arXiv subjects

L. Wang

Publications and source records attributed to L. Wang.

At least 307 records · Page 17Linked to original sources

arXiv2016

HELP: star formation as function of galaxy environmentwith Herschel

The Herschel Extragalactic Legacy Project (HELP) brings together a vast range of data from many astronomical observatories. Its main focus is on the Herschel data, which maps dust obscured star formation over 1300 deg$^2$. With this unprecedented combination of data sets, it is possible to investigate how the star formation vs stellar mass relation (main-sequence) of star-forming galaxies depends on environment. In this pilot study we explore this question between 0.1 < z < 3.2 using data in the COSMOS field. We estimate the local environment from a smoothed galaxy density field using the full photometric redshift probability distribution. We estimate star formation rates by stacking the SPIRE data from the Herschel Multi-tiered Extragalactic Survey (HerMES). Our analysis rules out the hypothesis that the main-sequence for star-forming systems is independent of environment at 1.5 < z < 2, while a simple model in which the mean specific star formation rate declines with increasing environmental density gives a better description. However, we cannot exclude a simple hypothesis in which the main-sequence for star-forming systems is independent of environment at z < 1.5 and z > 2. We also estimate the evolution of the star formation rate density in the COSMOS field and our results are consistent with previous measurements at z < 1.5 and z > 2 but we find a $1.4^{+0.3}_{-0.2}$ times higher peak value of the star formation rate density at $z \sim 1.9$.

astro-ph.GA↗

arXiv2016

Experimental Discovery of the First Nonsymmorphic Topological Insulator KHgSb

Topological insulators (TIs) host novel states of quantum matter, distinguished from trivial insulators by the presence of nontrivial conducting boundary states connecting the valence and conduction bulk bands. Up to date, all the TIs discovered experimentally rely on the presence of either time reversal or symmorphic mirror symmetry to protect massless Dirac-like boundary states. Very recently, it has been theoretically proposed that several materials are a new type of TIs protected by nonsymmorphic symmetry, where glide-mirror can protect novel exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of such new nonsymmorphic TI (NSTI) is still missing. Using angle-resolved photoemission spectroscopy, we reveal that such hourglass topology exists on the (010) surface of crystalline KHgSb while the (001) surface has no boundary state, which is fully consistent with first-principles calculations. We thus experimentally demonstrate that KHgSb is a NSTI hosting hourglass fermions. By expanding the classification of topological insulators, this discovery opens a new direction in the research of nonsymmorphic topological properties of materials.

cond-mat.mtrl-sci↗

arXiv2016

The gap of stellar mass in galaxy groups: another perspective of the Too-big-To-Fail problem in the Milky Way

The Milky Way presents the too-big-to-fail (TBTF) problem that there are two observed satellite galaxies with maximum circular velocity larger than 55km/s, and others have velocity less than 25km/s, but the cold dark matter model predicts there should be more than 10 subhaloes with velocity larger than 25km/s. Those massive subhaloes with $25km/s<V_{max}<55km/s$ should not have failed to form stars. The TBTF problem severely challenges the CDM model. Most efforts are seeking the effects of baryonic feedback, decreasing the mass of the Milky Way, changing the properties of dark matter, so as to assign the observed low-velocity satellites into the massive subhaloes found in simulations. However, the TBTF problem can be avoided if the MW have not accreted subhaloes with velocity between $25km/s<V_{max}<55km/s$ although the probability of such a gap is lower as $\sim 1\%$ and can not be tested against observations. In this work we study the gap in stellar mass of satellite galaxies using the SDSS group catalogue and a semi-analytical model. We find that there are 1-2\% of galaxy groups with a large gap in the stellar mass of their satellites. These 'big gap' groups have accreted less massive subhaloes in their formation history and naturally display a gap between their satellite galaxies. If extrapolating our results to the Milky Way is appropriate, we conclude that it is very likely that our Milky Way has not accreted enough massive subhaloes to host those low-velocity satellites, and the TBTF problem is naturally avoided.

astro-ph.GA↗

arXiv2016

Further results on the Hamilton-Waterloo problem

In this paper, we almost completely solve the existence of an almost resolvable cycle system with odd cycle length. We also use almost resolvable cycle systems as well as other combinatorial structures to give some new solutions to the Hamilton-Waterloo problem.

