Kinematic Lensing with the Roman Space Telescope

Wide-Field Science – Large
Elisabeth Krause / University of Arizona, PI

Weak gravitational lensing (WL) is one of the core probes of the Nancy Grace Roman Space Telescope to study multiple high-profile NASA science goals such as the origin and composition of the Universe and the processes of structure formation and galaxy evolution.

WL however is a very challenging measurement. Most importantly, the fact that the intrinsic shape of the lensed galaxy is unknown results in large statistical uncertainties of WL shear measurements. This so-called shape noise dilutes the desired measurement of the shear effect and as a consequence, traditional WL requires a large ensemble of galaxies to boost signal-to-noise of the shear signal. Unsurprisingly, this implies the inclusion of a substantial sample of faint galaxies that are affected by systematic uncertainties in shape and redshift measurement algorithms; perfectly controlling these systematics is impossible and can limit the constraining power of Roman cosmology.

Kinematic Lensing (KL) combines imaging and spectroscopic data into a new type of lensing inference. This reduces the shape noise uncertainty haunting WL by more than an order of magnitude. Further, the KL estimator automatically bypasses one of the most severe astrophysical systematics of weak lensing, so-called intrinsic galaxy alignments. The need for spectroscopic information, however, implies that the size of our KL galaxy sample will be smaller compared to that of standard WL. Nevertheless, the smaller KL galaxy sample will be significantly more robust to the two main observational systematics that are haunting standard WL galaxies. Firstly, the spectroscopic information renders redshift uncertainties obsolete; secondly, we can select large, bright galaxies, for which shape measurement uncertainties are well-controlled. Our team has run initial forecasts using the Roman spectroscopic sample as a KL galaxy sample, and we find that the KL constraining power on dark energy equation of state parameters is increased significantly over that of standard WL.

In this proposal we plan to develop a full KL inference pipeline that can ingest imaging and spectroscopic data from Roman and produce a corresponding KL shape catalog. We will also build the software infrastructure for the cosmological interpretation of the extracted KL signal, and we will use this infrastructure to create precision forecasts of KL science performance as a function of Roman survey strategy and systematics control.