arXiv subjects

Qirong Zhu

Publications and source records attributed to Qirong Zhu.

At least 19 recordsLinked to original sources

arXiv2025

VR-Assisted Guide Dog Training: A 360° PanoHaptic System for Right-Hand Commands Analysis

This paper presents a VR-based guide dog training system designed to assist novice trainers in understanding guide dog behavior and issuing appropriate training commands. Guide dogs play a vital role in supporting independent mobility for visually impaired individuals, yet the limited number of skilled trainers restricts their availability. Training is highly demanding, requiring accurate observation of the dog's status and precise command issuance, especially through right-hand gestures. While the trainer's left hand holds the harness to perceive haptic cues, the right hand is used to indicate directions, maintain attention, and provide comfort, with motion patterns varying by scenario and the dog's progress. Currently, novices learn mainly by observing experts or watching videos, which lacks immersion and makes it difficult to adopt the trainer's perspective for understanding behavior or synchronizing command timing. To address these limitations, the proposed system introduces a VR-based assistive platform integrating panoramic visuals and haptic feedback to create an immersive training environment. The visual module provides contextual guidance, including cues for command execution and real-time comparison of the user's posture with standard actions, while the haptic module delivers tactile feedback for command gestures. Users can re-experience training sessions across diverse scenarios and dog proficiency levels, allowing independent and repeated practice. By improving the timing, accuracy, and expressiveness of right-hand commands, the system aims to accelerate skill acquisition, enhance training quality, and mitigate the shortage of qualified trainers, ultimately increasing the availability of guide dogs for visually impaired individuals.

cs.HC↗

arXiv2024

Haptic Information Feedback Given to Handles in Guide Dog Training

In guide dog training, trainers use haptic information transmitted through the handle of the harness worn by the guide dog to understand the dog's state. They then apply appropriate force to the handle to train the dog to make correct judgments. This tactile experience can only be felt between the dog and the trainer, making it challenging to communicate the amount of force applied to others quantitatively. To solve this problem, this study proposes a method for real-time visualization of the force exerted on the handle and quantification of the handle movement through image processing, which can be applied to actual guide dog training.

cs.HC↗

arXiv2024

Environmental effects on Low Surface Brightness Galaxies in the IllustrisTNG simulation

Employing the TNG100 run of the IllustrisTNG project, we characterize the environment of Low Surface Brightness Galaxies (LSBGs) across varying scales, from their associated dark matter halos to their distribution within the broader cosmic structure. We find no significant differences in the halo concentration index $c_{200}$ between LSBGs and their High Surface Brightness (HSBGs) counterparts, with LSBGs residing in halos with higher spin parameter $λ$ and slightly more spherical shapes than HSBGs. LSBGs show a stronger alignment between the dark and stellar angular momentum vectors than their high surface brightness counterparts. The relative abundance of LSBGs within groups and clusters displays a central deficit, hinting at potential destruction upon reaching these core regions. Studying the density field, we find a preference for rotation-dominated LSBGs to reside in low-density environments, while dispersion-dominated LSBGs thrive in high-density regions where galaxy interactions govern their evolution, an observation corroborated by our analysis of the two-point correlation function $ξ(r)$. Our examination of the cosmic web reveals no significant differences in the distance to the closest large-scale structure, barring a few exceptions. This suggests a limited impact of large-scale spatial distribution on mechanisms driving LSBG evolution. All together, we conclude that the halo vicinity and local environment at the scale of galaxy clusters, where mechanisms such as galaxy mergers and tidal stripping, as well as stellar and gas accretion take place, is the most likely environment that favour the emergence of LSBGs with different morphologies, mostly driven by the presence or absence of important local interaction phenomena.

astro-ph.GA↗

arXiv2024

Feeling the Grass Grow: Making Midair Haptic Parameters Visible, Touchable and Controllable

