Searcharxiv⌕ Search

arXiv subjects

Zhiyuan Li

Publications and source records attributed to Zhiyuan Li.

At least 415 records · Page 23Linked to original sources
arXiv2016

Chandra Detection of X-ray Emission from Ultra-compact Dwarf Galaxies and Extended Star Clusters

We have conducted a systematic study of X-ray emission from ultra-compact dwarf (UCD) galaxies and extended star clusters (ESCs), based on archival {\sl Chandra} observations. Among a sample of 511 UCDs and ESCs complied from the literature, 17 X-ray counterparts with 0.5-8 keV luminosities above $\sim$$5\times10^{36} {\rm~erg~s^{-1}}$ are identified, which are distributed in eight early-type host galaxies. To facilitate comparison, we also identify X-ray counterparts of 360 globular clusters (GCs) distributed in four of the eight galaxies. The X-ray properties of the UCDs and ESCs are found to be broadly similar to those of the GCs. The incidence rate of X-ray-detected UCDs and ESCs, $(3.3\pm0.8)$\%, while lower than that of the X-ray-detected GCs [($7.0\pm0.4)$\%], is substantially higher than expected from the field populations of external galaxies. A stacking analysis of the individually undetected UCDs/ESCs further reveals significant X-ray signals, which corresponds to an equivalent 0.5-8 keV luminosity of $\sim$$4\times10^{35} {\rm~erg~s^{-1}}$ per source. Taken together, these provide strong evidence that the X-ray emission from UCDs and ESCs is dominated by low-mass X-ray binaries having formed from stellar dynamical interactions, consistent with the stellar populations in these dense systems being predominantly old. For the most massive UCDs, there remains the possibility that a putative central massive black hole gives rise to the observed X-ray emission.

astro-ph.GA↗
arXiv2015

X-ray emissivity of old stellar populations: a local group census

We study the unresolved X-ray emission in three Local Group dwarf elliptical galaxies (NGC 147, NGC 185 and NGC 205) using XMM-Newton observations, which most likely originates from a collection of weak X-ray sources, mainly cataclysmic variables and coronally active binaries. Precise knowledge of this stellar X-ray emission is crucial not only for understanding the relevant stellar astrophysics but also for disentangling and quantifying the thermal emission from diffuse hot gas in nearby galaxies.We find that the integrated X-ray emissivities of the individual dwarf ellipticals agree well with that of the Solar vicinity, supporting an often assumed but untested view that the X-ray emissivity of old stellar populations is quasi-universal in normal galactic environments, in which dynamical effects on the formation and destruction of binary systems are not important. The average X-ray emissivity of the dwarf ellipticals, including M32 studied in the literature, is measured to be $L_{0.5-2\ \rm {keV}}/M_{\ast} = (6.0 \pm 0.5 \pm 1.8) \times 10^{27} \ \rm{erg \ s^{-1} \ M_\odot^{-1}}$. We also compare this value to the integrated X-ray emissivities of Galactic globular clusters and old open clusters and discuss the role of dynamical effects in these dense stellar systems.

astro-ph.GA↗
arXiv2015

Modular Forms and Special Cubic Fourfolds

We study the degree of the special cubic fourfolds in the Hilbert scheme of cubic fourfolds via a computation of the generating series of Heegner divisors of even lattice of signature (2, 20).

math.AG↗
arXiv2015

Detection of a Compact Nuclear Radio Source in the Local Group Elliptical Galaxy M32

The Local Group compact elliptical galaxy M32 hosts one of the nearest candidate super-massive black holes (SMBHs), which has a previously suggested X-ray counterpart. Based on sensitive observations taken with the {\it Karl G. Jansky} Very Large Array (VLA), we detect for the first time a compact radio source coincident with the nucleus of M32, which exhibits an integrated flux density of $\sim$$47.3\pm6.1$ $μ$Jy at 6.6 GHz. We discuss several possibilities for the nature of this source, favoring an origin of the long-sought radio emission from the central SMBH, for which we also revisit the X-ray properties based on recently acquired {\sl Chandra} and {\sl XMM-Newton} data. Our VLA observations also discover radio emission from three previously known optical planetary nebulae in the inner region of M32.

astro-ph.GA↗
arXiv2015

Photometric Evidence of an Intermediate-age Stellar Population in the Inner Bulge of M31

