The Impact of Halo Radius Definition on Subhalo Occupation Variation

Dark Matter halo properties have been studied extensively within the virial radius of host halo systems, and previous research shows that there are correlations between host halo properties and subhalo occupation. This work explores how the correlation would change when one extends the definition of subhalo occupation out to 1.5 Mpc for Milky Way-mass host halos. We compute the correlations between four host halo properties (half-mass scale, concentration, peak-mass scale, and spin) and subhalo occupation with varying halo radius definitions. We find that the host halo properties impact satellite occupation beyond the virial radius and the locations at which the correlation peaks do not typically align with the virial radius or splashback radius. The behavior of the subhalo occupation variation as a function of radius, especially in the outskirts, is connected to the effect of halo assembly bias. However, there is no universal behavior in the subhalo occupation variation as the halo radius definition changes. We further find that using a ratio of the number of subhalos within an individual host system rather than total number counts can, to some extent, eliminate its concentration dependence. This method shows promise for using observed satellite counts to estimate host halo mass. Our analysis highlights the utility of extending observational surveys of satellite galaxies to beyond the virial radius.