We describe spectroscopic tomographic weak lensing measurements (spectrotomography) for two rich clusters of galaxies, A1767 and A2065, based on extensive spectroscopy and Subaru/Hyper Suprime-Cam (HSC) imaging. These detections represent the first use of spectrotomography based on archival Subaru/HSC imaging. The measurements depend only on galaxies with spectroscopic redshifts reported here. The approach cleanly separates cluster members from the background and suppresses systematics that may be introduced by the use of photometric background redshifts. We detect the tomographic shear signals at 3.1σ (A1767) and 3.5σ (A2065). The shear signal amplitudes are consistent with the cluster dynamical (caustic) masses, and they scale appropriately with source redshift. However, comparison with the first spectrophotometric detection of A2029 based on DECam imaging reveals some subtle potential systematic issues in deriving the shear signal for the relatively bright background galaxies used in the analysis. These issues may be important for understanding more extensive future applications of spectrotomography based on further Subaru imaging, as well as Euclid and LSST data. The total of three spectrophotometric detections (A1767, A2029, and A2065) sets the stage for broader application of the technique for unbiased cluster weak lensing mass determinations and potentially for a geometric cosmological test that is independent of other methods.

Introduction
Subaru/HSC Photometry and Shapes
Spectroscopy
The Weak Lensing Shear: Ellipticity Distributions and Mass Maps
    Examining Systematics in Ellipticity Measurements
    Weak Lensing Signal Detection based on Spectroscopic Background Galaxies
    Weak Lensing Maps
Tomography
    The Expected Tomographic Signal
    Significance of the Tomographic Detections
    Some Limitations of the Spectrotomography
Building a Platform for Spectrotomography and Cosmology
Conclusion
Acknowledgments
References

• Jubee Sohn(Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea, SNU Astronomy Research Center, Seoul National University, Seoul 08826, Republic of Korea) Corresponding author
• Ian P. Dell’Antonio(Physics Department, Brown University, Providence, RI 02912, USA)
• Margaret J. Geller(Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA)