Red Satellite Galaxies: Best Tracers of Host Mass Distribution in the Illustris-TNG100 Simulation

In a ΛCDM universe, the dark matter halos of galaxies are expected to follow a characteristic density profile, often parameterized as the Navarro, Frenk & White (NFW) profile. The degree to which the mass density of large galaxies follows an NFW profile should, therefore, be a strong observational test of the ΛCDM model. Small, faint galaxies, in orbit about large “host” galaxies, are possible tracers of their host’s mass density. The degree to which satellite galaxies trace their host’s mass density is, however, currently unresolved. Previous observational and theoretical studies have resulted in a mix of results, with some concluding that satellite galaxies are good tracers, and others concluding that they are not. Here we investigate the degree to which satellite galaxies trace the mass density around large host galaxies the high-resolution IllustrisTNG100-1 simulation. Due to the adoption of different feedback models, the TNG suite of simulations reproduces the observed red/blue color bimodality of galaxies, while the original Illustris suite of simulations does not. We compare results from the two simulations and show that the degree to which satellite galaxies trace their host’s mass density differs between the two simulations, which we attribute to the use of different feedback models. Further, we compare the number density profiles of the TNG100-1 satellites to the mass density profiles of their hosts. We find that, while no population of TNG100-1 satellites is a perfect tracer of host mass density, the best tracers are the red satellites of red hosts.