Benedikt Diemer, Surhud More, Andrey Kravtsov
A dark matter halo is commonly defined as a spherical overdensity of matter with respect to a reference density, such as the critical density or the mean matter density of the Universe. We show that such definitions lead to a spurious evolution in the halo's mass even if its physical density profile remains constant over time. This pseudo-evolution in mass is caused by the evolution of the reference density with redshift, and has little connection with the actual physical accretion of mass. We compute the pseudo-evolution of halos identified in a large N-body simulation from z=1 to 0, and show that it increases halo masses significantly across a wide range of halo masses and overdensities. Pseudo-evolution accounts for almost the entire mass evolution of halos with M200 smaller or equal 1E12 solar masses, while for larger halos it still accounts for about 50 percent of their overall mass evolution. We estimate the magnitude of the pseudo-evolution assuming that halo density profiles remain static in physical coordinates, and show that this simple model predicts the pseudo-evolution of simulated halos to a few percent accuracy. We discuss the impact of pseudo-evolution on the evolution of the halo mass function. We show that the non-evolution of the low-mass end of the halo mass function is the result of a fortuitous cancellation between pseudo-evolution and the absorption of small halos into larger hosts. We also show that the evolution of the low-mass end of the concentration-mass relation observed in simulations is almost entirely due to the pseudo-evolution of mass. Finally, we discuss the implications of our results for the interpretation of the evolution of various scaling relations between the observable properties of galaxies and galaxy clusters, and their halo masses.
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http://arxiv.org/abs/1207.0816
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