arXiv subjects

Qi Guo

Publications and source records attributed to Qi Guo.

At least 271 records · Page 15Linked to original sources

arXiv2017

Massive Quiescent Galaxies at z>3 in The Millennium Simulation Populated by A Semi-analytic Galaxy Formation Model

We take advantage of the statistical power of the large-volume dark-matter-only Millennium simulation, combined with a sophisticated semi-analytic galaxy formation model, to explore whether the recently reported $z=3.7$ quiescent galaxy ZF-COSMOS-20115 (ZF; Glazebrook et al. 2017) can be accommodated in current galaxy formation models. In our model, a population of quiescent galaxies (QGs) with stellar masses and star formation rates comparable to those of ZF naturally emerges at redshifts $z<4$. There are two and five ZF analogues at the redshift $3.86$ and $3.58$ in the Millennium simulation volume, respectively. We demonstrate that, while the $z>3.5$ massive QGs are rare (about 2\% of the galaxies with the similar stellar masses), the existing AGN feedback model implemented in the semi-analytic galaxy formation model can successfully explain the formation of the high-redshift QGs as it does on their lower redshift counterparts.

astro-ph.GA↗

arXiv2017

The segregation of baryons and dark matter during halo assembly

The standard galaxy formation theory assumes that baryons and dark matter are initially well-mixed before becoming segregated due to radiative cooling. We use non-radiative hydrodynamical simulations to explicitly examine this assumption and find that baryons and dark matter can also be segregated because of different physics obeyed by gas and dark matter during the build-up of the halo. As a result, baryons in many haloes do not originate from the same Lagrangian region as the dark matter. When using the fraction of corresponding dark matter and gas particles in the initial conditions (the "paired fraction") as a proxy of the dark matter and gas segregation strength of a halo, on average about $25$ percent of the baryonic and dark matter of the final halo are segregated in the initial conditions. This is at odds with the assumption of the standard galaxy formation model. A consequence of this effect is that the baryons and dark matter of the same halo initially experience different tidal torques and thus their angular momentum vectors are often misaligned. The degree of the misalignment is largely preserved during later halo assembly and can be understood with the tidal torque theory. The result challenges the precision of some semi-analytical approaches which utilize dark matter halo merger trees to infer properties of gas associated to dark matter haloes.

astro-ph.GA↗

arXiv2017

A Universe of Ultra-Diffuse Galaxies: Theoretical Predictions from $Λ$CDM Simulations

A particular population of galaxies have drawn much interest recently, which are as faint as typical dwarf galaxies but have the sizes as large as $L^*$ galaxies, the so called "ultra-diffuse galaxie" (UDGs). The lack of tidal features of UDGs in dense environments suggests that their host halos are perhaps as massive as that of the Milky Way. On the other hand, galaxy formation efficiency should be much higher in the halos of such masses. Here we use the model galaxy catalog generated by populating two large simulations: the Millennium-II cosmological simulation and Phoenix simulations of 9 big clusters with the semi-analytic galaxy formation model. This model reproduces remarkably well the observed properties of UDGs in the nearby clusters, including the abundance, profile, color, and morphology, etc. We search for UDG candidates using the public data and find 2 UDG candidates in our Local Group and 23 in our Local Volume, in excellent agreement with the model predictions. We demonstrate that UDGs are genuine dwarf galaxies, formed in the halos of $\sim 10^{10}M_{\odot}$. It is the combination of the late formation time and high-spins of the host halos that results in the spatially extended feature of this particular population. The lack of tidal disruption features of UDGs in clusters can also be explained by their late infall-time.

astro-ph.GA↗

arXiv2017

Fokas's Uniform Transform Method for linear systems

We demonstrate the use of the Unified Transform Method or Method of Fokas for boundary value problems for systems of constant-coefficient linear partial differential equations. We discuss how the apparent branch singularities typically appearing in the global relation are removable, allowing the method to proceed, in essence, as for scalar problems. We illustrate the use of the method with boundary value problems for the Klein-Gordon equation and the linearized Fitzhugh-Nagumo system. The case of wave equations is treated separately in an appendix.

