arXiv subjects

Yu Gao

Publications and source records attributed to Yu Gao.

At least 199 records · Page 11Linked to original sources

arXiv2022

Many-body Hilbert space scarring on a superconducting processor

Quantum many-body scarring (QMBS) -- a recently discovered form of weak ergodicity breaking in strongly-interacting quantum systems -- presents opportunities for mitigating thermalization-induced decoherence in quantum information processsing. However, the existing experimental realizations of QMBS are based on kinetically-constrained systems where an emergent dynamical symmetry "shields" such states from the thermalizing bulk of the spectrum. Here, we experimentally realize a distinct kind of QMBS phenomena by approximately decoupling a part of the many-body Hilbert space in the computational basis. Utilizing a programmable superconducting processor with 30 qubits and tunable couplings, we realize Hilbert space scarring in a non-constrained model in different geometries, including a linear chain as well as a quasi-one-dimensional comb geometry. By performing full quantum state tomography on 4-qubit subsystems, we provide strong evidence for QMBS states by measuring qubit population dynamics, quantum fidelity and entanglement entropy following a quench from initial product states. Our experimental findings broaden the realm of QMBS mechanisms and pave the way to exploiting correlations in QMBS states for applications in quantum information technology.

quant-ph↗

arXiv2022

Diagnosing Triggered Star Formation in the Galactic H II region Sh 2-142

Stars are formed by gravitational collapse, spontaneously or, in some cases under the constructive influence of nearby massive stars, out of molecular cloud cores. Here we present an observational diagnosis of such triggered formation processes in the prominent \ion{H}{2} region Sh\,2-142, which is associated with the young star cluster NGC\,7380, and with some bright-rimmed clouds as the signpost of photoionization of molecular cloud surfaces. Using near- (2MASS) and mid-infrared (WISE) colors, we identified candidate young stars at different evolutionary stages, including embedded infrared sources having spectral energy distributions indicative of active accretion. We have also used data from our optical observations to be used in SEDs, and from Gaia EDR3 to study the kinematics of young objects. With this young stellar sample, together with the latest CO line emission data (spectral resolution $\sim 0.16$~km~s$^{-1}$, sensitivity $\sim 0.5$~K), a positional and ageing sequence relative to the neighboring cloud complex, and to the bright-rimmed clouds, is inferred. The propagating stellar birth may be responsible, at least partially, for the formation of the cluster a few million years ago, and for the ongoing activity now witnessed in the cloud complex.

astro-ph.GA↗

arXiv2022

21-cm constraints on spinning primordial black holes

Hawking radiation from primordial black holes (PBH) can ionize and heat up neutral gas during the cosmic dark ages, leaving imprints on the global 21-cm signal of neutral hydrogen. We use the global 21-cm signal to constrain the abundance of spinning PBHs in mass range of $[2 \times 10^{13}, 10^{18}]$ grams. We consider several extended PBH distribution models. Our results show that 21-cm can set the most stringent PBH bounds in our mass window. Compared with constraints set by {\it Planck} cosmic microwave background (CMB) data, 21-cm limits are more stringent by about two orders of magnitudes. PBHs with higher spin are typically more strongly constrained. Our 21-cm constraints for the monochromatic mass distribution rule out spinless PBHs with initial mass below $1.5 \times 10^{17}\ rg$, whereas extreme Kerr PBHs with reduced initial spin of $a_0=0.999$ are excluded as the dominant dark matter component for masses below $6 \times 10^{17}\ rg$. We also derived limits for the log-normal, power-law and critical collapse PBH mass distributions.

astro-ph.CO↗

arXiv2022

Stability of peakons of the Camassa-Holm equation beyond wave breaking

Using a generalized framework that consists of evolution of the solution to the Camassa- Holm equation and its energy measure, we establish the global-in-time orbital stability of peakons with respect to the perturbed (energy) conservative solutions to the Camassa-Holm equation. Especially, we extend the H1-stability result obtained by Constantin and Strauss (Comm. Pure Appl. Math., 53(5), 603-610, 2000) globally-in-time, even after the perturbed solutions experience wave breaking. In addition, our result also shows that the singular part of the energy measure of the perturbed solutions will remain stable for all times.

