arXiv subjects

K. C. Wong

Publications and source records attributed to K. C. Wong.

33 records · Page 2Linked to original sources

arXiv2019

COSMOGRAIL XVIII: time delays of the quadruply lensed quasar WFI2033-4723

We present new measurements of the time delays of WFI2033-4723. The data sets used in this work include 14 years of data taken at the 1.2m Leonhard Euler Swiss telescope, 13 years of data from the SMARTS 1.3m telescope at Las Campanas Observatory and a single year of high-cadence and high-precision monitoring at the MPIA 2.2m telescope. The time delays measured from these different data sets, all taken in the R-band, are in good agreement with each other and with previous measurements from the literature. Combining all the time-delay estimates from our data sets results in Dt_AB = 36.2-0.8+0.7 days (2.1% precision), Dt_AC = -23.3-1.4+1.2 days (5.6%) and Dt_BC = -59.4-1.3+1.3 days (2.2%). In addition, the close image pair A1-A2 of the lensed quasars can be resolved in the MPIA 2.2m data. We measure a time delay consistent with zero in this pair of images. We also explore the prior distributions of microlensing time-delay potentially affecting the cosmological time-delay measurements of WFI2033-4723. There is however no strong indication in our measurements that microlensing time delay is neither present nor absent. This work is part of a H0LiCOW series focusing on measuring the Hubble constant from WFI2033-4723.

astro-ph.CO↗

arXiv2017

H0LiCOW VIII. A weak lensing measurement of the external convergence in the field of the lensed quasar HE$\,$0435$-$1223

We present a weak gravitational lensing measurement of the external convergence along the line of sight to the quadruply lensed quasar HE$\,$0435$-$1223. Using deep r-band images from Subaru-Suprime-Cam we observe galaxies down to a 3$σ$ limiting magnitude of $\sim 26$ mags resulting in a source galaxy density of 14 galaxies / arcmin$^2$ after redshift-based cuts. Using an inpainting technique and Multi-Scale Entropy filtering algorithm, we find that the region in close proximity to the lens has an estimated external convergence of $κ=-0.012^{+0.020}_{-0.013}$ and is hence marginally under-dense. We also rule out the presence of any halo with a mass greater than $M_{\rm vir}=1.6\times10^{14}h^{-1}M_\odot$ (68$\%$ confidence limit). Our results, consistent with previous studies of this lens, confirm that the intervening mass along the line of sight to HE$\,$0435$-$1223 does not affect significantly the cosmological results inferred from the time delay measurements of that specific object.

astro-ph.CO↗

arXiv2017

H0LiCOW V. New COSMOGRAIL time delays of HE0435-1223: $H_0$ to 3.8% precision from strong lensing in a flat $Λ$CDM model

We present a new measurement of the Hubble Constant H0 and other cosmological parameters based on the joint analysis of three multiply-imaged quasar systems with measured gravitational time delays. First, we measure the time delay of HE0435-1223 from 13-year light curves obtained as part of the COSMOGRAIL project. Companion papers detail the modeling of the main deflectors and line of sight effects, and how these data are combined to determine the time-delay distance of HE 0435-1223. Crucially, the measurements are carried out blindly with respect to cosmological parameters in order to avoid confirmation bias. We then combine the time-delay distance of HE0435-1223 with previous measurements from systems B1608+656 and RXJ1131-1231 to create a Time Delay Strong Lensing probe (TDSL). In flat $Λ$CDM with free matter and energy density, we find $H_0$ = 71.9 +2.4 -3.0 km/s/Mpc and $Ω_Λ$ = 0.62 +0.24 -0.35 . This measurement is completely independent of, and in agreement with, the local distance ladder measurements of H0. We explore more general cosmological models combining TDSL with other probes, illustrating its power to break degeneracies inherent to other methods. The TDSL and Planck joint constraints are $H_0$ = 69.2 +1.4 -2.2 km/s/Mpc, $Ω_Λ$ = 0.70 +0.01 -0.01 and $Ω_k$ = 0.003 +0.004 -0.006 in open $Λ$CDM and $H_0$ = 79.0 +4.4 -4.2 km/s/Mpc, $Ω_{de}$ = 0.77 +0.02 -0.03 and $w$ = -1.38 +0.14 -0.16 in flat $w$CDM. Combined with Planck and Baryon Acoustic Oscillation data, when relaxing the constraints on the numbers of relativistic species we find $N_{eff}$ = 3.34 +0.21 -0.21 and when relaxing the total mass of neutrinos we find 0.182 eV. In an open $w$CDM in combination with Planck and CMB lensing we find $H_0$ = 77.9 +5.0 -4.2 km/s/Mpc, $Ω_{de}$ = 0.77 +0.03 -0.03, $Ω_k$ = -0.003 +0.004 -0.004 and $w$ = -1.37 +0.18 -0.23.

