Weak lensing measurements are extremely challenging: the shear effect is degenerate with the unobservable intrinsic galaxy shape (with shape dispersion σε~0.25). This corresponds to a S/N ~ 0.01 per galaxy and makes traditional weak lensing a statistical measurement that requires averaging over large numbers of background galaxies.
Kinematic lensing breaks the shape-shear degeneracy by combining imaging and spectroscopic data into a new type of lensing inference. Spiral galaxies exhibit disk rotation patterns with a circular velocity that can be inferred from the easily observable galaxy luminosity (Tully-Fisher relation, Huff et al. 2013). Spectroscopic measurements of the rotation velocity of a disk galaxy enable us to infer the unobservable intrinsic galaxy shape in weak lensing measurements and measure shear on a per-galaxy basis.
KL Basic concepts
Kinematic Lensing will allow us to to constrain the intrinsic orientation of galaxies.
The basic idea for the kinematic lensing is depicted on the right. In an image, an inclined rotating circular disk has elliptical isophotes. When the image of this galaxy is sheared, the isophotes remain elliptical (in the weak shear limit, |g| << 1) with a new axis ratio and position angle. New photometric axes are inferred from this ellipse in the sheared image, and information about the original axes is lost. The case is different however, with galaxy kinematics measured via spectroscopy. The unsheared circular disk has kinematic axes that are perpendicular to one another and are aligned with the unsheared photometric axes. This cross shape becomes skewed when the velocity map is sheared; the kinematic axes are no longer perpendicular and they are misaligned with the photometric axes inferred from the sheared isophotal ellipse in the imaging data.
The fact that the velocity map transforms differently than the photometric image when shear is applied can be used to combine imaging and spectroscopic information to tightly constrain the intrinsic ellipticity of the source galaxies, thereby removing the largest statistical uncertainty in cosmic shear.
Kinematic Lensing - Roman Space Telescope
Forecast of Roman KL constraining power compared to Roman WL. Both techniques rely on different galaxy samples and are complementary.
In Xu et al 2023 we explore the KL concept in the context of the Roman Space Telescope and find that it would significantly boost the cosmological information compared to the Roman traditional WL measurement. KL and WL are complementary since the underlying galaxy samples are fundamentally different.
The figure to the right shows the improvement of KL (blue contours) with Roman ST over WL with Roman ST (black solid), which translates into an improvement of a factor of 3.65 in the relevant figure-of-merit, even though the KL galaxy sample is more than a factor of 10 smaller.
Our lab is leading a NASA Roman Wide Field Science team effort to explore the viability of a Roman KL survey and to implement a corresponding concept. We are simulating Roman grism observations for galaxies of different morphologies, we are building a Roman specific KL shear inference pipeline, and we are building the corresponding cosmology inference pipeline based on our CosmoLike software.
Kinematic Lensing - Pilot Measurements
We are pursuing multiple avenues to conduct pilot measurements of KL. The figure below shows our first measurement using Keck DEIMOS spectra and Subaru Telescope imaging. We also pursue KL measurements using existing data from the James Webb Space Telescope, the Dark Energy Spectroscopic Survey, and data we collect ourselves using the Binospec instrument on the MMT.
We use KL to measure a signal around the cluster Abell 2261. Our KL detection is based on 3 galaxies (pink), the traditional weak lensing shear profile is shown as black squares. Error bars on on KL measurement are comparable to those of several 10s to 100 galaxies using standard WL.