arXiv subjects

Xinzhong Er

Publications and source records attributed to Xinzhong Er.

51 records · Page 3Linked to original sources

arXiv2017

Cosmic tidal reconstruction

The gravitational coupling of a long-wavelength tidal field with small-scale density fluctuations leads to anisotropic distortions of the locally measured small-scale matter correlation function. Since the local correlation function is known to be statistically isotropic in the absence of such tidal interactions, the tidal distortions can be used to reconstruct the long-wavelength tidal field and large-scale density field in analogy with the cosmic microwave background lensing reconstruction. In this paper we present the theoretical framework of cosmic tidal reconstruction and test the reconstruction in numerical simulations. We find that the density field on large scales can be reconstructed with good accuracy and the cross-correlation coefficient between the reconstructed density field and the original density field is greater than 0.9 on large scales $(k\lesssim0.1\ h/\mathrm{Mpc})$, with the filter scale $\sim1.25\ \mathrm{Mpc}/h$. This is useful in the 21cm intensity mapping survey, where the long-wavelength radial modes are lost due to a foreground subtraction process.

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arXiv2017

Two families of astrophysical diverging lens models

In the standard gravitational lensing scenario, rays from a background source are bent in the direction of a foreground lensing mass distribution. Diverging lens behaviour produces deflections in the opposite sense to gravitational lensing, and is also of astrophysical interest. In fact, diverging lensing due to compact distributions of plasma has been proposed as an explanation for the extreme scattering events (ESEs) that produce frequency-dependent dimming of extra-galactic radio sources, and may also be related to the refractive radio-wave phenomena observed to affect the flux density of pulsars. In this work we study the behaviour of two families of astrophysical diverging lenses in the geometric optics limit, the power-law and the exponential plasma lenses. Generally, the members of these model families show distinct behaviour in terms of image formation and magnification, however the inclusion of a finite core for certain power-law lenses can produce a caustic and critical curve morphology that is similar to the well-studied Gaussian plasma lens. Both model families can produce dual radial critical curves, a novel distinction from the tangential distortion usually produced by gravitational (converging) lenses. The deflection angle and magnification of a plasma lens varies with the observational frequency, producing wavelength-dependent magnifications that alter the amplitudes and the shape of the light curves. Thus, multi-wavelength observations can be used to physically constrain the distribution of the electron density in such lenses.

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arXiv2016

Improving lensing cluster mass estimate with flexion

Gravitational lensing has long been considered as a valuable tool to determine the total mass of galaxy clusters. The shear profile as inferred from the statistics of ellipticity of background galaxies allows to probe the cluster intermediate and outer regions thus determining the virial mass estimate. However, the mass sheet degeneracy and the need for a large number of background galaxies motivate the search for alternative tracers which can break the degeneracy among model parameters and hence improve the accuracy of the mass estimate. Lensing flexion, i.e. the third derivative of the lensing potential, has been suggested as a good answer to the above quest since it probes the details of the mass profile. We investigate here whether this is indeed the case considering jointly using weak lensing, magnification and flexion. We use a Fisher matrix analysis to forecast the relative improvement in the mass accuracy for different assumptions on the shear and flexion signal - to - noise (S/N) ratio also varying the cluster mass, redshift, and ellipticity. It turns out that the error on the cluster mass may be reduced up to a factor 2 for reasonable values of the flexion S/N ratio. As a general result, we get that the improvement in mass accuracy is larger for more flattened haloes, but extracting general trends is a difficult because of the many parameters at play. We nevertheless find that flexion is as efficient as magnification to increase the accuracy in both mass and concentration determination.

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arXiv2014

Measurement of differential magnification

In gravitational lensing, the magnification effect changes the luminosity and size of a background galaxy. If the image sizes are not small compared to the scale over which the magnification and shear vary, higher-order distortions occur which are termed differential magnification. We give an approximation of the magnification gradient for several halo models. Assuming a symmetric distribution of source brightness, estimates for the differential magnification are obtained and then tested with simulations. One of the main uncertainties of our estimators comes from the finite resolution of the image. We study the strength of our method with the resolution of current and future telescopes. We point out that out method is a potential approach to estimate the first flexion, and can be used to study galaxy and cluster mass profiles.

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arXiv2013

Effects of plasma on gravitational lensing

We study gravitational lensing when plasma surrounds the lens. An extra deflection angle is induced by the plasma in addition to the deflection generated by gravity. An inhomogeneous plasma distribution generates a greater effect than a homogeneous one, and may cause significant effects to be detected in low frequency radio observations (a few hundred MHz). In particular, the lensed image positions will be different between optical and radio observations. The change of position due to a plasma can reach a few tens of milli-arcsec, which is readily detectable. One can use the position difference in different frequencies to estimate the density of plasma in the lens. The magnification ratios between multiple images are mainly determined by other properties of the lens, and are only weakly affected by the plasma. More importantly, we find that the strong lensing time delay will be affected by the plasma. Estimation of the Hubble constant from the time delay in low radio frequency observation may be slightly biased due to plasma in the lens. Unfortunately, the ionosphere of the Earth strongly affects low frequency radio observations. Thus our ability to detect the effect depends on how well we are able to calibrate out the ionosphere.