astro-ph.CO↗

arXiv2017

H0LiCOW II. Spectroscopic survey and galaxy-group identification of the strong gravitational lens system HE0435-1223

Galaxies located in the environment or on the line of sight towards gravitational lenses can significantly affect lensing observables, and can lead to systematic errors on the measurement of $H_0$ from the time-delay technique. We present the results of a systematic spectroscopic identification of the galaxies in the field of view of the lensed quasar HE0435-1223, using the W. M. Keck, Gemini and ESO-Very Large telescopes. Our new catalog triples the number of known galaxy redshifts in the vicinity of the lens, expanding to 102 the number of measured redshifts for galaxies separated by less than 3 arcmin from the lens. We complement our catalog with literature data to gather redshifts up to 15 arcmin from the lens, and search for galaxy groups or clusters projected towards HE0435-1223. We confirm that the lens is a member of a small group that includes at least 12 galaxies, and find 8 other group candidates near the line of sight of the lens. The flexion shift, namely the shift of lensed images produced by high order perturbation of the lens potential, is calculated for each galaxy/group and used to identify which objects produce the largest perturbation of the lens potential. This analysis demonstrates that i) at most three of the five brightest galaxies projected within 12 arcsec of the lens need to be explicitly used in the lens models, and ii) the groups can be treated in the lens model as an external tidal field (shear) contribution.

astro-ph.CO↗

arXiv2017

H0LiCOW I. $H_0$ Lenses in COSMOGRAIL's Wellspring: Program Overview

Strong gravitational lens systems with time delays between the multiple images allow measurements of time-delay distances, which are primarily sensitive to the Hubble constant that is key to probing dark energy, neutrino physics, and the spatial curvature of the Universe, as well as discovering new physics. We present H0LiCOW ($H_0$ Lenses in COSMOGRAIL's Wellspring), a program that aims to measure $H_0$ with $<3.5\%$ uncertainty from five lens systems (B1608+656, RXJ1131-1231, HE0435-1223, WFI2033-4723 and HE1104-1805). We have been acquiring (1) time delays through COSMOGRAIL and Very Large Array monitoring, (2) high-resolution Hubble Space Telescope imaging for the lens mass modeling, (3) wide-field imaging and spectroscopy to characterize the lens environment, and (4) moderate-resolution spectroscopy to obtain the stellar velocity dispersion of the lenses for mass modeling. In cosmological models with one-parameter extension to flat $Λ$CDM, we expect to measure $H_0$ to $<3.5\%$ in most models, spatial curvature $Ω_{\rm k}$ to 0.004, $w$ to 0.14, and the effective number of neutrino species to 0.2 (1$σ$ uncertainties) when combined with current CMB experiments. These are, respectively, a factor of $\sim15$, $\sim2$, and $\sim1.5$ tighter than CMB alone. Our data set will further enable us to study the stellar initial mass function of the lens galaxies, and the co-evolution of supermassive black holes and their host galaxies. This program will provide a foundation for extracting cosmological distances from the hundreds of time-delay lenses that are expected to be discovered in current and future surveys.

