The Splashback Radius of Halos from Particle Dynamics. II. Dependence on Mass, Accretion Rate, Redshift, and Cosmology

Abstract

The splashback radius R sp , the apocentric radius of particles on their first orbit after falling into a dark matter halo, has recently been suggested as a physically motivated halo boundary that separates accreting from orbiting material. Using the Sparta code presented in Paper I, we analyze the orbits of billions of particles in cosmological simulations of structure formation and measure R sp for a large sample of halos that span a mass range from dwarf galaxy to massive cluster halos, reach redshift 8, and include WMAP, Planck, and self-similar cosmologies. We analyze the dependence of R sp /R 200m and M sp /M 200m on the mass accretion rate Γ, halo mass, redshift, and cosmology. The scatter in these relations varies between 0.02 and 0.1 dex. While we confirm the known trend that R sp /R 200m decreases with Γ, the relationships turn out to be more complex than previously thought, demonstrating that R sp is an independent definition of the halo boundary that cannot trivially be reconstructed from spherical overdensity definitions. We present fitting functions for R sp /R 200m and M sp /M 200m as a function of accretion rate, peak height, and redshift, achieving an accuracy of 5% or better everywhere in the parameter space explored. We discuss the physical meaning of the distribution of particle apocenters and show that the previously proposed definition of R sp as the radius of the steepest logarithmic density slope encloses roughly three-quarters of the apocenters. Finally, we conclude that no analytical model presented thus far can fully explain our results.

The Splashback Feature around DES Galaxy Clusters: Galaxy Density and Weak Lensing Profiles

“…For v p >280 km s −1 subhalos, we find r sp =1.21 h −1 Mpc, while for the other two subhalo samples, we find r sp =1.47 h −1 Mpc. This ∼20% difference is consistent with that found in Diemer et al (2017).…”

“…The sharp decline in the profile can be understood as resulting from an absence of particles orbiting beyond the radius of second turnaround. 54 In simulations, a phase-space caustic cleanly separates matter that is experiencing second turnaround from matter that is on first infall, leading to a very sharp steepening in the halo profile (DK14; Adhikari et al 2014;Diemer et al 2017). As measurements from individual halos are noisy, to detect this sharp steepening, one needs to "stack," or average, over a large number of halos.…”

“…This is different from other common halo boundary definitions, such as R 200m (the radius within which the mean density is 200 times the mean density of the universe at that redshift), which need not be associated with any change in physical properties across the boundary. Furthermore, the location of the splashback feature has been shown in simulations to correlate with the halo accretion rate (DK14; Diemer et al 2017). Since the feature is, in principle, straightforward to measure in data, it could potentially be used to constrain halo accretion rates of clusters, which are otherwise challenging to measure.…”

TheThreeHundred project: backsplash galaxies in simulations of clusters

“…Previous work has hinted at this dependence of the backsplash population on the dynamical state of a cluster, by studying the splashback radius of clusters in different dynamical states, and accreting material at different rates. Numerous studies of the splashback feature in N -body simulations (Diemer & Kravtsov 2014;Diemer et al 2017) and in models of collapsing dark matter haloes (Adhikari et al 2014;More et al 2015) have found that the ratio between the splashback radius, Rsp, and R200 is smaller in rapidlyaccreting, unrelaxed clusters. Diemer & Kravtsov (2014) go on to explain that these clusters also typically form at later times, as indicated by Fig.…”

Phase-space structure of cold dark matter haloes inside splashback: multistream flows and self-similar solution

“…In N-body simulation, the location of this boundary corresponds to the first apocenter of the accreting dark matter particles, particularly referred to as the splashback radius. Because of its clear manifestation, the observational prospects and the theoretical understanding of the splashback feature as a unique signature of the CDM paradigm have attracted much attention (More et al 2015(More et al , 2016aShi 2016;Busch & White 2017;Diemer 2017;Diemer et al 2017;Adhikari et al 2018;Okumura et al 2018;Chang et al 2018;Contigiani et al 2019a,b).…”