Weak lensing using only galaxy position angles

Whittaker, L.; Brown, Michael L.; Battye, Richard A.

doi:10.1093/mnras/stu1858

Cited by 11 publications

(26 citation statements)

References 24 publications

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“…The terms Fn are defined in Whittaker et al (2014), but in this case, they are functions of the intrinsic ellipticities and depend upon the distribution of errors on obs −γ, which provide an estimate of int and have a contribution from errors on the measurements of obs and the estimate of γ. Let us express estimates of int aŝ…”

Section: B4 Derivation Of Equationsmentioning

confidence: 99%

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Estimating the weak-lensing rotation signal in radio cosmic shear surveys

Thomas

Whittaker

Camera

et al. 2017

Monthly Notices of the Royal Astronomical Society

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Weak lensing has become an increasingly important tool in cosmology and the use of galaxy shapes to measure cosmic shear has become routine. The weak-lensing distortion tensor contains two other effects in addition to the two components of shear: the convergence and rotation. The rotation mode is not measurable using the standard cosmic shear estimators based on galaxy shapes, as there is no information on the original shapes of the images before they were lensed. Due to this, no estimator has been proposed for the rotation mode in cosmological weak-lensing surveys, and the rotation mode has never been constrained. Here, we derive an estimator for this quantity, which is based on the use of radio polarisation measurements of the intrinsic position angles of galaxies. The rotation mode can be sourced by physics beyond ΛCDM, and also offers the chance to perform consistency checks of ΛCDM and of weak-lensing surveys themselves. We present simulations of this estimator and show that, for the pedagogical example of cosmic string spectra, this estimator could detect a signal that is consistent with the constraints from Planck. We examine the connection between the rotation mode and the shear B-modes and thus how this estimator could help control systematics in future radio weak-lensing surveys.

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Section: B4 Derivation Of Equationsmentioning

confidence: 99%

“…The variances and covariance of the mean trigonometric functions are given by (Whittaker et al 2014) …”

Section: B4 Derivation Of Equationsmentioning

confidence: 99%

Estimating the weak-lensing rotation signal in radio cosmic shear surveys

Thomas

Whittaker

Camera

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…where δα int is independent of the true α int , Whittaker et al (2014) show that estimates of α IA remain unbiased. However, estimates of γ IA , obtained by inverting the F1 γ IA function, become biased.…”

Section: Full Angle-only Estimator (Fao)mentioning

confidence: 84%

“…For this discussion we assume that the measurement error, δα int , is independent of the true IPA and distributed symmetrically about zero. If we then make the substitutionα int = α int + δα int , we can write the expectation value of the trigonometric functions of the IPA as (Whittaker et al 2014) cos 2α…”

Section: The Brown and Battye (Bb) Estimatormentioning

confidence: 99%

“…In Section 3 we extend the angle-only estimator, introduced by Whittaker et al (2014), to include IPA information and we also introduce a hybrid method that combines an angle-only estimate of the intrinsic alignment with full ellipticity information. In Section 4 we test the methods using simulations by reconstructing the shear and IA auto and cross-power spectra in three overlapping redshift bins.…”

Section: Introductionmentioning

confidence: 99%

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Separating weak lensing and intrinsic alignments using radio observations

Whittaker¹,

Brown²,

Battye³

2015

Monthly Notices of the Royal Astronomical Society

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We discuss methods for performing weak lensing using radio observations to recover information about the intrinsic structural properties of the source galaxies. Radio surveys provide unique information that can benefit weak lensing studies, such as HI emission, which may be used to construct galaxy velocity maps, and polarized synchrotron radiation; both of which provide information about the unlensed galaxy and can be used to reduce galaxy shape noise and the contribution of intrinsic alignments. Using a proxy for the intrinsic position angle of an observed galaxy, we develop techniques for cleanly separating weak gravitational lensing signals from intrinsic alignment contamination in forthcoming radio surveys. Random errors on the intrinsic orientation estimates introduce biases into the shear and intrinsic alignment estimates. However, we show that these biases can be corrected for if the error distribution is accurately known. We demonstrate our methods using simulations, where we reconstruct the shear and intrinsic alignment auto and cross-power spectra in three overlapping redshift bins. We find that the intrinsic position angle information can be used to successfully reconstruct both the lensing and intrinsic alignment power spectra with negligible residual bias.

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Image rotation from lensing

Francfort

Cusin

Durrer

2021

Class. Quantum Grav.

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Forthcoming radio surveys will include full polarisation information, which can be potentially useful for weak lensing observations. We propose a new method to measure the (integrated) gravitational field between a source and the observer, by looking at the angle between the morphology of a radio galaxy and the orientation of the polarisation. For this we use the fact that, while the polarisation of a photon is parallel transported along the photon geodesic, the infinitesimal shape of the source, e.g. its principal axis in the case of an ellipse, is Lie transported as described by the lens map. While at second order, the lens map usually contains a rotation, here we show that the presence of shear alone already induces an apparent rotation of the shape of an elliptical galaxy. As an example, we calculate the rotation of the shape vector with respect to the polarisation direction which is generated by a distribution of foreground Schwarzschild lenses. For radio galaxies, the intrinsic morphological orientation of a source and its polarised emission are correlated. It follows that observing both the polarisation and the morphological orientation provides information on both the unlensed source orientation and on the gravitational potential along the line of sight.

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Weak lensing using only galaxy position angles

Cited by 11 publications

References 24 publications

Estimating the weak-lensing rotation signal in radio cosmic shear surveys

Estimating the weak-lensing rotation signal in radio cosmic shear surveys

Separating weak lensing and intrinsic alignments using radio observations

Image rotation from lensing

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