math.AP↗

arXiv2017

Towards thinner convolutional neural networks through Gradually Global Pruning

Deep network pruning is an effective method to reduce the storage and computation cost of deep neural networks when applying them to resource-limited devices. Among many pruning granularities, neuron level pruning will remove redundant neurons and filters in the model and result in thinner networks. In this paper, we propose a gradually global pruning scheme for neuron level pruning. In each pruning step, a small percent of neurons were selected and dropped across all layers in the model. We also propose a simple method to eliminate the biases in evaluating the importance of neurons to make the scheme feasible. Compared with layer-wise pruning scheme, our scheme avoid the difficulty in determining the redundancy in each layer and is more effective for deep networks. Our scheme would automatically find a thinner sub-network in original network under a given performance.

cs.CV↗

arXiv2017

Attribute-controlled face photo synthesis from simple line drawing

Face photo synthesis from simple line drawing is a one-to-many task as simple line drawing merely contains the contour of human face. Previous exemplar-based methods are over-dependent on the datasets and are hard to generalize to complicated natural scenes. Recently, several works utilize deep neural networks to increase the generalization, but they are still limited in the controllability of the users. In this paper, we propose a deep generative model to synthesize face photo from simple line drawing controlled by face attributes such as hair color and complexion. In order to maximize the controllability of face attributes, an attribute-disentangled variational auto-encoder (AD-VAE) is firstly introduced to learn latent representations disentangled with respect to specified attributes. Then we conduct photo synthesis from simple line drawing based on AD-VAE. Experiments show that our model can well disentangle the variations of attributes from other variations of face photos and synthesize detailed photorealistic face images with desired attributes. Regarding background and illumination as the style and human face as the content, we can also synthesize face photos with the target style of a style photo.

cs.CV↗

arXiv2016

Galaxy formation in the Planck cosmology - IV. Mass and environmental quenching, conformity and clustering

We study the quenching of star formation as a function of redshift, environment and stellar mass in the galaxy formation simulations of Henriques et al. (2015), which implement an updated version of the Munich semi-analytic model (L-GALAXIES) on the two Millennium Simulations after scaling to a Planck cosmology. In this model massive galaxies are quenched by AGN feedback depending on both black hole and hot gas mass, and hence indirectly on stellar mass. In addition, satellite galaxies of any mass can be quenched by ram-pressure or tidal stripping of gas and through the suppression of gaseous infall. This combination of processes produces quenching efficiencies which depend on stellar mass, host halo mass, environment density, distance to group centre and group central galaxy properties in ways which agree qualitatively with observation. Some discrepancies remain in dense regions and close to group centres, where quenching still seems too efficient. In addition, although the mean stellar age of massive galaxies agrees with observation, the assumed AGN feedback model allows too much ongoing star formation at late times. The fact that both AGN feedback and environmental effects are stronger in higher density environments leads to a correlation between the quenching of central and satellite galaxies which roughly reproduces observed conformity trends inside haloes.

astro-ph.GA↗

arXiv2016

The growth of the central region by acquisition of counter-rotating gas in star-forming galaxies

Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Based on new measurements with integral field spectroscopy of a large representative galaxy sample, we find an appreciable fraction of counter-rotators among blue galaxies (9 out of 489 galaxies). The central regions of blue counter-rotators show younger stellar populations and more intense, ongoing star formation than their outer parts, indicating ongoing growth of the central regions. The result offers observational evidence that the acquisition of external gas in blue galaxies is possible; the interaction with pre-existing gas funnels the gas into nuclear regions (< 1 kpc) to form new stars.

astro-ph.GA↗

arXiv2016

SDSS-IV MaNGA: Properties of galaxies with kinematically decoupled stellar and gaseous components

We study the properties of 66 galaxies with kinematically misaligned gas and stars from MaNGA survey. The fraction of kinematically misaligned galaxies varies with galaxy physical parameters, i.e. M*, SFR and sSFR. According to their sSFR, we further classify these 66 galaxies into three categories, 10 star-forming, 26 "Green Valley" and 30 quiescent ones. The properties of different types of kinematically misaligned galaxies are different in that the star-forming ones have positive gradient in D4000 and higher gas-phase metallicity, while the green valley/quiescent ones have negative D4000 gradients and lower gas-phase metallicity on average. There is evidence that all types of the kinematically misaligned galaxies tend to live in more isolated environment. Based on all these observational results, we propose a scenario for the formation of star forming galaxies with kinematically misaligned gas and stars - the progenitor accretes misaligned gas from a gas-rich dwarf or cosmic web, the cancellation of angular momentum from gas-gas collisions between the pre-existing gas and the accreted gas largely accelerates gas inflow, leading to fast centrally-concentrated star-formation. The higher metallicity is due to enrichment from this star formation. For the kinematically misaligned green valley and quiescent galaxies, they might be formed through gas-poor progenitors accreting kinematically misaligned gas from satellites which are smaller in mass.