We explore the assembly history of the M31 bulge within a projected major-axis radius of 180" (~680 pc) by studying its stellar populations in Hubble Space Telescope WFC3 and ACS observations. Colors formed by comparing near-ultraviolet vs. optical bands are found to become bluer with increasing major-axis radius, which is opposite to that predicted if the sole sources of near-ultraviolet light were old extreme horizontal branch stars with a negative radial gradient in metallicity. Spectral energy distribution fits require a metal-rich intermediate-age stellar population (300 Myr to 1 Gyr old, ~solar metallicity) in addition to the dominant old population. The radial gradients in age and metallicity of the old stellar population are consistent with those in previous works. For the intermediate-age population, we find an increase in age with radius and a mass fraction that increases up to 2% at 680 pc away from the center. We exclude contamination from the M31 disk and/or halo as the main origin for this population. Our results thus suggest that intermediate-age stars exist beyond the central 5" (19 pc) of M31 and contribute ~1% of the total stellar mass in the bulge. These stars could be related to the secular growth of the M31 bulge.

astro-ph.GA↗
arXiv2015

Asymptotic behavior of solutions to space-time fractional diffusion equations

This article discusses the analyticity and the long-time asymptotic behavior of solutions to space-time fractional diffusion equations in $\mathbb{R}^d$. By a Laplace transform argument, we prove that the decay rate of the solution as $t\to\infty$ is dominated by the order of the time-fractional derivative. We consider the decay rate also in a bounded domain.

math.AP↗
arXiv2015

The Noether-Lefschetz conjecture and generalizations

We prove the Noether-Lefschetz conjecture on the moduli space of quasi-polarized K3 surfaces. This is deduced as a particular case of a general theorem that states that low degree cohomology classes of arithmetic manifolds of orthogonal type are dual to the classes of special cycles, i.e. sub-arithmetic manifolds of the same type. For compact manifolds this was proved in \cite{BMM11}, here we extend the results of \cite{BMM11} to non-compact manifolds. This allows us to apply our results to the moduli spaces of quasi-polarized K3 surfaces.

math.AG↗
arXiv2015

Old supernova dust factory revealed at the Galactic center

Dust formation in supernova ejecta is currently the leading candidate to explain the large quantities of dust observed in the distant, early Universe. However, it is unclear whether the ejecta-formed dust can survive the hot interior of the supernova remnant (SNR). We present infrared observations of ~0.02 $M_\odot$ of warm (~100 K) dust seen near the center of the ~10,000 yr-old Sgr A East SNR at the Galactic center. Our findings signify the detection of dust within an older SNR that is expanding into a relatively dense surrounding medium ($n_e$ ~ 100 $\mathrm{cm}^{-3}$) and has survived the passage of the reverse shock. The results suggest that supernovae may indeed be the dominant dust production mechanism in the dense environment of early Universe galaxies.

astro-ph.SR↗
arXiv2014

Integral Hodge classes on fourfolds fiberd by quadric bundles

We discuss the space of sections and certain bisections on a quadric surfaces bundle $X$ over a smooth curve. The Abel-Jacobi from these spaces to the intermediate Jacobian will be shown to be dominant with rationally connected fibers. As an application, we prove that the integral Hodge conjecture holds for degree four integral Hodge classes of fourfolds fibered by quadric bundles over a smooth curve. This gives an alternative proof of a result of Colliot-Th{é}l{è}ne and Voisin.

math.AG↗
arXiv2014

Initial-boundary value problems for multi-term time-fractional diffusion equations with positive constant coefficients

In this paper, we investigate the well-posedness and the long-time asymptotic behavior for the initial-boundary value problem for multi-term time-fractional diffusion equations, where the time differentiation consists of a finite summation of Caputo derivatives with decreasing orders in (0,1) and positive constant coefficients. By exploiting several important properties of multinomial Mittag-Leffler functions, various estimates follow from the explicit solutions in form of these special functions. Then the uniqueness and continuous dependency upon initial value and source term are established, from which the continuous dependence of solution of Lipschitz type with respect to various coefficients is also verified. Finally, by a Laplace transform argument, it turns out that the decay rate of the solution as time tends to infinity is dominated by the minimum order of the time-fractional derivatives.

math.AP↗
arXiv2014

Generic Properties of Random Gene Regulatory Networks

Modeling gene regulatory networks (GRNs) is an important topic in systems biology. Although there has been much work focusing on various specific systems, the generic behavior of GRNs with continuous variables is still elusive. In particular, it is not clear typically how attractors partition among the three types of orbits: steady state, periodic and chaotic, and how the dynamical properties change with network's topological characteristics. In this work, we first investigated these questions in random GRNs with different network sizes, connectivity, fraction of inhibitory links and transcription regulation rules. Then we searched for the core motifs that govern the dynamic behavior of large GRNs. We show that the stability of a random GRN is typically governed by a few embedding motifs of small sizes, and therefore can in general be understood in the context of these short motifs. Our results provide insights for the study and design of genetic networks.

