Spectrotomographic Weak Lensing of redMaPPer Galaxy Clusters

Abstract

Galaxy clusters are among the potentially most constraining probes of cosmology and stand out for the breadth of scientific questions they are useful for in astrophysics and cosmology. The principal challenge with using observations of galaxy clusters to constrain cosmological physics is the multifaceted problem of characterizing cluster selection and mass. This challenge is especially critical to making full use of the large statistical samples of lower-mass clusters identified readily from large imaging surveys of the sky. Calibrating the masses of optically selected samples of clusters relies on weak gravitational lensing to yield mean mass for a given observable proxy of mass. While weak lensing stands out for yielding a low-bias constraint on mean mass at a given observable, it is nevertheless subject to significant systematic uncertainties. Uncertainty in the photometric redshift distribution of background galaxies used for cluster weak lensing is a significant contributor to uncertainty in cluster mass. In this work, we present a first detection of weak gravitational lensing using a purely spectroscopic source galaxy sample over large sky area by combining the Dark Energy Survey Year 3 and Dark Energy Spectroscopic Instrument Year 3 datasets to measure excess surface density around redMaPPer galaxy clusters. The DESI DR2 spectroscopic dataset provides a sufficient number of distant spectroscopic galaxy redshifts to measure weak lensing without the need to rely on photometric redshifts, leading to a measurement with lower systematic uncertainty and enabling a new empirical test of photometric redshift bias. We detect spectrotomographic weak lensing at a signal-to-noise ratio of 26. Moreover, our comparison of photometric with spectroscopic weak lensing enables an assessment of assumptions made during photometric weak lensing, namely regarding the quantification of source galaxy sample contamination and photometric redshift calibration. We find consistency in spectrotomographic and photometric weak lensing. Finally, we comment on further future applications enabled by spectrotomographic lensing in the era of Stage IV galaxy surveys limited by systematic, rather than statistical, uncertainties in the galaxy lensing signal.