Satellite Galaxies in the Illustris-1 Simulation: Poor Tracers of the Mass Distribution

Number density profiles are computed for the satellites of relatively isolated host galaxies in the Illustris-1 simulation. The mean total mass density of the hosts is well fitted by a Navarro–Frenk–White (NFW) profile. The number density profile for the complete satellite sample is inconsistent with NFW, and on scales ≲0.5 r200, the satellites do not trace the hosts' mass. This differs substantially from previous results from semianalytic galaxy formation models. The shape of the satellite number density profile depends on the luminosities of the hosts and the satellites, and on the host virial mass. The number density profile for the faintest satellites is well fitted by an NFW profile, but the concentration is much less than the mean host mass density. The number density profile for the brightest satellites exhibits a steep increase in slope for host-satellite distances ≲0.1 r200, in qualitative agreement with recent observational studies that find a steep increase in the satellite number density at small host-satellite distances. On scales ≳0.1 r200 the satellites of the faintest hosts trace the host mass reasonably well. On scales ≲0.4 r200, the satellites of the brightest hosts do not trace the host mass, and the satellite number density increases steeply for host-satellite distances ≲0.1 r200. The discrepancy between the satellite number density profile and the host mass density is most pronounced for the most massive systems, with the satellite number density falling far below that of the mass density on scales ≲0.5 r200.