astro-ph.CO↗

arXiv2014

Rubbery layer on polystyrene surface well below the glassy temperature

The dynamics of polymer surfaces is a controversial subject. Many experimental evidences support the existence of a surface mobile layer, while there are a few equally strong experimental evidences that contradict it. Here we show, through careful studies of the topological evolution of nanometer-size ridges and ditches on rubbed polystyrene surfaces, that there is indeed a surface rubbery layer whose mobility decreases rapidly with depth. The segments in the layer can only be displaced by a finite distance comparable to the depth. The temperature and depth dependence of the surface effective compliance is quantitatively determined. At tens of degrees below the bulk glass transition temperature the thickness of the surface rubbery layer is only a fraction of nanometer while the rest of the polymer sample remains frozen. Such surface mobile layer can qualitatively explain the seemingly contradictory behaviors of polymer surfaces and resolve most of the controversy among the results obtained from different experimental approaches.

cond-mat.mtrl-sci↗

arXiv2013

Dark energy with gravitational lens time delays

Strong lensing gravitational time delays are a powerful and cost effective probe of dark energy. Recent studies have shown that a single lens can provide a distance measurement with 6-7 % accuracy (including random and systematic uncertainties), provided sufficient data are available to determine the time delay and reconstruct the gravitational potential of the deflector. Gravitational-time delays are a low redshift (z~0-2) probe and thus allow one to break degeneracies in the interpretation of data from higher-redshift probes like the cosmic microwave background in terms of the dark energy equation of state. Current studies are limited by the size of the sample of known lensed quasars, but this situation is about to change. Even in this decade, wide field imaging surveys are likely to discover thousands of lensed quasars, enabling the targeted study of ~100 of these systems and resulting in substantial gains in the dark energy figure of merit. In the next decade, a further order of magnitude improvement will be possible with the 10000 systems expected to be detected and measured with LSST and Euclid. To fully exploit these gains, we identify three priorities. First, support for the development of software required for the analysis of the data. Second, in this decade, small robotic telescopes (1-4m in diameter) dedicated to monitoring of lensed quasars will transform the field by delivering accurate time delays for ~100 systems. Third, in the 2020's, LSST will deliver 1000's of time delays; the bottleneck will instead be the aquisition and analysis of high resolution imaging follow-up. Thus, the top priority for the next decade is to support fast high resolution imaging capabilities, such as those enabled by the James Webb Space Telescope and next generation adaptive optics systems on large ground based telescopes.

astro-ph.CO↗

arXiv2012

Weyl fluid dark matter model tested on the galactic scale by weak gravitational lensing

The higher dimensional Weyl curvature induces on the brane a new source of gravity. This Weyl fluid of geometrical origin (reducing in the spherically symmetric, static configuration to a dark radiation and dark pressure) modifies space-time geometry around galaxies and has been shown to explain the flatness of galactic rotation curves. Independent observations for discerning between the Weyl fluid and other dark matter models are necessary. Gravitational lensing could provide such a test. Therefore we study null geodesics and weak gravitational lensing in the dark radiation dominated region of galaxies in a class of spherically symmetric brane-world metrics. We find that the lensing profile in the brane-world scenario is distinguishable from dark matter lensing, despite both the brane-world scenario and dark matter models fitting the rotation curve data. In particular, in the asymptotic regions light deflection is 18% enhanced as compared to dark matter halo predictions. For a linear equation of state of the Weyl fluid we further find a critical radius, below which brane-world effects reduce, while above it they amplify light deflection. This is in contrast to any dark matter model, the addition of which always increases the deflection angle.

