Shear with Radio Galaxy
Polarization & Kinematics

Cail Daley

Nice TOSCA Meeting

November 8th, 2024

Outline

  • Motivation

  • Kinematic Lensing (KL)

  • Radio Galaxy Polarization

  • Relevance to TOSCA

Motivation

In cosmic shear analyses, intrinsic galaxy shape is unknown.

  • Shape noise: source of variance

    • need lots of galaxies to beat down noise

  • Intrinsic alignments: source of bias

Estimates of intrinsic shape can mitigate both!

  • Kinematics: intrinsic position angle / ellipticity / symmetries

  • Polarization: intrinsic position angle

Kinematic Lensing (KL)

Intuitive Picture

Hopp & Witmann (2024)

A Bit of History

  • Blain et al. (2002): first proposal of KL
    • ideal instruments predicted to be ALMA & SKA!

  • Morales (2006): proposed KL with HI surveys
    • relative to continuum surveys:
      • line-to-continuum, HI \(\dd N / \dd S\), low-SNR estimators

First KL Measurement (Gurri et al. 2020)

  • 18 hand-picked galaxy-galaxy systems: \(\ev{\gamma} = 0.0201 \pm 0.0079\)

    • primarily sensitive to \(\gamma_\times\); \(\gamma_+\) degenerate with inclination and scale radius

  • kinematic shape noise dominates uncertainty:
    \(\sigma_k \sim 0.03\) vs. \(\sigma_p \sim 0.2\), so need \(\sim\) 50 times fewer galaxies*

  • Lower kinematic shape noise when gas & stellar velocity fields match

  • Point to SKA as enabler of KL at scale!

KL State of the Art (R. S. et al. 2024)

  • 141 target galaxies \(\to\) 3 after cuts.
  • use Tully-Fisher to estimate inclination \(\implies\) intrinsic ellipticity
    • \(\gamma_+\) constrained via Tully-Fisher
    • \(\gamma_\times\) via kinematic-photometric misalignment
  • per-galaxy SNR > 1, and 10\(\times\) reduction in shape noise!

KL Symmetries (Hopp & Witmann 2024)

Almost Feasible with WALLABY?

21 CM H1 survey, 30” resolution, up to z=0.1

WALLABY website, data from Serra et al. (2015)

Murugeshan et al (2024)

Feasible with ALMA

The ALMA-ALPAKA survey I (Rizzo et al. 2023)

high-resolution CO and [CI] kinematics of star-forming galaxies at z = 0.5-3.5

Radio Galaxy Polarization

Intuition

  • Star-forming galaxies dominate observed sources

    • synchrotron emission driven by large-scale galactic magnetic fields
      \(\implies\) polarization position angle

  • Nearby spiral polarization fractions: 1-10% (Stil et al. 2008)

  • polarization angle not affected by lensing

    • but Faraday effect, and cosmic birefringence..

Stil et al. (2008)

Polarization: Some Examples

Slides from a presentation by David Mulcahy

Polarized thermal emission from dust in a galaxy at redshift 2.6
(Geach et al. 2023)

A Bit of History

  • Kronberg et al. 1991, Kronberg, Dyer & Roeser 1996, Burns et al. 2004:
    lensing measurements with polarized radio jets

A Bit of History

  • Brown & Battye (2011a, 2011b): first polarization shear estimator

A Bit of History

  • Whitaker et al. (2015, 2018):
    • improve upon B&B11 estimator
    • quadratic estimator
      using pol. vectors combined with finite-difference gradients of Stokes I
    • estimates of rotation from birefringence:
      2.03º \(\pm\) 0.75º (authors caution Farrady rotation systematics)
2PCF upper limits from 30 sources

Weak Lensing Rotation

Thomas et al. 2016

  • Tensor & vector gravitational potentials allow for
    a rotation mode \(\omega\) in addition to \(\gamma\) and \(\kappa\):

    • only measurable if source-plane position angle can be estimated

  • Second-order effect in \(\Lambda\)CDM, post-Born/lens-lens coupling

    • should be equivalent to shear \(B\)-modes
      \(\implies\) systematics (or \(\Lambda\)CDM) null test

  • Can solve simultaneously for shear and rotation from lensing

A new observable for cosmic shear

Francfort, Durrer, & Cusin (2022)

Estimator based on correlation function of lensing-induced rotation itself.

For a single galaxy:
\(\alpha\) is position angle; \(\delta \alpha\) is lensing-induced rotation \[\Theta = \frac{2 - \epsilon^2}{\epsilon^2} \delta \alpha = \gamma_2 \cos 2 \alpha - \gamma_1 \sin 2 \alpha\]

For a pair of galaxies 1,2: \[\Xi = \Theta(\vb n_1, \alpha_1, z_1) \Theta(\vb n_2, \alpha_2, z_2)\]

Estimator averaging over \(\Xi, \Xi'\) pairs (4-pt?):

Summary

Observables:

  • Optical photometric shape

  • Radio intensity shape

  • Optical kinematics

  • Radio kinematics

  • Radio polarization

Seems not-quite-there with current datasets..

  • perhaps estimator proof-of-concept / recovery of null shear

  • could hand-pick systems,
    but probably not yet interesting for cosmology in the near future

Relevance to TOSCA

What interesting things can we do in the timescale of TOSCA?

  • Joint shear reconstruction (with ML?)

    • ASKAP pathfinders (WALLABY, POSSUM)

    • ALMA proposal, or archival data scraping?

  • Calibration

    • what can we do if we know something about the intrinsic shape
      of 0.1% of Euclid galaxies?

Thank you!