Constraining the Splashback Radius in IllustrisTNG

Galaxy clusters are the largest structures in the universe and are used as an effective cosmological probe. A common definition of cluster mass is M200, defined as the mass enclosed with within R200 -- the radius within which the mass density is 200 times greater than the critical density of the universe. However, since R200 is not directly observable, M200 measurements are challenging to determine. As such, with the upcoming releases of cluster surveys such as Euclid and LSST, constraining cosmological parameters using cluster mass is challenging. The splashback radius, Rsp, is a more physically-motivated definition based on infalling material reaching the apocenter of their first orbit. More practically, Rsp can also be understood as the radius at which the density profiles of accreting halos is at its steepest. By analyzing cluster properties across the three resolutions of the largest IllustrisTNG simulation (TNG300), we investigate the efficacy of mapping various calculations of Rsp to retrieve underlying cluster data. We map relations between 2D and 3D calculations of Rsp, along with calculations of Rsp using stellar and dark matter particles. In more accurately defining cluster radii observationally, we can more easily define a cluster mass function that can help better constrain cosmological parameters such as σ8 and ΩM .