The Splashback Boundary Of Haloes In Hydrodynamic Simulations

The splashback radius, R-sp, is a physically motivated halo boundary that separates infalling and collapsed matter of haloes. We study R-sp in the hydrodynamic and dark matter-only IllustrisTNG simulations. The most commonly adopted signature of R-sp is the radius at which the radial density profiles are steepest. Therefore, we explicitly optimize our density profile fit to the profile slope and find that this leads to a similar to 5 per cent larger radius compared to other optimizations. We calculate R-sp for haloes with masses between 10(13 and 15) M-circle dot as a function of halo mass, accretion rate, and redshift. R-sp decreases with mass and with redshift for haloes of similar M-200m in agreement with previous work. We also find that R-sp/R-200m decreases with halo accretion rate. We apply our analysis to dark matter, gas, and satellite galaxies associated with haloes to investigate the observational potential of R-sp. The radius of steepest slope in gas profiles is consistently smaller than the value calculated from dark matter profiles. The steepest slope in galaxy profiles, which are often used in observations, tends to agree with dark matter profiles but is lower for less massive haloes. We compare R-sp in hydrodynamic and N-body dark matter-only simulations and do not find a significant difference caused by the addition of baryonic physics. Thus, results from dark matter-only simulations should be applicable to realistic haloes.