ON PHYSICAL SCALES OF DARK MATTER HALOS

It is common practice to describe formal size and mass scales of dark matter halos as spherical overdensities with respect to an evolving density threshold. Here, we critically investigate the evolutionary eects of several such commonly used denitions and compare them to the halo evolution within xed physical scales as well as to the evolution of other intrinsic physical properties of dark matter halos. It is shown that, in general, the traditional way of characterizing sizes and masses of halos dramatically overpredicts the degree of evolution in the last 10 Gyr, especially for low-mass halos. This pseudo-evolution leads to the illusion of growth even though there are no major changes within xed physical scales. Such formal size denitions also serve as proxies for the virialized region of a halo in the literature. In general, those spherical overdensity scales do not coincide with the virialized region. A physically more precise nomenclature would be to simply characterize them by their very denition instead of calling such formal size and mass denitions ’virial’. In general, we nd a discrepancy between the evolution of the underlying physical structure of dark matter halos seen in cosmological structure formation simulations and pseudo-evolving formal virial quantities. We question the importance of the role of formal virial quantities currently ubiquitously used in descriptions, models and relations that involve properties of dark matter structures. Concepts and relations based on pseudo-evolving formal virial quantities do not properly reect the actual evolution of dark matter halos and lead to an inaccurate picture of the physical evolution of our universe. Subject headings: galaxies: halos | galaxies: structure | galaxies: kinematics and dynamics | galaxies: evolution | dark matter | methods: numerical