math.AP↗

arXiv2022

Fluid Simulation System Based on Graph Neural Network

Traditional computational fluid dynamics calculates the physical information of the flow field by solving partial differential equations, which takes a long time to calculate and consumes a lot of computational resources. We build a fluid simulation simulator based on the graph neural network architecture. The simulator has fast computing speed and low consumption of computing resources. We regard the computational domain as a structural graph, and the computational nodes in the structural graph determine neighbor nodes through adaptive sampling. Building deep learning architectures with attention graph neural networks. The fluid simulation simulator is trained according to the simulation results of the flow field around the cylinder with different Reynolds numbers. The trained fluid simulation simulator not only has a very high accuracy for the prediction of the flow field in the training set, but also can extrapolate the flow field outside the training set. Compared to traditional CFD solvers, the fluid simulation simulator achieves a speedup of 2-3 orders of magnitude. The fluid simulation simulator provides new ideas for the rapid optimization and design of fluid mechanics models and the real-time control of intelligent fluid mechanisms.

physics.flu-dyn↗

arXiv2022

Cold Gas in Massive Galaxies as A Critical Test of Black Hole Feedback Models

Black hole feedback has been widely implemented as the key recipe to quench star formation in massive galaxies in modern semi-analytic models and hydrodynamical simulations. As the theoretical details surrounding the accretion and feedback of black holes continue to be refined, various feedback models have been implemented across simulations, with notable differences in their outcomes. Yet, most of these simulations have successfully reproduced some observations, such as stellar mass function and star formation rate density in the local Universe. We use the recent observation on the change of neutral hydrogen gas mass (including both ${\rm H_2}$ and ${\rm HI}$) with star formation rate of massive central disc galaxies as a critical constraint of black hole feedback models across several simulations. We find that the predictions of IllustrisTNG agree with the observations much better than the other models tested in this work. This favors IllustrisTNG's treatment of active galactic nuclei - where kinetic winds are driven by black holes at low accretion rates - as more plausible amongst those we test. In turn, this also indirectly supports the idea that the massive central disc galaxy population in the local Universe was likely quenched by AGN feedback.

astro-ph.GA↗

arXiv2022

A 2-3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946

The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new $2-4$" (${\sim}75{-}150$ pc at $7.7$ Mpc) observations at 2 and 3 mm covering the central $50$" (${\sim}1.9$ kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO$^+$, HNC, CS, HC$_3$N, N$_2$H$^+$, C$_2$H, CH$_3$OH, and H$_2$CO. We complemented these with published 1mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density ${Σ_{\mathrm{SFR}}}$ on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 $-$ the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO$^+$, and HNC). We find that HCO$^+$ correlates best with ${Σ_{\mathrm{SFR}}}$, but the dense gas fraction ($f_{\mathrm{dense}}$) and star formation efficiency of the dense gas (${\mathrm{SFE_{dense}}}$) fits show different behaviours than expected from large-scale disc observations.The SBE has a higher ${Σ_{\mathrm{SFR}}}$, $f_{\mathrm{dense}}$, and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2-1)/CO(1-0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO$^+$/HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies.

astro-ph.GA↗

arXiv2021

Joint space-time analyticity of mild solutions to the Navier-Stokes equations

In this paper, we show the optimal decay rate estimates of the space-time derivatives and the joint space-time analyticity of solutions to the Navier-Stokes equations. As it is known from the Hartogs's theorem, for a complex function with two complex variables, the joint analyticity with respect to two variables can be derived from combining of analyticity with respect to each variable. However, as a function of two real variables for space and time, the joint space-time analyticity of solutions to the Navier-Stokes equations cannot be directly obtained from the combination of space analyticity and time analyticity. Our result seems to be the first quantitative result for the joint space-time analyticity of solutions to the Navier-Stokes equations, and the proof only involves real variable methods. Moreover, the decay rate estimates also yield the bounds on the growth (in time) of radius of space analyticity, time analyticity, and joint space-time analyticity of solutions.