math.CO↗

arXiv2016

Nematicity in LaFeAsO$_{1-x}$F$_x$

Orbital ordering has recently emerged as another important state in iron based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO$_{1-x}$F$_x$ with a focus on resistivity, Nernst effect, thermal expansion, and $^{75}$As NMR data. The transport properties at the underdoped level $x=0.05$ exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e. at about 160 K. Furthermore, the $^{75}$As-NMR spin-lattice relaxation rate $1/(T_1T)$ reveals a progressive slowing down of spin fluctuations. Yet, long-range magnetic order and also a detectable orthorhombic lattice distortion are absent. Thus, we conclude from the data that short-range orbital-nematic ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all anomalies in the transport and also the indications of slow spin fluctuations disappear close to optimal doping $x=0.1$ which suggests that in LaFeAsO$_{1-x}$F$_x$ the nematic phase actually competes with superconductivity.

cond-mat.supr-con↗

arXiv2016

Galaxy And Mass Assembly (GAMA): Accurate Panchromatic Photometry from Optical Priors using LAMBDAR

We present the Lambda Adaptive Multi-Band Deblending Algorithm in R (LAMBDAR), a novel code for calculating matched aperture photometry across images that are neither pixel- nor PSF-matched, using prior aperture definitions derived from high resolution optical imaging. The development of this program is motivated by the desire for consistent photometry and uncertainties across large ranges of photometric imaging, for use in calculating spectral energy distributions. We describe the program, specifically key features required for robust determination of panchromatic photometry: propagation of apertures to images with arbitrary resolution, local background estimation, aperture normalisation, uncertainty determination and propagation, and object deblending. Using simulated images, we demonstrate that the program is able to recover accurate photometric measurements in both high-resolution, low-confusion, and low-resolution, high-confusion, regimes. We apply the program to the 21-band photometric dataset from the Galaxy And Mass Assembly (GAMA) Panchromatic Data Release (PDR; Driver et al. 2016), which contains imaging spanning the far-UV to the far-IR. We compare photometry derived from LAMBDAR with that presented in Driver et al. (2016), finding broad agreement between the datasets. Nonetheless, we demonstrate that the photometry from LAMBDAR is superior to that from the GAMA PDR, as determined by a reduction in the outlier rate and intrinsic scatter of colours in the LAMBDAR dataset. We similarly find a decrease in the outlier rate of stellar masses and star formation rates using LAMBDAR photometry. Finally, we note an exceptional increase in the number of UV and mid-IR sources able to be constrained, which is accompanied by a significant increase in the mid-IR colour-colour parameter-space able to be explored.

astro-ph.GA↗

arXiv2016

Candidate Gravitationally Lensed Dusty Star-forming Galaxies in the Herschel Wide Area Surveys

We present a list of candidate gravitationally lensed dusty star-forming galaxies (DSFGs) from the HerMES Large Mode Survey (HeLMS) and the Herschel Stripe 82 Survey (HerS). Together, these partially overlapping surveys cover 372 deg$^{2}$ on the sky. After removing local spiral galaxies and known radio-loud blazars, our candidate list of lensed DSFGs is composed of 77 sources with 500 $μ$m flux densities ($S_{500}$) greater than 100 mJy. Such sources are dusty starburst galaxies similar to the first bright Sub Millimeter Galaxies (SMGs) discovered with SCUBA. We expect a large fraction of this list to be strongly lensed, with a small fraction made up of bright SMG-SMG mergers that appear as Hyper-Luminous Infrared Galaxies (HyLIRGs; $\rm L_{IR}>10^{13} L_{\odot}$). Thirteen of the 77 candidates have spectroscopic redshifts from CO spectroscopy with ground-based interferometers, putting them at $z>1$ and well above the redshift of the foreground lensing galaxies. The surface density of our sample is 0.21 $\pm$ 0.03 deg$^{-2}$. We present follow-up imaging of a few of the candidates confirming their lensing nature. The sample presented here is an ideal tool for higher resolution imaging and spectroscopic observations to understand detailed properties of starburst phenomena in distant galaxies.

