Mass functions of dark matter halos from maximum entropy distributions for self-gravitating collisionless flow
The halo-mediated inverse mass cascade is a key feature of the intermediate statistically steady state for self-gravitating collisionless flow (SG-CFD). A broad spectrum of halos and halo groups are necessary to form from inverse mass cascade for long-range interaction system to maximize its entropy. The limiting velocity ($\textbf X$), speed ($\textbf Z$), and energy ($\textbf E$) distributions of collisionless particles can be obtained analytically from a maximum entropy principle. Halo mass function, i.e. the spectrum of halo mass, is a fundamental quantity for structure formation and evolution. Instead of basing mass functions on simplified spherical/elliptical collapse models, it is possible to reformulate mass function as an intrinsic distribution to maximize system entropy during the everlasting statistically steady state. Starting from halo-based description of non-equilibrium collisionless flow, distributions of particle virial dispersion ($\textbf H$), square of particle velocity ($\textbf P$), and number of halos ($\textbf J$) are proposed. Their statistical properties and connections with the limiting velocity distribution ($\textbf X$) are well studied and established. With $\textbf H$ being essentially the halo mass function, two limiting cases of $\textbf H$ distribution are analyzed for large halos ($\textbf H_\infty$) and small halos ($\textbf H_s$), respectively. For large halos, $\textbf H_\infty$ is shown to also be a maximum entropy distribution. For small halos, $\textbf H_s$ approximates the $\textbf P$ distribution and recovers the Press-Schechter mass function. The full solution of $\textbf H$ distribution depends on the limiting distribution ($\textbf X$) that maximizes system entropy and the exact models of halo velocity dispersions.