An IllustrisTNG view of the caustic technique for galaxy cluster mass estimation

The TNG300-1 run of the IllustrisTNG simulations includes 1697 clusters of galaxies with M_200c > 10¹⁴ M_⊙ covering the redshift range 0.01 − 1.04. We built mock spectroscopic redshift catalogs of simulated galaxies within these clusters and applied the caustic technique to estimate the cumulative cluster mass profiles. We computed the total true cumulative mass profile from the 3D simulation data, calculated the ratio of caustic mass to total 3D mass as a function of cluster-centric distance, and identified the radial range where this mass ratio is roughly constant. The ratio of 3D to caustic mass on this plateau defines ℱ_β. The filling factor, ℱ_β = 0.41 ± 0.08, is constant on a plateau that covers a wide cluster-centric distance range, (0.6 − 4.2) R_200c. This calibration is insensitive to redshift. The calibrated caustic mass profiles are unbiased, with an average uncertainty of 23%. At R_200c, the average M^C/M^3D = 1.03 ± 0.22; at 2 R_200c, the average M^C/M^3D = 1.02 ± 0.23. Simulated galaxies are unbiased tracers of the mass distribution. IllustrisTNG is a broad statistical platform for application of the caustic technique to large samples of clusters with spectroscopic redshifts for ≳200 members in each system. These observations will allow extensive comparisons with weak-lensing masses and will complement other techniques for measuring the growth rate of structure in the Universe.