In this paper, we present an ultrasound mid-air haptic interaction system that integrates a designed visualization of haptic parameters while maintaining ease of control. The design of corresponding haptic parameters for real-world tactile textures is a complex task. Furthermore, users often face difficulties in simultaneously controlling multi-dimensional haptic parameters to achieve the desired vibration feedback. To address these challenges, the SLS optimization method facilitates user control of these multi-dimensional parameters through a simple one-dimensional slider. Concurrently, our system employs the "Growing Grass" metaphor to visualize haptic parameter adjustments in real-time. This approach combining visual and haptic sensations can bring richer experiences and generate a realistic sensation of touching a grassy surface. Our objective is to enhance users' intuitive comprehension of haptic parameters through this innovative system.

cs.HC↗

arXiv2022

Evolution of Massive Black Hole Binaries in Collisionally Relaxed Nuclear Star Clusters -- Impact of Mass Segregation

Massive Black Hole (MBH) binaries are considered to be one of the most important sources of Gravitational Waves (GW) that can be detected by GW detectors like LISA. However, there are a lot of uncertainties in the dynamics of MBH binaries in the stages leading up to the GW-emission phase. It has been recently suggested that Nuclear Star Clusters (NSCs) could provide a viable route to overcome the final parsec problem for MBH binaries at the center of galaxies. NSCs are collisional systems where the dynamics would be altered by the presence of a mass spectrum. In this study, we use a suite of high-resolution $N$-body simulations with over 1 million particles to understand how collisional relaxation under the presence of a mass spectrum of NSC particles affects the dynamics of the MBH binary under the merger of two NSCs. We consider MBH binaries with different mass ratios and additional non-relaxed models. We find that mass-segregation driven by collisional relaxation can lead to accelerated hardening in lower mass ratio binaries but has the opposite effect in higher mass ratio binaries. Crucially, the relaxed models also demonstrate much lower eccentricities at binary formation and negligible growth during hardening stages leading to longer merger timescales. The results are robust and highlight the importance of collisional relaxation on changing the dynamics of the binary. Our models are state-of-the-art, use zero softening, and high enough particle numbers to model NSCs realistically.

astro-ph.GA↗

arXiv2022

The formation of low surface brightness galaxies in the IllustrisTNG simulation

We explore the nature of low surface brightness galaxies (LSBGs) in the hydrodynamic cosmological simulation TNG100 of the IllustrisTNG project, selecting a sample of LSBGs ($r$-band effective surface brightness $μ_r > 22.0$ mag arcsec$^{-2}$) at $z=0$ over a wide range of stellar masses ($M_{\ast} = 10^{9}$-$10^{12}$ M$_\odot$). We find LSBGs of all stellar masses, although they are particularly prevalent at $M_{\ast} < 10^{10}$ M$_\odot$. We show that the specific star formation rates of LSBGs are not significantly different from those of high surface brightness galaxies (HSBGs) but, as a population, LSBGs are systematically less massive and more extended than HSBGs, and tend to display late-type morphologies according to a kinematic criterion. At fixed stellar mass, we find that haloes hosting LSBGs are systematically more massive and have a higher baryonic fraction than those hosting HSBGs. We find that LSBGs have higher stellar specific angular momentum and halo spin parameter values compared to HSBGs, as suggested by previous works. We track the evolution of these quantities back in time, finding that the spin parameters of the haloes hosting LSBGs and HSBGs exhibit a clear bifurcation at $z \sim 2$, which causes a similar separation in the evolutionary tracks of other properties such as galactic angular momentum and effective radius, ultimately resulting in the values observed at $z =$ 0. The higher values of specific stellar angular momentum and halo spin in LSBGs seem to be responsible for their extended nature, preventing material from collapsing into the central regions of the galaxies, also causing LSBGs to host less massive black holes at their centres.