q-bio.MN↗
arXiv2014

Uniqueness for inverse problems of determining orders of multi-term time-fractional derivatives of diffusion equation

This article proves the uniqueness for two kinds of inverse problems of identifying fractional orders in diffusion equations with multiple time-fractional derivatives by pointwise observation. By means of eigenfunction expansion and Laplace transform, we reduce the uniqueness for our inverse problems to the uniqueness of expansions of some special function and complete the proof.

math.AP↗
arXiv2014

Picard groups on moduli of K3 surfaces with Mukai models

We discuss the Picard group of moduli space $\mathcal{K}_g$ of quasi-polarized K3 surfaces of genus $g\leq 12$ and $g\neq 11$. In this range, $\mathcal{K}_g$ is unirational and a general element in $\mathcal{K}_g$ is a complete intersection with respect to a vector bundle on a homogenous space, by the work of Mukai. In this paper, we find generators of the Picard group $Pic_\mathbb{Q}(\mathcal{K}_g)$ using Noether-Lefschetz theory. This verifies the Noether-Lefschetz conjecture on moduli of K3 surfaces in these cases.

math.AG↗
arXiv2014

The Panchromatic Hubble Andromeda Treasury. VII. The Steep Mid-Ultraviolet to Near-Infrared Extinction Curve in the Central 200 pc of the M31 Bulge

We measure the extinction curve in the central 200 pc of M31 at mid-ultraviolet to near-infrared wavelengths (from 1928A to 1.5μm), using Swift/UVOT and HST WFC3/ACS observations in thirteen bands. Taking advantage of the high angular resolution of the HST WFC3 and ACS detectors, we develop a method to simultaneously determine the relative extinction and the fraction of obscured starlight for five dusty complexes located in the circumnuclear region. The extinction curves of these clumps (RV =2.4-2.5) are steeper than the average Galactic one (RV =3.1), but are similar to optical and near-infrared curves recently measured toward the Galactic Bulge (RV~2.5). This similarity suggests that steep extinction curves may be common in the inner bulge of galaxies. In the ultraviolet, the extinction curves of these clumps are also unusual. We find that one dusty clump (size <2 pc) exhibits a strong UV bump (extinction at 2175A), more than three standard deviation higher than that predicted by common models. Although the high stellar metallicity of the M31 bulge indicates that there are sufficient carbon and silicon to produce large dust grains, the grains may have been destroyed by supernova explosions or past activity of the central super-massive black hole, resulting in the observed steepened extinction curve.

astro-ph.GA↗
arXiv2013

Initial-boundary value problems for linear diffusion equation with multiple time-fractional derivatives

In this paper, we discuss initial-boundary value problems for linear diffusion equation with multiple time-fractional derivatives. By means of the Mittag-Leffler function and the eigenfunction expansion, we reduce the problem to an integral equation for a solution, and we apply the fixed-point theorem to prove the unique existence and the Hölder regularity of solution. For the case of the homogeneous equation, the solution can be analytically extended to a sector $\{z\in\mathbb{C};z\neq0,|\arg z|<\fracπ{2}\}$. In the case where all the coefficients of the time-fractional derivatives are positive constants, by the Laplace transform and the analyticity, we can prove that if a function satisfies the fractional diffusion equation and the homogeneous Neumann boundary condition on arbitrary subboundary as well as the homogeneous Dirichlet boundary condition on the whole boundary, then it vanishes identically.

math.AP↗
arXiv2013

Evidence for A Parsec-scale Jet from The Galactic Center Black Hole: Interaction with Local Gas

Despite strong physical reasons that they should exist and decades of search, jets from the Galactic Center Black Hole, Sgr A*, have not yet been convincingly detected. Based on high-resolution Very Large Array images and ultra-deep imaging-spectroscopic data produced by the Chandra X-ray Observatory, we report new evidence for the existence of a parsec-scale jet from Sgr A*, by associating a linear feature G359.944-0.052, previously identified in X-ray images of the Galactic Center, with a radio shock front on the Eastern Arm of the Sgr A West HII region. We show that the shock front can be explained in terms of the impact of a jet having a sharp momentum peak along the Galaxy's rotation axis, whereas G359.944-0.052, a quasi-steady feature with a power-law spectrum, can be understood as synchrotron radiation from shock-induced ultrarelativistic electrons cooling in a finite post-shock region downstream along the jet path. Several interesting implications of the jet properties are discussed.

astro-ph.HE↗