Entropy Evolution Towards DARKexp of IllustrisTNG Dark Matter Halos

Abstract: Dark matter (DM) halos simulated via N-body techniques are known to exhibit nearly universal profiles at equilibrium; however, their origin remains uncertain despite thorough investigation. This work aims to probe the origin of simulated DM halo structure by testing DARKexp, a first-principles approach that describes the maximum entropy state of collisionless, self-gravitating systems in equilibrium via their energy distributions, and proposes an entropy functional that is expected to increase during evolution. We fit the DARKexp energy distribution to a set of massive equilibrium halos from the cosmological simulation IllustrisTNG. For the first time, we calculate the entropy of these halos as a function of cosmological time by tracking halos that are in equilibrium at z = 0.0, to z = 3.0 and calculating their entropy at various epochs between. We find that DARKexp provides an excellent fit to the energy distributions of equilibrium DM halos and that such halos exhibit an overall increase in entropy during evolution. Our results indicate that DM halos evolve to become their maximum entropy state at equilibrium and that this state is described by DARKexp.