astro-ph.GA↗

arXiv2021

Fast Multipole Methods for $N$-body Simulations of Collisional Star Systems

Direct $N$-body simulations of star clusters are accurate but expensive, largely due to the numerous $\mathcal{O} (N^2)$ pairwise force calculations. To solve the post-million-body problem, it will be necessary to use approximate force solvers, such as tree codes. In this work, we adapt a tree-based, optimized Fast Multipole Method (FMM) to the collisional $N$-body problem. The use of a rotation-accelerated translation operator and an error-controlled cell opening criterion leads to a code that can be tuned to arbitrary accuracy. We demonstrate that our code, Taichi, can be as accurate as direct summation when $N> 10^4$. This opens up the possibility of performing large-$N$, star-by-star simulations of massive stellar clusters, and would permit large parameter space studies that would require years with the current generation of direct summation codes. Using a series of tests and idealized models, we show that Taichi can accurately model collisional effects, such as dynamical friction and the core-collapse time of idealized clusters, producing results in strong agreement with benchmarks from other collisional codes such as NBODY6++GPU or PeTar. Parallelized using OpenMP and AVX, Taichi is demonstrated to be more efficient than other CPU-based direct $N$-body codes for simulating large systems. With future improvements to the handling of close encounters and binary evolution, we clearly demonstrate the potential of an optimized FMM for the modeling of collisional stellar systems, opening the door to accurate simulations of massive globular clusters, super star clusters, and even galactic nuclei.

astro-ph.GA↗

arXiv2020

The Formation of the First Quasars. I. The Black Hole Seeds, Accretion and Feedback Models

Supermassive black holes (SMBHs) of $\sim 10^9\, M_\odot$ are generally believed to be the central engines of the luminous quasars observed at $z\gtrsim6$, but their astrophysical origin remains elusive. The $z\gtrsim$ quasars reside in rare density peaks, which poses several challenges to uniform hydrodynamic simulations. To investigate the formation of these distant quasars, we perform a suite of zoom-in simulations on a favorable halo, with a mass of $\sim 10^{13}\, M_\odot$ at $z = 6$ and a history of multiple major mergers, ideal for BH growth. We test BH seeds of $10 - 10^6\, M_\odot$, and various accretion and feedback models, including thin-disk and slim-disk accretion. We find, contrary to previous studies, that light seeds of $\lesssim 10^3\, M_\odot$ fail to grow to $10^8\, M_\odot$ by $z\sim 6$ even with super-critical accretion; that the hyper-Eddington mode leads to lower accretion rates than the Eddington-limited case due to stronger feedback, resulting in significantly smaller BHs by two orders of magnitude; and that while the super-critical model boosts the growth of low-spin BHs, for high-spin BHs the mass may be reduced due to increased radiative feedback. Our simulations show that the first $10^8 - 10^9\, M_\odot$ SMBHs may grow from heavy seeds of $\gtrsim 10^4\, M_\odot$ via Eddington-limited or mild super-critical accretion facilitated by gas-rich mergers and self-regulated by feedback, and they co-evolve with their host galaxies, producing bright quasars such as those at $z\sim$6 and ULAS J1342+0928, currently the most distant quasar at z = 7.54.

astro-ph.GA↗

arXiv2020

ART$^2$: A 3D Parallel Multi-wavelength Radiative Transfer Code for Continuum and Atomic and Molecular Lines

ART$^2$ is a 3D multi-wavelength Monte Carlo radiative transfer (RT) code that couples continuum and emission lines to track the propagation of photons and their interactions with the interstellar medium (ISM). The previous version of ART$^2$, which included continuum and Ly$α$ line, has been extensively applied to hydrodynamics simulations to study multi-band properties of galaxies and ISM. Here, we describe new implementations of non-local thermodynamic equilibrium RT of molecular and atomic fine structure emission lines, and the parallelization of the code using a number of novel methods. The new ART$^2$ can efficiently and self-consistently produce a full spectrum that includes both continuum and lines such as [CII], [NII], [OIII], Ly$α$, and CO. These essential features, together with the multi-phase ISM model and the adaptive grid, make ART$^2$ a multi-purpose code to study multi-wavelength properties of a wide range of astrophysical systems from planetary disks to large-scale structures. To demonstrate the capability of the new ART$^2$, we applied it to two hydrodynamics simulations: the zoom-in Milky Way Simulation to obtain panchromatic properties of individual galaxies, and the large-scale IllustrisTNG100 Simulation to obtain global properties such as the line intensity mappings. These products are essential for a broad array of studies such as the correlations between physical and panchromatic properties and their evolution. By enabling direct comparison between numerical simulations and multi-band observations, ART$^2$ provides a crucial theoretical framework for the interpretations of existing observations, the plan for future surveys, and the synergy between multi-band galaxy surveys and line intensity mappings. Therefore, ART$^2$ is a powerful and versatile tool to bridge the gap between theories and observations of cosmic structures.