math.AP↗

arXiv2021

Faster Maxflow via Improved Dynamic Spectral Vertex Sparsifiers

We make several advances broadly related to the maintenance of electrical flows in weighted graphs undergoing dynamic resistance updates, including: 1. More efficient dynamic spectral vertex sparsification, achieved by faster length estimation of random walks in weighted graphs using Morris counters [Morris 1978, Nelson-Yu 2020]. 2. A direct reduction from detecting edges with large energy in dynamic electric flows to dynamic spectral vertex sparsifiers. 3. A procedure for turning algorithms for estimating a sequence of vectors under updates from an oblivious adversary to one that tolerates adaptive adversaries via the Gaussian-mechanism from differential privacy. Combining these pieces with modifications to prior robust interior point frameworks gives an algorithm that on graphs with $m$ edges computes a mincost flow with edge costs and capacities in $[1, U]$ in time $\widetilde{O}(m^{3/2-1/58} \log^2 U)$. In prior and independent work, [Axiotis-Mądry-Vladu FOCS 2021] also obtained an improved algorithm for sparse mincost flows on capacitated graphs. Our algorithm implies a $\widetilde{O}(m^{3/2-1/58} \log U)$ time maxflow algorithm, improving over the $\widetilde{O}(m^{3/2-1/328}\log U)$ time maxflow algorithm of [Gao-Liu-Peng FOCS 2021].

cs.DS↗

arXiv2021

The HASHTAG project: The First Submillimeter Images of the Andromeda Galaxy from the Ground

Observing nearby galaxies with submillimeter telescopes on the ground has two major challenges. First, the brightness is significantly reduced at long submillimeter wavelengths compared to the brightness at the peak of the dust emission. Second, it is necessary to use a high-pass spatial filter to remove atmospheric noise on large angular scales, which has the unwelcome by-product of also removing the galaxy's large-scale structure. We have developed a technique for producing high-resolution submillimeter images of galaxies of large angular size by using the telescope on the ground to determine the small-scale structure (the large Fourier components) and a space telescope (Herschel or Planck) to determine the large-scale structure (the small Fourier components). Using this technique, we are carrying out the HARP and SCUBA-2 High Resolution Terahertz Andromeda Galaxy Survey (HASHTAG), an international Large Program on the James Clerk Maxwell Telescope, with one aim being to produce the first high-fidelity high-resolution submillimeter images of Andromeda. In this paper, we describe the survey, the method we have developed for combining the space-based and ground-based data, and present the first HASHTAG images of Andromeda at 450 and 850um. We also have created a method to predict the CO(J=3-2) line flux across M31, which contaminates the 850um band. We find that while normally the contamination is below our sensitivity limit, the contamination can be significant (up to 28%) in a few of the brightest regions of the 10 kpc ring. We therefore also provide images with the predicted line emission removed.

astro-ph.GA↗

arXiv2021

An Extended Primal-Dual Algorithm Framework for Nonconvex Problems with Application to Nonlinear Imaging

We propose an extended primal-dual algorithm framework for solving a general nonconvex optimization model. This work is motivated by image reconstruction problems in a class of nonlinear imaging, where the forward operator can be formulated as a nonlinear convex function with respect to the reconstructed image. Using the proposed framework, we put forward six specific iterative schemes, and present their detailed mathematical explanation. We also establish the relationship to existing algorithms. Moreover, under proper assumptions, we analyze the convergence of the schemes for the general model when the optimal dual variable regarding the nonlinear operator is non-vanishing. As a representative, the image reconstruction for spectral computed tomography is used to demonstrate the effectiveness of the proposed algorithm framework. By special properties of the concrete problem, we further prove the convergence of these customized schemes when the optimal dual variable regarding the nonlinear operator is vanishing. Finally, the numerical experiments show that the proposed algorithm has good performance on image reconstruction for various data with non-standard scanning configuration.

math.OC↗

arXiv2021

Observation of a symmetry-protected topological time crystal with superconducting qubits

