Model-free analysis of quadruply imaged gravitationally lensed systems and substructured galaxies

Woldesenbet, Addishiwot G.; Williams, Liliya L. R.

doi:10.1093/mnras/stv1960

Cited by 11 publications

(16 citation statements)

References 33 publications

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“…Rather than thinking of external shear as a physical observable quantity, perhaps it makes most sense to instead think of it as a simple first order fitting parameter that represents information from several unknown effects. Whatever the case, Woldesenbet & Williams (2015) document the effects of external shear on the distribution of quads relative to the FSQ. Since this shear can provide deviations from the FSQ, we can experiment with nonzero values of shear and see if this is able to better match observations.…”

Section: Nonzero External Shearmentioning

confidence: 99%

The impact of ΛCDM substructure and baryon-dark matter transition on the image positions of quad galaxy lenses

Gomer

Williams

2017

Monthly Notices of the Royal Astronomical Society

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The positions of multiple images in galaxy lenses are related to the galaxy mass distribution. Smooth elliptical mass profiles were previously shown to be inadequate in reproducing the quad population. In this paper, we explore the deviations from such smooth elliptical mass distributions. Unlike most other work, we use a model-free approach based on the relative polar image angles of quads, and their position in 3D space with respect to the Fundamental Surface of Quads. The FSQ is defined by quads produced by elliptical lenses. We have generated thousands of quads from synthetic populations of lenses with substructure consistent with ΛCDM simulations, and found that such perturbations are not sufficient to match the observed distribution of quads relative to the FSQ. The result is unchanged even when subhalo masses are increased by a factor of ten, and the most optimistic lensing selection bias is applied. We then produce quads from galaxies created using two components, representing baryons and dark matter. The transition from the mass being dominated by baryons in inner radii to being dominated by dark matter in outer radii can carry with it asymmetries, which would affect relative image angles. We run preliminary experiments using lenses with two elliptical mass components with nonidentical axis ratios and position angles, perturbations from ellipticity in the form of nonzero Fourier coefficients a 4 and a 6 , and artificially offset ellipse centers as a proxy for asymmetry at image radii. We show that combination of these effects is a promising way of accounting for quad population properties. We conclude that the quad population provides a unique and sensitive tool for constraining detailed mass distribution in the centers of galaxies.

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Section: Nonzero External Shearmentioning

confidence: 99%

The impact of ΛCDM substructure and baryon-dark matter transition on the image positions of quad galaxy lenses

Gomer

Williams

2017

Monthly Notices of the Royal Astronomical Society

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“…Motivated to make a comparison to observables, we return to our model-free statistical measures of a population of quads: the radial position of images relative to the outermost image and the deviation from the FSQ resulting from the angular distribution of images. Woldesenbet & Williams (2015) showed that deviations from the FSQ for a single system can be caused by external shear or by deviations to the elliptical shape (Gomer & Williams 2018), while we have shown that shear can also reproduce the distribution or radial ratios. If shear were responsible for both quantities, then the innermost radial ratio and the deviation from the FSQ should be correlated.…”

Section: Role Of Shearmentioning

confidence: 61%

“…Meanwhile, the distribution of observed quads has significant spread from the FSQ. External shear causes the distribution to split above and below the FSQ, but is insufficient to account for the observed distribution (Woldesenbet & Williams 2015). However, the perturbations to the potential we use in our various tests can reproduce the observed distribution (Gomer & Williams 2018).…”

Section: Comparison With Observed Quadsmentioning

confidence: 87%

“…The observed galaxy-scale quad population has significant deviation from this surface, confirming that real lenses are not simply elliptical mass distributions. Even more interestingly, Woldesenbet & Williams (2015) then showed that the addition of external shear was insufficient to bridge the gap, and that while many individual quads can be described as ellipse+shear, the population of quads cannot be reproduced-it must come, at least in part, from lenses with more complicated mass distributions. Gomer & Williams (2018) expanded on this analysis, finding that the observed population cannot be accounted for by including ΛCDM substructure, even if the mass of each clump is increased by a factor of 10.…”

Section: Ellipse+shear Assumptionmentioning

confidence: 99%

“…Rather than comparing properties recovered from modeling, such as ellipticity or shear, we would like to compare quad populations independent of the modeling process. As such, we look at statistical distributions of image properties, namely the distribution of relative image angles relative to the FSQ (Woldesenbet & Williams 2015) and the radial distance ratios. Any set of mock quads which seeks to represent a real population should at the minimum match the statistical properties of the observed population of quads.…”

Section: Comparison With Observed Quadsmentioning

confidence: 99%

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Galaxy-lens determination of H0: the effect of the ellipse + shear modelling assumption

Gomer

Williams

2021

Monthly Notices of the Royal Astronomical Society

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Galaxy lenses are frequently modeled as an elliptical mass distribution with external shear and isothermal spheres to account for secondary and line-of-sight galaxies. There is statistical evidence that some fraction of observed quads are inconsistent with these assumptions, and require a dipole-like contribution to the mass with respect to the light. Simplifying assumptions about the shape of mass distributions can lead to the incorrect recovery of parameters such as H0. We create several tests of synthetic quad populations with different deviations from an elliptical shape, then fit them with an ellipse+shear model, and measure the recovered values of H0. Kinematic constraints are not included. We perform two types of fittings- one with a single point source and one with an array of sources emulating an extended source. We carry out two model-free comparisons between our mock quads and the observed population. One result of these comparisons is a statistical inconsistency not yet mentioned in the literature: the image distance ratios with respect to the lens center of observed quads appear to span a much wider range than those of synthetic or simulated quads. Bearing this discrepancy in mind, our mock populations can result in biases on H0 $\sim 10{{\ \rm per\ cent}}$.

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Essentials of Strong Gravitational Lensing

Saha,

Sluse,

Wagner

et al. 2024

Space Sci Rev

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Of order one in $10^{3}$ 10 3 quasars and high-redshift galaxies appears in the sky as multiple images as a result of gravitational lensing by unrelated galaxies and clusters that happen to be in the foreground. While the basic phenomenon is a straightforward consequence of general relativity, there are many non-obvious consequences that make multiple-image lensing systems (aka strong gravitational lenses) remarkable astrophysical probes in several different ways. This article is an introduction to the essential concepts and terminology in this area, emphasizing physical insight. The key construct is the Fermat potential or arrival-time surface: from it the standard lens equation, and the notions of image parities, magnification, critical curves, caustics, and degeneracies all follow. The advantages and limitations of the usual simplifying assumptions (geometrical optics, small angles, weak fields, thin lenses) are noted, and to the extent possible briefly, it is explained how to go beyond these. Some less well-known ideas are discussed at length: arguments using wavefronts show that much of the theory carries over unchanged to the regime of strong gravitational fields; saddle-point contours explain how even the most complicated image configurations are made up of just two ingredients. Orders of magnitude, and the question of why strong lensing is most common for objects at cosmological distance, are also discussed. The challenges of lens modeling, and diverse strategies developed to overcome them, are discussed in general terms, without many technical details.

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Model-free analysis of quadruply imaged gravitationally lensed systems and substructured galaxies

Cited by 11 publications

References 33 publications

The impact of ΛCDM substructure and baryon-dark matter transition on the image positions of quad galaxy lenses

The impact of ΛCDM substructure and baryon-dark matter transition on the image positions of quad galaxy lenses

Galaxy-lens determination of H0: the effect of the ellipse + shear modelling assumption

Essentials of Strong Gravitational Lensing

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