astro-ph.GA↗

arXiv2018

Formation of a Malin 1 analogue in IllustrisTNG by stimulated accretion

The galaxy Malin 1 contains the largest stellar disk known but the formation mechanism of this structure has been elusive. In this paper, we report a Malin 1 analogue in the 100 Mpc IllustrisTNG simulation and describe its formation history. At redshift zero, this massive galaxy, having a maximum circular velocity $V_{\rm max}$ of 430 ${\rm km\ s^{-1}}$, contains a 100 kpc gas/stellar disk with morphology similar to Malin 1. The simulated galaxy reproduces well many observed features of Malin 1's vast disk, including its stellar ages, metallicities, and gas rotation curve. We trace the extended disk back in time and find that a large fraction of the cold gas at redshift zero originated from the cooling of hot halo gas, triggered by the merger of a pair of intruding galaxies. Our finding provides a novel way to form large galaxy disks as extreme as Malin 1 within the current galaxy formation framework.

astro-ph.GA↗

arXiv2017

A momentum conserving $N$-body scheme with individual timesteps

$N$-body simulations study the dynamics of $N$ particles under the influence of mutual long-distant forces such as gravity. In practice, $N$-body codes will violate Newton's third law if they use either an approximate Poisson solver or individual timesteps. In this study, we construct a novel $N$-body scheme by combining a fast multipole method (FMM) based Poisson solver and a time integrator using a hierarchical Hamiltonian splitting (HHS) technique. We test our implementation for collision-less systems using several problems in galactic dynamics. As a result of the momentum conserving nature of these two key components, the new $N$-body scheme is also momentum conserving. Moreover, we can fully utilize the $\mathcal O(\textit N)$ complexity of FMM with the integrator. With the restored force symmetry, we can improve both angular momentum conservation and energy conservation substantially. The new scheme will be suitable for many applications in galactic dynamics and structure formation. Our implementation, in the code Taichi, is publicly available at https://bitbucket.org/qirong_zhu/taichi_public/.

astro-ph.IM↗

arXiv2017

Primordial Black Holes as Dark Matter: Constraints From Compact Ultra-Faint Dwarfs

The ground-breaking detections of gravitational waves from black hole mergers by LIGO have rekindled interest in primordial black holes (PBHs) and the possibility of dark matter being composed of PBHs. It has been suggested that PBHs of tens of solar masses could serve as dark matter candidates. Recent analytical studies demonstrated that compact ultra-faint dwarf galaxies can serve as a sensitive test for the PBH dark matter hypothesis, since stars in such a halo-dominated system would be heated by the more massive PBHs, their present-day distribution can provide strong constraints on PBH mass. In this study, we further explore this scenario with more detailed calculations, using a combination of dynamical simulations and Bayesian inference methods. The joint evolution of stars and PBH dark matter is followed with a Fokker-Planck code PhaseFlow. We run a large suite of such simulations for different dark matter parameters, then use a Markov Chain Monte Carlo approach to constrain the PBH properties with observations of ultra-faint galaxies. We find that two-body relaxation between the stars and PBH drives up the stellar core size, and increases the central stellar velocity dispersion. Using the observed half-light radius and velocity dispersion of stars in the compact ultra-faint dwarf galaxies as joint constraints, we infer that these dwarfs may have a cored dark matter halo with the central density in the range of 1-2 $\rm{M_{\odot}/pc^3}$, and that the PBHs may have a mass range of 2-14 $\rm{M_{\odot}}$ if they constitute all or a substantial fraction of the dark matter.

astro-ph.CO↗

arXiv2017

Is there a disk of satellites around the Milky Way?

