Statistical mechanics of collisionless orbits. I. origin of central cusps in dark-matter halos

TY - JOUR

T1 - Statistical mechanics of collisionless orbits. I. origin of central cusps in dark-matter halos

AU - Hjorth, Jens

AU - Williams, L.L.R.

PY - 2010/10/10

Y1 - 2010/10/10

N2 - We present an equilibrium statistical mechanical theory of collisionless self-gravitational systems with isotropic velocity distributions. Compared with existing standard theories, we introduce two changes: (1) the number of possible microstates is computed in energy (orbit) space rather than phase space and (2) low occupation numbers are treated more appropriately than they are using Stirling's approximation. Combined, the two modifications predict that the relaxed parts of collisionless self-gravitating systems, such as dark-matter halos, have a differential energy distribution N(e) [exp(0 - e)-1], dubbed "DARKexp." Such systems have central power-law density cusps (r) r-1, which suggests a statistical mechanical origin of cusps in simulated dark-matter halos.

AB - We present an equilibrium statistical mechanical theory of collisionless self-gravitational systems with isotropic velocity distributions. Compared with existing standard theories, we introduce two changes: (1) the number of possible microstates is computed in energy (orbit) space rather than phase space and (2) low occupation numbers are treated more appropriately than they are using Stirling's approximation. Combined, the two modifications predict that the relaxed parts of collisionless self-gravitating systems, such as dark-matter halos, have a differential energy distribution N(e) [exp(0 - e)-1], dubbed "DARKexp." Such systems have central power-law density cusps (r) r-1, which suggests a statistical mechanical origin of cusps in simulated dark-matter halos.

U2 - 10.1088/0004-637X/722/1/851

DO - 10.1088/0004-637X/722/1/851

M3 - Journal article

SN - 0004-637X

VL - 722

SP - 851

EP - 855

JO - Astrophysical Journal

JF - Astrophysical Journal

IS - 1

ER -