Beware the Nonuniqueness of Einstein Rings

Saha, Prasenjit; Williams, Liliya L. R.

doi:10.1086/321151

Cited by 28 publications

(31 citation statements)

References 16 publications

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“…Saha & Williams 2004;Diego et al 2005;Liesenborgs et al 2006;Coe et al 2008) fit the lensing observables perfectly and derive ensembles of models reproducing existing data for sets of individual multiplyimaged sources. These techniques effectively explore degeneracies between lens models (Saha & Williams 2001, 2006, but with the drawback that many non-physical models (e.g., dynamically unstable or with arbitrary substructure) can be obtained. As emphasized by Coe et al (2008), there is no unambiguous set of criteria to define a priori whether a model is physical or not.…”

Section: Discussionmentioning

confidence: 99%

Source-position transformation: an approximate invariance in strong gravitational lensing

Schneider

Sluse

2014

A&A

149

146

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The main obstacle that gravitational lensing has in determining accurate masses of deflectors, or in determining precise estimates for the Hubble constant, is the degeneracy of lensing observables with respect to the mass-sheet transformation (MST). The MST is a global modification of the mass distribution which leaves all image positions, shapes, and flux ratios invariant, but which changes the time delay. Here we show that another global transformation of lensing mass distributions exists which leaves image positions and flux ratios almost invariant, and of which the MST is a special case. As is the case for the MST, this new transformation only applies if one considers only those source components that are at the same distance from us. Whereas for axi-symmetric lenses this source position transformation exactly reproduces all strong lensing observables, it does so only approximately for more general lens situations. We provide crude estimates for the accuracy with which the transformed mass distribution can reproduce the same image positions as the original lens model, and present an illustrative example of its performance. This new invariance transformation is most likely the reason why the same strong lensing information can be accounted for with rather different mass models.

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Section: Discussionmentioning

confidence: 99%

Source-position transformation: an approximate invariance in strong gravitational lensing

Schneider

Sluse

2014

A&A

149

146

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“…The first is to use a non-parametric model for the projected mass distribution, imposing only a minimum number of physically-motivated requirements such as monotonicity, and thereby generate large numbers of mass models which are exactly consistent with the data. This was pioneered by Saha and Williams in a series of papers [179, 237, 180, 177] in which pixellated models of galaxy mass distributions were used. Although pixellated models are useful for exploring the space of allowed models, they do not break the essential degeneracy.…”

Section: One-step Distance Methodsmentioning

confidence: 99%

The Hubble Constant

Jackson

2015

Living Rev Relativ

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I review the current state of determinations of the Hubble constant, which gives the length scale of the Universe by relating the expansion velocity of objects to their distance. There are two broad categories of measurements. The first uses individual astrophysical objects which have some property that allows their intrinsic luminosity or size to be determined, or allows the determination of their distance by geometric means. The second category comprises the use of all-sky cosmic microwave background, or correlations between large samples of galaxies, to determine information about the geometry of the Universe and hence the Hubble constant, typically in a combination with other cosmological parameters. Many, but not all, object-based measurements give H0 values of around 72–74 km s−1 Mpc−1, with typical errors of 2–3 km s−1 Mpc−1. This is in mild discrepancy with CMB-based measurements, in particular those from the Planck satellite, which give values of 67–68 km s−1 Mpc−1 and typical errors of 1–2 km s−1 Mpc−1. The size of the remaining systematics indicate that accuracy rather than precision is the remaining problem in a good determination of the Hubble constant. Whether a discrepancy exists, and whether new physics is needed to resolve it, depends on details of the systematics of the object-based methods, and also on the assumptions about other cosmological parameters and which datasets are combined in the case of the all-sky methods.

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“…The plot of arrival‐time contours also provides a simple model for the ring‐like extended image (cf. Saha & Williams 2001) which in this case arises from the extended source A . Detailed comparison would, however, be very difficult because the ring is faint and in many places overlaid with the light from the ellipticals.…”

Section: Mass Reconstructionsmentioning

confidence: 99%

Light/mass offsets in the lensing cluster Abell 3827: evidence for collisional dark matter?

Williams

Saha

2011

Monthly Notices of the Royal Astronomical Society

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If dark matter has a non‐zero self‐interaction cross‐section, then dark matter haloes of individual galaxies in cluster cores should experience a drag force from the ambient dark matter of the cluster, which will not affect the stellar components of galaxies, and thus will lead to a separation between the stellar and dark matter. If the cross‐section is only a few decades below its current astrophysically determined upper limit, then kpc‐scale separations should result. However, such separations will be observable only under very favourable conditions. Abell 3827 is a nearby late stage cluster merger with four massive central ellipticals within 20 kpc of each other. The 10 strong lensing images tightly surrounding the ellipticals provide an excellent set of constraints for a free‐form lens reconstruction. Our free‐form mass maps show a massive dark extended clump, about 6 kpc from one of the ellipticals. The robustness of this result has been tested with many reconstructions, and confirmed with experiments using synthetic lens mass distributions. Interpreted in terms of dark matter collisionality, our result yields σ/m≳ 4.5 10−7 (t/1010 yr)−2 cm2 g−1, where t is the merger’s age.

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Beware the Nonuniqueness of Einstein Rings

Cited by 28 publications

References 16 publications

Source-position transformation: an approximate invariance in strong gravitational lensing

Source-position transformation: an approximate invariance in strong gravitational lensing

The Hubble Constant

Light/mass offsets in the lensing cluster Abell 3827: evidence for collisional dark matter?

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