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arXiv2013

Biases in physical parameter estimates through differential lensing magnification

We study the lensing magnification effect on background galaxies. Differential magnification due to different magnifications of different source regions of a galaxy will change the lensed composite spectra. The derived properties of the background galaxies are therefore biased. For simplicity, we model galaxies as a superposition of an axis-symmetric bulge and a face-on disk in order to study the differential magnification effect on the composite spectra. We find that some properties derived from the spectra (e.g., velocity dispersion, star formation rate and metallicity) are modified. Depending on the relative positions of the source and the lens, the inferred results can be either over- or under-estimates of the true values. In general, for an extended source at strong lensing regions with high magnifications, the inferred physical parameters (e.g. metallicity) can be strongly biased. Therefore detailed lens modelling is necessary to obtain the true properties of the lensed galaxies.

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arXiv2013

Gravitational lensing effects on sub-millimetre galaxy counts

We study the effects on the number counts of sub-millimetre galaxies due to gravitational lensing. We explore the effects on the magnification cross section due to halo density profiles, ellipticity and cosmological parameter (the power-spectrum normalisation $σ_8$). We show that the ellipticity does not strongly affect the magnification cross section in gravitational lensing while the halo radial profiles do. Since the baryonic cooling effect is stronger in galaxies than clusters, galactic haloes are more concentrated. In light of this, a new scenario of two halo population model is explored where galaxies are modeled as a singular isothermal sphere profile and clusters as a Navarro, Frenk and White (NFW) profile. We find the transition mass between the two has modest effects on the lensing probability. The cosmological parameter $σ_8$ alters the abundance of haloes and therefore affects our results. Compared with other methods, our model is simpler and more realistic. The conclusions of previous works is confirm that gravitational lensing is a natural explanation for the number count excess at the bright end.

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arXiv2012

Large Size Scatter of Passively Evolving Lensed Galaxies at z~2 in CLASH

In a systematic search over 11 cluster fields from Cluster Lensing And Supernova survey with Hubble (CLASH) we identify ten passively evolving massive galaxies at redshift z~2.We derive the stellar properties of these galaxies using HST WFC3/ACS multiband data, together with Spitzer IRAC observations. We also deduce the optical rest-frame effective radius of these high redshift objects. The derived stellar masses and measured effective radii have been corrected by the lensing magnification factors, which are estimated by simply adopting the spherical NFW model for the foreground cluster lens. The observed near-IR images, obtained by HST WFC3 camera with high spatial resolution and lensed by the foreground clusters, enable us to study the structures of such systems. Nine out of ten galaxies have on average three times smaller effective radius than local ETGs of similar stellar masses, in agreement with previous works at redshift 1.5 < z < 2.5. Combined with literature data for z~2, we find that the mass-normalized effective radius scales with redshift as re/M^0.56 \propto (1 + z)^{-1.13}. We confirm that their size distribution shows a large scatter: from normal size to ~5 times smaller compared to local ETGs with similar stellar masses. The 1-σ scatter σ_{logre} of the size distribution is 0.22 and 0.34 at z~1.6 and z~2.1,respectively.The observed large size scatter has to be carefully taken into account in galaxy evolution model predictions.

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arXiv2012

Gravitational lensing properties of isothermal universal halo profile

N-body simulations predict that dark matter halos with different mass scales are described by a universal model, the Navarro-Frenk-White (NFW) density profiles. As a consequence of baryonic cooling effects, the halos will become more concentrated, and similar to an isothermal sphere over large range in radii ($\sim 300$ $h^{-1}$kpc). The singular isothermal sphere model however has to be truncated artificially at large radii since it extends to infinity. We model a massive galaxy halo as a combination of an isothermal sphere and an NFW density profile. We give an approximation for the mass concentration at different baryon fractions and present exact expressions for the weak lensing shear and flexion for such a halo. We compare the lensing properties with a Singular Isothermal Sphere and NFW profiles. We find that the combined profile can generate higher order lensing signals at small radii and is more efficient in generating strong lensing events. In order to distinguish such a halo profile from the SIS or NFW profiles, one needs to combine strong and weak lensing constraints on small and large radii.