We report the observation of a symmetry-protected topological time crystal, which is implemented with an array of programmable superconducting qubits. Unlike the time crystals reported in previous experiments, where spontaneous breaking of the discrete time translational symmetry occurs for local observables throughout the whole system, the topological time crystal observed in our experiment breaks the time translational symmetry only at the boundaries and has trivial dynamics in the bulk. More concretely, we observe robust long-lived temporal correlations and sub-harmonic temporal response for the edge spins up to 40 driving cycles. We demonstrate that the sub-harmonic response is independent of whether the initial states are random product states or symmetry-protected topological states, and experimentally map out the phase boundary between the time crystalline and thermal phases. Our work paves the way to exploring peculiar non-equilibrium phases of matter emerged from the interplay between topology and localization as well as periodic driving, with current noisy intermediate-scale quantum processors.

quant-ph↗

arXiv2021

The effect of active galactic nuclei on the cold interstellar medium in distant star-forming galaxies

In the framework of a systematic ALMA study of IR-selected main-sequence and starburst galaxies at z~1-1.7 at typical ~1" resolution, we report on the effects of mid-IR- and X-ray-detected active galactic nuclei (AGN) on the reservoirs and excitation of molecular gas in a sample of 55 objects. We find detectable nuclear activity in ~30% of the sample. The presence of dusty tori influences the IR SED of galaxies, as highlighted by the strong correlation among the AGN contribution to the total IR luminosity budget (fAGN = LIR,AGN/LIR), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR (S1.2mm/S24um) observed color, with consequences on the empirical SFR estimates. Nevertheless, we find only marginal effects of AGN on the CO (J=2,4,5,7) or neutral carbon ([CI](1-0), [CI](2-1)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full SLEDs. The [CI] and CO emission up to J=5,7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, we highlight the existence of a large variety of line luminosities and ratios despite the homogeneous selection. In particular, we find a sparse group of AGN-dominated sources with the highest LIR,AGN/LIR,SFR ratios, >3, that are more luminous in CO(5-4) than what is predicted by the L'CO(5-4)-LIR,SFR relation, which might be the result of the nuclear activity. For the general population, our findings translate into AGN having minimal effects on quantities such as gas and dust fractions and SFEs. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8x larger dust optical depths. In general, this is consistent with a marginal impact of the nuclear activity on the gas reservoirs and star formation in average star-forming AGN hosts with LIR>5e11 Lsun, typically underrepresented in surveys of quasars and SMGs.

astro-ph.GA↗

arXiv2021

TOOD: Task-aligned One-stage Object Detection

One-stage object detection is commonly implemented by optimizing two sub-tasks: object classification and localization, using heads with two parallel branches, which might lead to a certain level of spatial misalignment in predictions between the two tasks. In this work, we propose a Task-aligned One-stage Object Detection (TOOD) that explicitly aligns the two tasks in a learning-based manner. First, we design a novel Task-aligned Head (T-Head) which offers a better balance between learning task-interactive and task-specific features, as well as a greater flexibility to learn the alignment via a task-aligned predictor. Second, we propose Task Alignment Learning (TAL) to explicitly pull closer (or even unify) the optimal anchors for the two tasks during training via a designed sample assignment scheme and a task-aligned loss. Extensive experiments are conducted on MS-COCO, where TOOD achieves a 51.1 AP at single-model single-scale testing. This surpasses the recent one-stage detectors by a large margin, such as ATSS (47.7 AP), GFL (48.2 AP), and PAA (49.0 AP), with fewer parameters and FLOPs. Qualitative results also demonstrate the effectiveness of TOOD for better aligning the tasks of object classification and localization. Code is available at https://github.com/fcjian/TOOD.

cs.CV↗

arXiv2021

On-chip Acousto Thermal Shift Assay for Rapid and Sensitive Assessment of Protein Thermodynamic Stability