astro-ph.GA↗

arXiv2016

Four new planets around giant stars and the mass-metallicity correlation of planet-hosting stars

CONTEXT. Exoplanet searches have demonstrated that giant planets are preferentially found around metal-rich stars and that their fraction increases with the stellar mass. AIMS. During the past six years, we have conducted a radial velocity follow-up program of 166 giant stars, to detect substellar companions, and characterizing their orbital properties. Using this information, we aim to study the role of the stellar evolution in the orbital parameters of the companions, and to unveil possible correlations between the stellar properties and the occurrence rate of giant planets. METHODS. Using FEROS and CHIRON spectra, we have computed precision radial velocities and we have derived atmospheric and physical parameters for all of our targets. Additionally, velocities computed from UCLES spectra are presented here. By studying the periodic radial velocity signals, we have detected the presence of several substellar companions. RESULTS. We present four new planetary systems around the giant stars HIP8541, HIP74890, HIP84056 and HIP95124. Additionally, we find that giant planets are more frequent around metal-rich stars, reaching a peak in the detection of $f$ = 16.7$^{+15.5}_{-5.9}$% around stars with [Fe/H] $\sim$ 0.35 dex. Similarly, we observe a positive correlation of the planet occurrence rate with the stellar mass, between M$_\star$ $\sim$ 1.0 -2.1 M$_\odot$, with a maximum of $f$ = 13.0$^{+10.1}_{-4.2}$%, at M$_\star$ = 2.1 M$_\odot$. CONCLUSIONS. We conclude that giant planets are preferentially formed around metal-rich stars. Also, we conclude that they are more efficiently formed around more massive stars, in the mass range of M$_\star$ $\sim$ 1.0 - 2.1 M$_\odot$. These observational results confirm previous findings for solar-type and post-MS hosting stars, and provide further support to the core-accretion formation model.

astro-ph.EP↗

arXiv2016

NIHAO VI. The hidden discs of simulated galaxies

Detailed studies of galaxy formation require clear definitions of the structural components of galaxies. Precisely defined components also enable better comparisons between observations and simulations. We use a subsample of eighteen cosmological zoom-in simulations from the NIHAO project to derive a robust method for defining stellar kinematic discs in galaxies. Our method uses Gaussian Mixture Models in a 3D space of dynamical variables. The NIHAO galaxies have the right stellar mass for their halo mass, and their angular momenta and Sérsic indices match observations. While the photometric disc-to-total ratios are close to 1 for all the simulated galaxies, the kinematic ratios are around ~0.5. Thus, exponential structure does not imply a cold kinematic disc. Above log(M*)~9.5, the decomposition leads to thin discs and spheroids that have clearly different properties, in terms of angular momentum, rotational support, ellipticity, [Fe/H] and [O/Fe]. At log(M*)<9.5, the decomposition selects discs and spheroids with less distinct properties. At these low masses, both the discs and spheroids have exponential profiles with high minor-to-major axes ratios, i.e. thickened discs.

astro-ph.GA↗

arXiv2016

Topological superconductor with a large Chern number and a large bulk excitation gap in single layer graphene

We show that a two-dimensional topological superconductor (TSC) can be realized in a hybrid system with a conventional $s$-wave superconductor proximity-coupled to a quantum anomalous Hall (QAH) state from the Rashba and exchange effects in single layer graphene. With very low or even zero doping near the Dirac points, i.e., two inequivalent valleys, this TSC has a Chern number as large as four, which supports four Majorana edge modes. More importantly, we show that this TSC has a robust topologically nontrivial bulk excitation gap, which can be larger or even one order of magnitude larger than the proximity-induced superconducting gap. This unique property paves a way for the application of QAH insulators as seed materials to realize robust TSCs and Majorana modes.

cond-mat.mes-hall↗

arXiv2016

A Search of Low-Mass WIMPs with p-type Point Contact Germanium Detector in the CDEX-1 Experiment