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arXiv2012

Estimation of halo ellipticity using spin-3 flexion

Estimating the ellipticity of dark matter haloes at the galaxy or galaxy-cluster scale can provide important constraints on the formation of galaxies or clusters, as well as on the nature of dark matter. We show in this paper that the spin-3 gravitational flexion can add useful information on the ellipticity of lensing haloes. We introduce a general formalism to decompose fields with arbitrary spin into radial and tangential components. The ratio of the tangential and radial flexion components directly estimates the lens ellipticity. We point out that any centroid offset will significantly bias our estimate, which on the other hand can be used to determine the centre of the lens halo.

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arXiv2011

Measurement of halo properties with weak lensing shear and flexion

We constrain properties of cluster haloes by performing likelihood analysis using lensing shear and flexion data. We test our analysis using two mock cluster haloes: an isothermal ellipsoid (SIE) model and a more realistic elliptical Navarro-Frenk-White (eNFW) model. For both haloes, we find that flexion is more sensitive to the halo ellipticity than shear. The introduction of flexion information significantly improves the constraints on halo ellipticity, orientation and mass. We also point out that there is a degeneracy between the mass and the ellipticity of SIE models in the lensing signal.

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arXiv2011

Estimate of halo ellipticity as a function of radius with flexions

The cold dark matter theory predicts triaxial dark matter haloes. The radial distribution of halo ellipticity depends on baryonic processes and the nature of dark matter particles (collisionless or collisional). Here we show that we can use lensing flexion ratios to measure the halo ellipticity as a function of radius. We introduce a weight function and study the relationship between the first and second order statistics of flexion ratios, both of which can be used to reduce the bias in the estimate of ellipticity. we perform numerical tests for our method, and demonstrate that it can reduce the bias and determine the halo ellipticity as a function of radius. We also point out that the minimum mean flexion ratio can be used to trace the centres of galaxy clusters.

astro-ph.CO↗

arXiv2010

Estimate of dark halo ellipticity by lensing flexion

Aims. The predictions of the ellipticity of the dark matter halos from models of structure formation are notoriously difficult to test with observations. A direct measurement would give important constraints on the formation of galaxies, and its effect on the dark matter distribution in their halos. Here we show that galaxy-galaxy flexion provides a direct and potentially powerful method for determining the ellipticity of (an ensemble of) elliptical lenses. Methods. We decompose the spin-1 flexion into a radial and a tangential component. Using the ratio of tangential-to- radial flexion, which is independent of the radial mass profile, the mass ellipticity can be estimated. Results. An estimator for the ellipticity of the mass distribution is derived and tested with simulations. We show that the estimator is slightly biased. We quantify this bias, and provide a method to reduce it. Furthermore, a parametric fitting of the flexion ratio and orientation provides another estimate for the dark halo ellipticity, which is more accurate for individual lenses Overall, galaxy-galaxy flexion appears as a powerful tool for constraining the ellipticity of mass distributions.

astro-ph.CO↗

arXiv2010

Mass reconstruction by gravitational shear and flexion

Galaxy clusters are considered as excellent probes for cosmology. For that purpose, their mass needs to be measured and their structural properties needs to be understood. We propose a method for galaxy cluster mass reconstruction which combines information from strong lensing, weak lensing shear and flexion. We extend the weak lensing analysis to the inner parts of the cluster and, in particular, improve the resolution of substructure. We use simulations to show that the method recovers the mass density profiles of the cluster. We find that the weak lensing flexion is sensitive to substructure. After combining the flexion data into the joint weak and strong lensing analysis, we can resolve the cluster properties with substructures.

astro-ph.CO↗

arXiv2007

Weak lensing goes bananas: What flexion really measures

In weak gravitational lensing, the image distortion caused by shear measures the projected tidal gravitational field of the deflecting mass distribution. To lowest order, the shear is proportional to the mean image ellipticity. If the image sizes are not small compared to the scale over which the shear varies, higher-order distortions occur, called flexion. For ordinary weak lensing, the observable quantity is not the shear, but the reduced shear, owing to the mass-sheet degeneracy. Likewise, the flexion itself is unobservable. Rather, higher-order image distortions measure the reduced flexion, i.e., derivatives of the reduced shear. We derive the corresponding lens equation in terms of the reduced flexion and calculate the resulting relation between brightness moments of source and image. Assuming an isotropic distribution of source orientations, estimates for the reduced shear and flexion are obtained; these are then tested with simulations. In particular, the presence of flexion affects the determination of the reduced shear. The results of these simulations yield the amount of bias of the estimators, as a function of the shear and flexion. We point out and quantify a fundamental limitation of the flexion formalism, in terms of the product of reduced flexion and source size. If this product increases above the derived threshold, multiple images of the source are formed locally, and the formalism breaks down. Finally, we show how a general (reduced) flexion field can be decomposed into its four components: two of them are due to a shear field, carrying an E- and B-mode in general. The other two components do not correspond to a shear field; they can also be split up into corresponding E- and B-modes.

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