gr-qc↗

arXiv2011

Inflating wormholes in the braneworld models

The braneworld model, in which our Universe is a three-brane embedded in a five-dimensional bulk, allows the existence of wormholes, without any violation of the energy conditions. A fundamental ingredient of traversable wormholes is the violation of the null energy condition (NEC). However, in the brane world models, the stress energy tensor confined on the brane, threading the wormhole, satisfies the NEC. In conventional general relativity, wormholes existing before inflation can be significantly enlarged by the expanding spacetime. We investigate the evolution of an inflating wormhole in the brane world scenario, in which the wormhole is supported by the nonlocal brane world effects. As a first step in our study we consider the possibility of embedding a four-dimensional brane world wormhole into a five dimensional bulk. The conditions for the embedding are obtained by studying the junction conditions for the wormhole geometry, as well as the full set of the five dimensional bulk field equations. For the description of the inflation we adopt the chaotic inflation model. We study the dynamics of the brane world wormholes during the exponential inflation stage, and in the stage of the oscillating scalar field. A particular exact solution corresponding to a zero redshift wormhole is also obtained. The resulting evolution shows that while the physical and geometrical parameters of a zero redshift wormhole decay naturally, a wormhole satisfying some very general initial conditions could turn into a black hole, and exist forever.

gr-qc↗

arXiv2011

Galaxy Number Counts and Implications for Strong Lensing

We compare galaxy number counts in HST/ACS fields containing moderate-redshift (0.2<z<1.0) strong gravitational lenses with those in two control samples: (1) the first square degree of the COSMOS survey, comprising 259 ACS fields and (2) 20 "pure parallel" fields randomly located on the sky. Through a Bayesian analysis we determine the expectation values (mu_0) and confidence levels of the underlying number counts for a range of apertures and magnitude bins. Our analysis has produced the following results: (i) We infer that our control samples are not consistent, with the number counts in the COSMOS sample being significantly higher than in the pure parallel sample for 21 <= F814W <= 23. This result matches those found in previous analyses of COSMOS data using different techniques. (ii) We find that small-size apertures, centered on strong lenses, are overdense compared with randomly placed apertures in the control samples, even compared to the COSMOS sample. Correcting for the local clustering of elliptical galaxies, based on the average two-point correlation function reduces this overdensity to the 1-2 sigma level. Thus, the overdensity of galaxies seen along a typical line of sight to a lens can be explained mostly by the natural clustering of galaxies, rather than being due to lenses lying along otherwise biased lines of sight. However, a larger sample of lenses is needed to see if the remaining bias persists when the lens-field uncertainties are smaller. (iii) There is considerable scatter in the lines of sight to _individual_ lens systems, but quantities that are linearly dependent on the external convergence (e.g., H_0) should become unbiased if the extra galaxies that cause the bias can be accounted for in the lens models either through direct modeling or via an informed prior on the external convergence. The number counts can used to set such an informed prior.

astro-ph.CO↗

arXiv2010

Inflation and late time acceleration in braneworld cosmological models with varying brane tension

Braneworld models with variable brane tension $λ$ introduce a new degree of freedom that allows for evolving gravitational and cosmological constants, the latter being a natural candidate for dark energy. We consider a thermodynamic interpretation of the varying brane tension models, by showing that the field equations with variable $λ$ can be interpreted as describing matter creation in a cosmological framework. The particle creation rate is determined by the variation rate of the brane tension, as well as by the brane-bulk energy-matter transfer rate. We investigate the effect of a variable brane tension on the cosmological evolution of the Universe, in the framework of a particular model in which the brane tension is an exponentially dependent function of the scale factor. The resulting cosmology shows the presence of an initial inflationary expansion, followed by a decelerating phase, and by a smooth transition towards a late accelerated de Sitter type expansion. The varying brane tension is also responsible for the generation of the matter in the Universe (reheating period). The physical constraints on the model parameters, resulted from the observational cosmological data, are also investigated.

gr-qc↗

arXiv2008

Reheating the Universe in Braneworld Cosmological Models with bulk-brane energy transfer