The "Disk of satellites" (DoS) around Milky Way is a highly debated topic with conflicting interpretations of observations and their theoretical models. We perform a comprehensive analysis of all dwarfs detected in the Milky Way and find that the DoS structure depends strongly on the plane identification method and the sample size. In particular, we demonstrate that a smaller sample size produces a higher anisotropy of the spatial distribution and a stronger clustering of the angular momentum of the satellites. Moreover, we calculate the evolution of the 11 classical satellites with proper motion measurements and find that the thin DoS they currently reside in is transient. Furthermore, we analyze two cosmological simulations using the same initial conditions of a Milky Way-sized galaxy, an N-body run with dark matter only and a hydrodynamic one with both baryonic and dark matter, and find that the hydrodynamic simulation produces more anisotropic distribution of satellites than the N-body one. Our results suggest that an anisotropic distribution of satellites in galaxies can originate from baryonic processes in the hierarchical structure formation model, but the claimed highly-flattened, coherently-rotating DoS of the Milky Way may be a small-number selection effect. These findings may help resolve the contradictory claims of DoS in other galaxies and the discrepancy among numerical simulations.

astro-ph.GA↗

arXiv2017

The formation and evolution of star clusters in interacting galaxies

Observations of globular clusters show that they have universal lognormal mass functions with a characteristic peak at $\sim 2\times 10^{5}\, {\rm{M_{\odot}}}$, but the origin of this peaked distribution is highly debated. Here we investigate the formation and evolution of star clusters in interacting galaxies using high-resolution hydrodynamical simulations performed with two different codes in order to mitigate numerical artifacts. We find that massive star clusters in the range of $\sim 10^{5.5} - 10^{7.5}\, {\rm{M_{\odot}}}$ form preferentially in the highly-shocked regions produced by galaxy interactions. The nascent cluster-forming clouds have high gas pressures in the range of $P/k \sim 10^8 - 10^{12}\, \rm{K}\,\rm{cm^{-3}}$, which is $\sim 10^4 - 10^8$ times higher than the typical pressure of the interstellar medium but consistent with recent observations of a pre-super star cluster cloud in the Antennae Galaxies. Furthermore, these massive star clusters have quasi-lognormal initial mass functions with a peak around $\sim 10^{6}\, {\rm{M_{\odot}}}$. The number of clusters declines with time due to destructive processes, but the shape and the peak of the mass functions do not change significantly during the course of galaxy collisions. Our results suggest that gas-rich galaxy mergers may provide a favorable environment for the formation of massive star clusters such as globular clusters, and that the lognormal mass functions and the unique peak may originate from the extreme high-pressure conditions of the birth clouds and may survive the dynamical evolution.

astro-ph.GA↗

arXiv2017

Gas Cooling in Hydrodynamic Simulations with An Exact Time Integration Scheme

We implement and test the exact time integration method proposed by Townsend 2009 for gas cooling in cosmological hydrodynamic simulations. The errors using this time integrator for the internal energy are limited by the resolution of the cooling tables and are insensitive to the size of the timestep, improving accuracy relative to explicit or implicit schemes when the cooling time is short. We compare results with different time integrators for gas cooling in cosmological hydrodynamic simulations. We find that the temperature of the gas in filaments before accreting into dark matter halos to form stars, obtained with the exact cooling integration, lies close to the equilibrium where radiative cooling balances heating from the UV background. For comparison, the gas temperature without the exact integrator shows substantial deviations from the equilibrium relation. Galaxy stellar masses with the exact cooling technique agree reasonably well, but are systematically lower than the results obtained by the other integration schemes, reducing the need for feedback to suppress star formation. Our implementation of the exact cooling technique is provided and can be easily incorporated into any hydrodynamic code.

