Article (Scientific journals)
Ambiguities in gravitational lens models: the density field from the source position transformation
Unruh, Sandra; Schneider, Peter; Sluse, Dominique
2017In Astronomy and Astrophysics, 601, p. 77-13
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Keywords :
cosmological parameters; gravitational lensing: strong
Abstract :
[en] Strong gravitational lensing is regarded as the most precise technique to measure the mass in the inner region of galaxies or galaxy clusters. In particular, the mass within one Einstein radius can be determined with an accuracy of the order of a few percent or better, depending on the image configuration. For other radii, however, degeneracies exist between galaxy density profiles, precluding an accurate determination of the enclosed mass. The source position transformation (SPT), which includes the well-known mass-sheet transformation (MST) as a special case, describes this degeneracy of the lensing observables in a more general way. In this paper we explore properties of an SPT, removing the MST to leading order, that is we consider degeneracies which have not been described before. The deflection field \ahat(čθ) resulting from an SPT is not curl-free in general, and thus not a deflection that can be obtained from a lensing mass distribution. Starting from a variational principle, we construct lensing potentials that give rise to a deflection field \atilde, which differs from \ahat by less than an observationally motivated upper limit. The corresponding mass distributions from these "valid" SPTs are studied: their radial profiles are modified relative to the original mass distribution in a significant and non-trivial way, and originally axi-symmetric mass distributions can obtain a finite ellipticity. These results indicate a significant effect of the SPT on quantitative analyses of lens systems. We show that the mass inside the Einstein radius of the original mass distribution is conserved by the SPT; hence, as is the case for the MST, the SPT does not affect the mass determination at the Einstein radius. Furthermore, we analyse a degeneracy between two lens models, empirically found previously, and show that this degeneracy can be interpreted as being due to an SPT. Thus, degeneracies between lensing mass distributions are not just a theoretical possibility, but do arise in actual lens modeling.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Unruh, Sandra;  Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121, Bonn, Germany
Schneider, Peter;  Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121, Bonn, Germany
Sluse, Dominique  ;  Université de Liège > Département d'astrophys., géophysique et océanographie (AGO) > Département d'astrophys., géophysique et océanographie (AGO)
Language :
English
Title :
Ambiguities in gravitational lens models: the density field from the source position transformation
Publication date :
01 May 2017
Journal title :
Astronomy and Astrophysics
ISSN :
0004-6361
eISSN :
1432-0746
Publisher :
EDP Sciences, Les Ulis, France
Volume :
601
Pages :
77-13
Peer reviewed :
Peer Reviewed verified by ORBi
Funders :
BELSPO - SPP Politique scientifique - Service Public Fédéral de Programmation Politique scientifique
Available on ORBi :
since 25 May 2017

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