The emergence of the cosmological composition (the reheating era) after the inflationary period is analyzed in the framework of the braneworld models, in which our Universe is a three-brane embedded in a five-dimensional bulk, by assuming the possibility of the brane-bulk energy exchange. The inflaton field is assumed to decay into normal matter only, while the dark matter is injected into the brane from the bulk. To describe the reheating process we adopt a phenomenological approach, by describing the decay of the inflaton field by a friction term proportional to the energy density of the field. After the radiation dominated epoch the model reduces to the standard four dimensional cosmological model. The modified field equations are analyzed analytically and numerically in both the extra-dimensions dominate reheating phase (when the quadratic terms in energy density dominate the dynamics), and in the general case. The evolution profiles of the matter, of the scalar field and of the scale factor of the universe are obtained for different values of the parameters of the model, and of the equations of state of the normal and dark matter, respectively. The equation describing the time evolution of the ratio of the energy density of the dark and of the normal matter is also obtained. The ratio depends on the rate of the energy flow between the bulk and the brane. The observational constraint of an approximately constant ratio of the dark and of the baryonic matter requires that the dark matter must be non-relativistic (cold). The model predicts a reheating temperature of the order of $3\times 10^6$ GeV, a brane tension of the order of $10^{25}$ GeV$^4$, and the obtained composition of the universe is consistent with the observational data.

gr-qc↗

arXiv2007

Lens Galaxy Properties of SBS1520+530: Insights from Keck Spectroscopy and AO Imaging

We report on an investigation of the SBS 1520+530 gravitational lens system and its environment using archival HST imaging, Keck spectroscopic data, and Keck adaptive-optics imaging. The AO imaging has allowed us to fix the lens galaxy properties with a high degree of precision when performing the lens modeling, and the data indicate that the lens has an elliptical morphology and perhaps a disk. The new spectroscopic data suggest that previous determinations of the lens redshift may be incorrect, and we report an updated, though inconclusive, value z_lens = 0.761. We have also spectroscopically confirmed the existence of several galaxy groups at approximately the redshift of the lens system. We create new models of the lens system that explicitly account for the environment of the lens, and we also include improved constraints on the lensing galaxy from our adaptive-optics imaging. Lens models created with these new data can be well-fit with a steeper than isothermal mass slope (alpha = 2.29, with the density proportional to r^-alpha) if H_0 is fixed at 72 km/s/Mpc; isothermal models require H_0 ~ 50 km/s/Mpc. The steepened profile may indicate that the lens is in a transient perturbed state caused by interactions with a nearby galaxy.

astro-ph↗

arXiv1999

Nonequilibrium ac Josephson effect in mesoscopic Nb-InAs-Nb junctions

Microwave irradiation of Nb-InAs-Nb junctions reveals frequency-doubled Josephson currents which persist to high temperatures, in the absence of a critical current. A nonequilibrium dynamical model, based on time-dependent Andreev bound states, successfully accounts for the resulting half-integer Shapiro step and an enhancement in the conductance near zero bias.

cond-mat.supr-con↗

arXiv1998

Current-voltage characteristics of semiconductor-coupled superconducting weak links with large electrode separations

We have studied the current-voltage characteristics of superconducting weak links in which the coupling medium is the 2-D electron gas in InAs-based semiconductor quantum wells, with relatively large (typically 0.5 micron) separations between niobium electrodes. The devices exhibit Josephson-like current-voltage characteristics; however, the falloff of the differential resistance with decreasing temperature is thermally activated, and is orders of magnitude slower than for more conventional weak links. Most unexpectedly, the thermal activation energies are found to be proportional to the width of the device, taken perpendicular to the current flow. This behavior falls outside the range of established theories; we propose that it is a fluctuation effect caused by giant shot noise associated with multiple Andreev reflections. The possibility of non-equilibrium effects is discussed.

cond-mat.supr-con↗