astro-ph.GA↗

arXiv2017

Growth of First Galaxies: Impacts of Star Formation and Stellar Feedback

We present the results of cosmological hydrodynamic simulations with zoom-in initial conditions, and investigate the formation of the first galaxies and their evolution towards observable galaxies at $z \sim 6$. We focus on three different galaxies which end up in halos with masses $M_{h} = 2.4 \times10^{10}~h^{-1}\; M_{\odot}$ (Halo-10), $1.6 \times10^{11}~h^{-1}\; M_{\odot}$ (Halo-11) and $0.7 \times10^{12}~h^{-1} M_{\odot}$ (Halo-12) at z=6. Our simulations also probe impacts of different sub-grid assumptions, i.e., SF efficiency and cosmic reionization, on SF histories in the first galaxies. We find that star formation occurs intermittently due to supernova (SN) feedback at z > 10, and then it proceeds more smoothly as the halo mass grows at lower redshifts. Galactic disks are destroyed due to SN feedback, while galaxies in simulations with no-feedback or lower SF efficiency models can sustain galactic disk for long periods > 10 Myr. The expulsion of gas at the galactic center also affects the inner dark matter density profile. However, SN feedback does not seem to keep the shallow profile of dark matter for a long period. Our simulated galaxies in Halo-11 and Halo-12 reproduce the star formation rates (SFR) and stellar masses of observed Lyman-$α$ emitters (LAEs) at z = 7-8 fairly well given observational uncertainties. In addition, we investigate the effect of UV background radiation on star formation as an external feedback source, and find that earlier reionization extends the quenching time of star formation due to photo-ionization heating, but does not affect the stellar mass at z=6.

astro-ph.GA↗

arXiv2017

The nature of disk of satellites around Milky Way-like galaxies

It has been suggested that the satellite galaxies of the Milky Way reside in a highly-flattened, kinematically-coherent plane called Disk of Satellites (DoS). The origin of the DoS, however, has been hotly debated, and a number of conflicting claims have been reported in the literature on whether or not the DoS is consistent with predictions from the standard Lambda Cold Dark Matter ($Λ$CDM) cosmological model. Here we investigate this issue by comparing a high-resolution, hydrodynamic $Λ$CDM simulation of a Milky Way sized galaxy with its dark matter only counterpart. We find the following results: (1) The abundance and distribution of satellite galaxies around a host galaxy is significantly different in the hydro simulation compared to its N-body counterpart; (2) No clear coherent rotation is found in the satellite system, as the fractions of corotating and counter-corotating satellites remain comparable across cosmic time; (3) The satellite distribution evolves significantly with time, from nearly isotropic at high redshift to anisotropic at the present day; (4) The DoS properties strongly depend on sample selection and plane identification methods. Our results imply that the spatially-thin and coherently-rotating DoS reported in Milky Way and other galaxies may be a selection effect of small sample size.

astro-ph.GA↗

arXiv2017

Baryonic impact on the dark matter orbital properties of Milky Way-sized haloes

We study the orbital properties of dark matter haloes by combining a spectral method and cosmological simulations of Milky Way-sized galaxies. We compare the dynamics and orbits of individual dark matter particles from both hydrodynamic and $N$-body simulations, and find that the fraction of box, tube and resonant orbits of the dark matter halo decreases significantly due to the effects of baryons. In particular, the central region of the dark matter halo in the hydrodynamic simulation is dominated by regular, short-axis tube orbits, in contrast to the chaotic, box and thin orbits dominant in the $N$-body run. This leads to a more spherical dark matter halo in the hydrodynamic run compared to a prolate one as commonly seen in the $N$-body simulations. Furthermore, by using a kernel based density estimator, we compare the coarse-grained phase-space densities of dark matter haloes in both simulations and find that it is lower by $\sim0.5$ dex in the hydrodynamic run due to changes in the angular momentum distribution, which indicates that the baryonic process that affects the dark matter is irreversible. Our results imply that baryons play an important role in determining the shape, kinematics and phase-space density of dark matter haloes in galaxies.

astro-ph.GA↗