astro-ph.GA↗

arXiv2016

Total Ionizing Dose and Synergistic Effect of Magnetoresistive Random Access Memory

Magetoresistive Random Access Memory (MRAM) was irradiated by 60Co γ-rays and electron beam. The test of synergistic effect was performed under additional magnetic field when irradiation. We analyzed Total Ionizing Dose (TID) and synergistic damage mechanism of MRAM. DC, AC and function parameters of the memory were tested in radiation and annealing by Very Large Scale Integrated circuit (VLSI) test system. The radiation sensitive parameters were obtained through analyzing the data. Because magnetic field imposed on MRAM when the test of synergistic effect, Shallow Trench Isolation (STI) leakage or Frenkel-Poole emission of synergistic effect was smaller than that of TID, and radiation damage of synergistic effect was lower than that of TID.

physics.space-ph↗

arXiv2016

Chaoticons described by nonlocal nonlinear Schrodinger equation

It is shown that the unstable evolutions of the Hermite-Gauss-type stationary solutions for the nonlocal nonlinear Schrodinger equation with the exponential-decay response function can evolve into chaotic states. This new kind of entities are referred to as chaoticons because they exhibit not only chaotic properties (with positive Lyapunov exponents and spatial decoherence) but also soliton-like properties (with invariant statistic width and interaction of quasi-elastic collisions).

nlin.PS↗

arXiv2016

Every Filter Extracts A Specific Texture In Convolutional Neural Networks

Many works have concentrated on visualizing and understanding the inner mechanism of convolutional neural networks (CNNs) by generating images that activate some specific neurons, which is called deep visualization. However, it is still unclear what the filters extract from images intuitively. In this paper, we propose a modified code inversion algorithm, called feature map inversion, to understand the function of filter of interest in CNNs. We reveal that every filter extracts a specific texture. The texture from higher layer contains more colours and more intricate structures. We also demonstrate that style of images could be a combination of these texture primitives. Two methods are proposed to reallocate energy distribution of feature maps randomly and purposefully. Then, we inverse the modified code and generate images of diverse styles. With these results, we provide an explanation about why Gram matrix of feature maps \cite{Gatys_2016_CVPR} could represent image style.

cs.CV↗

arXiv2016

Modulational instability in nonlocal Kerr media with sine-oscillatory response

We discuss the modulational instability (MI) of plane waves in nonlocal Kerr media with the sine-oscillation response function, which can model the nematic liquid crystal with negative dielectric anisotropy. The results in the framework of the nonlocal nonlinear Schrödinger equation show that the MI in this system has two extraordinary properties. First, MI exists in both cases of negative Kerr coefficient $(n_{2}<0)$ and positive Kerr coefficient $(n_{2}>0)$. Second, the maximum gain of MI is not affected by the intensity of the plane wave, and the wave number %spatial frequency of the maximum gain depends on the oscillation period of the response function. We also explore, for the first time to the best of our knowledge, the physical mechanism of MI in the framework of the four-wave mixing (FWM). %FWM interaction. By introducing a phase mismatch term $\triangle k$ and a (nonlinear) growth factor $γ$ (both of them have clear physical meanings), we find that the necessary and sufficient condition for the MI appearance is that the phase mismatching during the FWM process should be small enough such that $|\triangle k|<2|γ|$. Basing on the discover, we present a new scenario to uniformly and consistently explain why the MI can happen for the focusing local Kerr media and cannot for the defocusing local Kerr media, as well as the MI phenomena in the nonlocal cases.

nlin.PS↗

arXiv2016

Galaxies in the EAGLE hydrodynamical simulation and in the Durham and Munich semi-analytical models