The CDEX-1 experiment conducted a search of low-mass (< 10 GeV/c2) Weakly Interacting Massive Particles (WIMPs) dark matter at the China Jinping Underground Laboratory using a p-type point-contact germanium detector with a fiducial mass of 915 g at a physics analysis threshold of 475 eVee. We report the hardware set-up, detector characterization, data acquisition and analysis procedures of this experiment. No excess of unidentified events are observed after subtraction of known background. Using 335.6 kg-days of data, exclusion constraints on the WIMP-nucleon spin-independent and spin-dependent couplings are derived.

hep-ex↗

arXiv2015

Dust properties of Lyman break galaxies at $z\sim3$

We explore from a statistical point of view the far-infrared (far-IR) and sub-millimeter (sub-mm) properties of a large sample of LBGs (22,000) at z~3 in the COSMOS field. The large number of galaxies allows us to split it in several bins as a function of UV luminosity, UV slope, and stellar mass to better sample their variety. We perform stacking analysis in PACS (100 and 160 um), SPIRE (250, 350 and 500 um) and AzTEC (1.1 mm) images. Our stacking procedure corrects the biases induced by galaxy clustering and incompleteness of our input catalogue in dense regions. We obtain the full IR spectral energy distributions (SED) of subsamples of LBGs and derive the mean IR luminosity as a function of UV luminosity, UV slope, and stellar mass. The average IRX is roughly constant over the UV luminosity range, with a mean of 7.9 (1.8 mag). However, it is correlated with UV slope, and stellar mass. We investigate using a statistically-controlled stacking analysis as a function of (stellar mass, UV slope) the dispersion of the IRX-UVslope and IRX-M* plane. Our results enable us to study the average relation between star-formation rate (SFR) and stellar mass, and we show that our LBG sample lies on the main sequence of star formation at z~3.

astro-ph.GA↗

arXiv2015

The HerMES sub-millimetre local and low-redshift luminosity functions

We used wide area surveys over 39 deg$^2$ by the HerMES collaboration, performed with the Herschel Observatory SPIRE multi-wavelength camera, to estimate the low-redshift, $0.02<z<0.5$, monochromatic luminosity functions (LFs) of galaxies at 250, 350 and 500$\,μ$m. SPIRE flux densities were also combined with Spitzer photometry and multi-wavelength archival data to perform a complete SED fitting analysis of SPIRE detected sources to calculate precise k-corrections, as well as the bolometric infrared (8-1000$\,μ$m) luminosity functions and their low-$z$ evolution from a combination of statistical estimators. Integration of the latter prompted us to also compute the local luminosity density (LLD) and the comoving star formation rate density (SFRD) for our sources, and to compare them with theoretical predictions of galaxy formation models. The luminosity functions show significant and rapid luminosity evolution already at low redshifts, $0.02<z<0.2$, with L$_{IR}^* \propto (1+z)^{6.0\pm0.4}$ and $Φ_{IR}^* \propto (1+z)^{-2.1\pm0.4}$, L$_{250}^* \propto (1+z)^{5.3\pm0.2}$ and $Φ_{250}^* \propto (1+z)^{-0.6\pm0.4}$ estimated using the IR bolometric and the 250$\,μ$m LFs respectively. Converting our IR LD estimate into an SFRD assuming a standard Salpeter IMF and including the unobscured contribution based on the UV dust-uncorrected emission from local galaxies, we estimate a SFRD scaling of SFRD$_0+0.08 z$, where SFRD$_0\simeq (1.9\pm 0.03)\times 10^{-2} [\mathrm{M}_\odot\,\mathrm{Mpc}^{-3}]$ is our total SFRD estimate at $z\sim0.02$.

astro-ph.GA↗

arXiv2015

Macro deformation twins in single-crystal aluminum

Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond nanotwins. Here, we present the first experimental demonstration of macro deformation twins in single-crystal aluminum formed under ultrahigh strain-rate ($\sim$10$^6$ s$^{-1}$), large shear strain (200$\%$) via dynamic equal channel angular pressing. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates.

