation of proto-globular clusters.

astro-ph.CO↗

arXiv2018

How do stars affect $ψ$DM halos?

Wave dark matter ($ψ$DM) predicts a compact soliton core and a granular halo in every galaxy. This work presents the first simulation study of an elliptical galaxy by including both stars and $ψ$DM, focusing on the systematic changes of the central soliton and halo granules. With the addition of stars in the inner halo, we find the soliton core consistently becomes more prominent by absorbing mass from the host halo than that without stars, and the halo granules become "non-isothermal", "hotter" in the inner halo and "cooler" in the outer halo, as opposed to the isothermal halo in pure $ψ$DM cosmological simulations. Moreover, the composite (star+$ψ$DM) mass density is found to follow a $r^{-2}$ isothermal profile near the half-light radius in most cases. Most striking is the velocity dispersion of halo stars that increases rapidly toward the galactic center by a factor of at least 2 inside the half-light radius caused by the deepened soliton gravitational potential, a result that compares favorably with observations of elliptical galaxies and bulges in spiral galaxies. However in some rare situations we find a phase segregation turning a compact distribution of stars into two distinct populations with high and very low velocity dispersions; while the high-velocity component mostly resides in the halo, the very low-velocity component is bound to the interior of the soliton core, resembling stars in faint dwarf spheroidal galaxies.

astro-ph.GA↗

arXiv2016

Observation of quantum Hall plateau-plateau transition and scaling behavior of the zeroth Landau level in graphene p-n-p junctions

We report distinctive magnetotransport properties of a graphene p-n-p junction prepared by controlled diffusion of metallic contacts. In most cases, materials deposited on a graphene surface introduce substantial carrier scattering, which greatly reduces the high mobility of intrinsic graphene. However, we show that an oxide layer only weakly perturbs the carrier transport, which enables fabrication of a high-quality graphene p-n-p junction through a one-step and resist-free method. The measured conductance-gate voltage $(G-V_G)$ curves can be well described by a metal contact model, which confirms the charge density depinning due to the oxide layer. The graphene p-n-p junction samples exhibit pronounced quantum Hall effect, a well-defined transition point of the zeroth Landau level (LL), and scaling behavior. The scaling exponent obtained from the evolution of the zeroth LL width as a function of temperature exhibits a relatively low value of $κ=0.21\pm0.01$. Moreover, we calculate the energy level for the LLs based on the distribution of plateau-plateau transition points, further validating the assignment of the LL index of the QH plateau-plateau transition.

cond-mat.mes-hall↗

arXiv2014

Understanding the Core-Halo Relation of Quantum Wave Dark Matter, $ψ$DM, from 3D Simulations

We examine the nonlinear structure of gravitationally collapsed objects that form in our simulations of wavelike cold dark matter ($ψ$DM), described by the Schrödinger-Poisson (SP) equation with a particle mass $\sim 10^{-22} {\rm eV}$. A distinct gravitationally self-bound solitonic core is found at the center of every halo, with a profile quite different from cores modeled in the warm or self-interacting dark matter scenarios. Furthermore, we show that each solitonic core is surrounded by an extended halo composed of large fluctuating dark matter granules which modulate the halo density on a scale comparable to the diameter of the solitonic core. The scaling symmetry of the SP equation and the uncertainty principle tightly relate the core mass to the halo specific energy, which, in the context of cosmological structure formation, leads to a simple scaling between core mass ($M_c$) and halo mass ($M_h$), $M_c \propto a^{-1/2} M_h^{1/3}$, where $a$ is the cosmic scale factor. We verify this scaling relation by (i) examining the internal structure of a statistical sample of virialized halos that form in our 3D cosmological simulations, and by (ii) merging multiple solitons to create individual virialized objects. Sufficient simulation resolution is achieved by adaptive mesh refinement and graphic processing units acceleration. From this scaling relation, present dwarf satellite galaxies are predicted to have kpc sized cores and a minimum mass of $\sim 10^8 {M_\odot}$, capable of solving the small-scale controversies in the cold dark matter model. Moreover, galaxies of $2\times10^{12} {M_\odot}$ at $z=8$ should have massive solitonic cores of $\sim 2\times10^9 {M_\odot}$ within $\sim 60 {\rm pc}$. Such cores can provide a favorable local environment for funneling the gas that leads to the prompt formation of early stellar spheroids and quasars.

astro-ph.GA↗

arXiv2013

Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility of our device. It is found that at the direct I-QH transition, the product of the quantum mobility and is about 0.37 which is considerably smaller than 1. In contrast, at Bc, the longitudinal resistivity is close to the Hall resistivity, i.e., the product of the classical mobility and the crossing field is about 1. Therefore our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.

cond-mat.mes-hall↗

arXiv2011

Vortex Turbulence in Linear Schroedinger Wave Mechanics

Quantum turbulence that exhibits vortex creation, annihilation and interactions is demonstrated as an exact solution of the time-dependent, free-particle Schrödinger equation evolved from a smooth random-phased initial condition. Relaxed quantum turbulence in 2D and 3D exhibits universal scaling in the steady-state energy spectrum as k-1 in small scales. Due to the lack of dissipation, no evidence of the Kolmogorov-type forward energy cascade in 3D or the inverse energy cascade in 2D is found, but the rotational and potential flow components do approach equi-partition in the scaling regime. In addition, the 3D vortex line-line correlation exhibits universal behaviour, scaled as \Deltar^-2, where \Deltar is the separation between any two vortex line elements, in fully developed turbulence. We also show that the quantum vortex is not frozen to the matter, nor is the vortex motion induced by other vortices via Biot-Savart's law. Thus, the quantum vortex is actually a nonlinear wave, propagating at a speed very different from a classical vortex.

quant-ph↗

arXiv2009

High-Resolution Simulation on Structure Formation with Extremely Light Bosonic Dark Matter

An alternative bosonic dark matter model is examined in detail via high-resolution simulations. These bosons have particle mass of order $10^{-22}eV$ and are non-interacting. If they do exist and can account for structure formation, these bosons must be condensed into the Bose-Einstein state and described by a coherent wave function. This matter, also known as Fuzzy Dark Matter (Hu, Barkana & Gruzinov 2000),, is speculated to be able, first, to eliminate the sub-galactic halos to solve the problem of over-abundance of dwarf galaxies, and, second, to produce flat halo cores in galaxies suggested by some observations. Our simulation results show that although this extremely light bosonic dark matter indeed suppresses low-mass halos, it can, to the contrary of expectation, yield singular halo cores. The density profile of the singular halo is almost identical to the halo profile of Navarro, Frenk & White (1997). Such a profile seems to be universal, in that it can be produced via either accretion or merger.

astro-ph↗

arXiv2002

Effects of Preheated Clusters on the CMB Spectrum

Mounting evidence from $x$-ray observations reveals that bound objects should receive some form of energy in the past injected from non-gravitaional sources. We report that an instantaneous heating scheme, for which gases in dense regions were subjected to a temperature jump of 1keV at $z=2$ whereas those in rarified regions remained intact, can produce bound objects obeying the observed mass-temperature and luminosity-temperature relations. Such preheating lowers the peak Sunyaev-Zeldovich (SZ) power by a factor of 2 and exacerbates the need for the normalization of matter fluctuations $σ_8$ to assume an extreme high value $(\sim 1.1)$ for the SZ signals to account for the excess anisotropy on 5-arcminute scale detected by the Cosmic Background Imager in the cosmic microwave background radiation.

astro-ph↗