We compare global predictions from the EAGLE hydrodynamical simulation, and two semi-analytic (SA) models of galaxy formation, L-GALAXIES and GALFORM. All three models include the key physical processes for the formation and evolution of galaxies and their parameters are calibrated against a small number of observables at $z\approx 0$. The two SA models have been applied to merger trees constructed from the EAGLE dark matter only simulation. We find that at $z\leq 2$, both the galaxy stellar mass functions for stellar masses $M_*<10^{10.5} M_{\odot}$ and the median specific star formation rates (sSFRs) in the three models agree to better than $0.4$~dex. The evolution of the sSFR predicted by the three models closely follows the mass assembly history of dark matter haloes. In both EAGLE and L-GALAXIES there are more central passive galaxies with $M_*<10^{9.5} M_{\odot}$ than in L-GALAXIES. This difference is related to galaxies that have entered and then left a larger halo and which are treated as satellites in GALFORM. In the range $0 10^{9.5} M_{\odot}$ differ in some instances by an order of magnitude, while the stellar mass-size relation in EAGLE is a factor of $\approx 2$ tighter than for the two SA models. Our results suggest the need for a revision of how SA models treat the effect of baryonic self-gravity on the underlying dark matter. The treatment of gas flows in the models needs to be revised based on detailed comparison with observations to understand in particular the evolution of the stellar mass-metallicity relation.

astro-ph.GA↗

arXiv2016

Superconductivity at 7.8 K in the ternary LaRu2As2 compound

Here we report the discovery of superconductivity in the ternary LaRu2As2 compound. The polycrystalline LaRu2As2 samples were synthesized by the conventional solid state reaction method. Powder X-ray diffraction analysis indicates that LaRu2As2 crystallizes in the ThCr2Si2-type crystal structure with the space group I4/mmm (No. 139), and the refined lattice parameters are a = 4.182(6) Å and c = 10.590(3) Å. The temperature dependent resistivity measurement shows a clear superconducting transition with the onset Tc (critical temperature) at 7.8 K, and zero resistivity happens at 6.8 K. The upper critical field at zero temperature m0Hc2(0) was estimated to be 1.6 T from the resistivity measurement. DC magnetic susceptibility measurement shows a bulk superconducting Meissner transition at 7.0 K, and the isothermal magnetization measurement indicates that LaRu2As2 is a type-II superconductor.

cond-mat.supr-con↗

arXiv2016

Superconductivity in undoped CaFe2As2 single crystals

Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method, and the crystals were quenched in ice-water rapidly after high temperature growth. The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature dependent X-ray diffraction measurements. Superconductivity below 25 K was observed in the collapsed phase by resistivity and magnetization measurements. The isothermal magnetization curve measured at 2 K indicates that this is a typical type-II superconductor. For comparison, we systematically characterized the properties of the furnace cooled, quenched, and post-annealed single crystals, and found strong internal crystallographic strain existing in the quenched samples, which is the key for the occurrence of superconductivity in the undoped CaFe2As2 single crystals.

cond-mat.supr-con↗

arXiv2016

Multi-qubit non-adiabatic holonomic controlled quantum gates in decoherence-free subspaces

Non-adiabatic holonomic quantum gate in decoherence-free subspaces is of greatly practical importance due to its built-in fault tolerance, coherence stabilization virtues, and short run-time. Here we propose some compact schemes to implement two- and three-qubit controlled unitary quantum gates and Fredkin gate. For the controlled unitary quantum gates, the unitary operator acting on the target qubit is an arbitrary single-qubit gate operation. The controlled quantum gates can be directly implemented using non-adiabatic holonomy in decoherence-free subspaces and the required resource for the decoherence-free subspace encoding is minimal by using only two neighboring physical qubits undergoing collective dephasing to encode a logical qubit.

quant-ph↗

arXiv2015

Superconductivity at 3.85 K in BaPd2As2 with the ThCr2Si2-type structure

The single crystal of ThCr2Si2-type BaPd2As2 was successfully prepared by a self-flux method, and the crystal structure was characterized by X-ray diffraction method with the lattice parameters a = 4.489(2) Å and c = 10.322(3) Å. Bulk superconductivity below a Tc (critical temperature) of 3.85 K was revealed in this compound by the measurements of resistivity, magnetic susceptibility and specific heat, and this Tc is much higher than those of other isostructural Pd-based superconductors.

cond-mat.supr-con↗