Thermal shift assays (TSAs) have been extensively used to study thermodynamics of proteins and provide an efficient means to assess protein-ligand binding or protein-protein interaction. However, existing TSAs have limitations such as time consuming, labor intensive, or low sensitivity. Here we introduce a novel acousto thermal shift assay (ATSA), the first ultrasound enabled TSA, for real-time analysis of protein thermodynamic stability. It capitalizes the novel coupling of unique acoustic mechanisms to achieve protein unfolding, concentration, and measurement on a single microfluidic chip within minutes. Compared to conventional TSA methods, our ATSA technique enabled ultra-fast (at least 30 times faster), highly sensitive (7-34 folds higher), and label-free monitoring of protein-ligand interactions and protein stability. ATSA paves new avenues for protein analysis in biology, medicine and fast diagnosis.

physics.bio-ph↗

arXiv2021

Acoustically Manipulating Internal Structure of Disk-in-Sphere Endoskeletal Droplets

Manipulation of micro/nano particles has been well studied and demonstrated by optical, electromagnetic, and acoustic approaches, or their combinations. Manipulation of internal structure of droplet/particle is rarely explored and remains challenging due to its complicated nature. Here we demonstrated the manipulation of internal structure of disk-in-sphere endoskeletal droplets using acoustic wave for the first time. We developed a model to investigate the physical mechanisms behind this novel phenomenon. Theoretical analysis of the acoustic interactions indicated that these assembly dynamics arise from a balance of the primary and secondary radiation forces. Additionally, the disk orientation was found to change with acoustic driving frequency, which allowed on-demand, reversible adjusting disk orientations with respect to the substrate. This novel dynamic behavior leads to unique reversible arrangements of the endoskeletal droplets and their internal architecture, which may provide a new avenue for directed assembly of novel hierarchical colloidal architectures and intracellular organelles or intra-organoid structures.

physics.app-ph↗

arXiv2021

NOEMA Observations of CO Emission in Arp 142 and Arp 238

Previous studies have shown significant differences in the enhancement of the star-formation rate (SFR) and the star-formation efficiency (SFE=SFR/M_mol) between spiral-spiral and spiral-elliptical mergers. In order to shed light on the physical mechanism of these differences, we present NOEMA observations of the molecular gas distribution and kinematics (linear resolutions of ~ 2kpc) in two representative close major-merger star-forming pairs: the spiral-elliptical pair Arp142 and the spiral-spiral pair Arp238. The CO in Arp142 is widely distributed over a highly distorted disk without any nuclear concentration, and an off-centric ring-like structure is discovered in channel maps. The SFE varies significantly within Arp142, with a starburst region (Region 1) near the eastern tip of the distorted disk showing an SFE ~0.3 dex above the mean of the control sample of isolated galaxies, and the SFE of the main disk (Region 4) 0.43 dex lower than the mean of the control sample. In contrast, the CO emission in Arp238 is detected only in two compact sources at the galactic centers. Compared to the control sample, Arp238-E shows an SFE enhancement of more than 1 dex whereas Arp238-W has an enhancement of ~0.7 dex. We suggest that the extended CO distribution and the large SFE variation in Arp142 are due to an expanding large-scale ring triggered by a recent high-speed head-on collision between the spiral galaxy and the elliptical galaxy, and the compact CO sources with high SFEs in Arp238 are associated with nuclear starbursts induced by gravitational tidal torques in a low-speed coplanar interaction.

astro-ph.GA↗

arXiv2021

Regularity structure of conservative solutions to the Hunter-Saxton equation

In this paper we characterize the regularity structure, as well as show the global-in-time existence and uniqueness, of (energy) conservative solutions to the Hunter-Saxton equation by using the method of characteristics. The major difference between the current work and previous results is that we are able to characterize the singularities of energy measure and their nature in a very precise manner. In particular, we show that singularities, whose temporal and spatial locations are also explicitly given in this work, may only appear at at most countably many times, and are completely determined by the absolutely continuous part of initial energy measure. Our mathematical analysis is based on using the method of characteristics in a generalized framework that consists of the evolutions of solution to the Hunter-Saxton equation and the energy measure. This method also provides a clear description of the semi-group property for the solution and energy measure for all times.

math.AP↗