cond-mat.mtrl-sci↗

arXiv2015

The Pan-Pacific Planet Search III: Five companions orbiting giant stars

We report a new giant planet orbiting the K giant HD 155233, as well as four stellar-mass companions from the Pan-Pacific Planet Search, a southern hemisphere radial velocity survey for planets orbiting nearby giants and subgiants. We also present updated velocities and a refined orbit for HD 47205b (7 CMa b), the first planet discovered by this survey. HD 155233b has a period of 885$\pm$63 days, eccentricity e=0.03$\pm$0.20, and m sin i=2.0$\pm$0.5 M_jup. The stellar-mass companions range in m sin i from 0.066 M_sun to 0.33 M_sun. Whilst HD 104358B falls slightly below the traditional 0.08 M_sun hydrogen-burning mass limit, and is hence a brown dwarf candidate, we estimate only a 50% a priori probability of a truly substellar mass.

astro-ph.EP↗

arXiv2015

Stratospheric Imaging of Polar Mesospheric Clouds: A New Window on Small-Scale Atmospheric Dynamics

Instabilities and turbulence extending to the smallest dynamical scales play important roles in the deposition of energy and momentum by gravity waves throughout the atmosphere. However, these dynamics and their effects have been impossible to quantify to date due to lack of observational guidance. Serendipitous optical images of polar mesospheric clouds at ~82 km obtained by star cameras aboard a cosmology experiment deployed on a stratospheric balloon provide a new observational tool, revealing instability and turbulence structures extending to spatial scales < 20 m. At 82 km, this resolution provides sensitivity extending to the smallest turbulence scale not strongly influenced by viscosity: the "inner scale" of turbulence, $l_0\sim$10($ν^3$/$ε$)$^{1/4}$. Such images represent a new window into small-scale dynamics that occur throughout the atmosphere but are impossible to observe in such detail at any other altitude. We present a sample of images revealing a range of dynamics features, and employ numerical simulations that resolve these dynamics to guide our interpretation of several observed events.

astro-ph.IM↗

arXiv2015

Spin orbit coupling controlled spin pumping effect

Effective spin mixing conductance (ESMC) across the nonmagnetic metal (NM)/ferromagnet interface, spin Hall conductivity (SHC) and spin diffusion length (SDL) in the NM layer govern the functionality and performance of pure spin current devices with spin pumping technique. We show that all three parameters can be tuned significantly by the spin orbit coupling (SOC) strength of the NM layer in systems consisting of ferromagnetic insulating Y3Fe5O12 layer and metallic Pd1-xPtx layer. Surprisingly, the ESMC is observed to increase significantly with x changing from 0 to 1.0. The SHC in PdPt alloys, dominated by the intrinsic term, is enhanced notably with increasing x. Meanwhile, the SDL is found to decrease when Pd atoms are replaced by heavier Pt atoms, validating the SOC induced spin flip scattering model in polyvalent PdPt alloys. The capabilities of both spin current generation and spin charge conversion are largely heightened via the SOC. These findings highlight the multifold tuning effects of the SOC in developing the new generation of spintronic devices.

cond-mat.mtrl-sci↗

arXiv2015

Spectral Models for Early Time SN 2011fe Observations

We use observed UV through near IR spectra to examine whether SN 2011fe can be understood in the framework of Branch-normal SNe Ia and to examine its individual peculiarities. As a benchmark, we use a delayed-detonation model with a progenitor metallicity of Z_solar/20. We study the sensitivity of features to variations in progenitor metallicity, the outer density profile, and the distribution of radioactive nickel. The effect of metallicity variations in the progenitor have a relatively small effect on the synthetic spectra. We also find that the abundance stratification of SN 2011fe resembles closely that of a delayed detonation model with a transition density that has been fit to other Branch-normal Type Ia supernovae. At early times, the model photosphere is formed in material with velocities that are too high, indicating that the photosphere recedes too slowly or that SN 2011fe has a lower specific energy in the outer ~0.1 M_sun than does the model. We discuss several explanations for the discrepancies. Finally, we examine variations in both the spectral energy distribution and in the colors due to variations in the progenitor metallicity, which suggests that colors are only weak indicators for the progenitor metallicity, in the particular explosion model that we have studied. We do find that the flux in the U band is significantly higher at maximum light in the solar metallicity model than in the lower metallicity model and the lower metallicity model much better matches the observed spectrum.

astro-ph.SR↗