G is the gravitational constant. M is the mass under consideration. v M {\displaystyle {v_{M}}} is the velocity of the object under consideration, in a frame where the center of gravity of the matter field is initially at rest. X = v M / ( 2 σ ) {\displaystyle X=v_{M}/({\sqrt {2}}\sigma )} is the ratio of the velocity of the object under consideration to the modal velocity of the Maxwellian distribution. ( σ {\displaystyle \sigma } is the velocity dispersion). erf {\displaystyle {\mbox{erf}}} is the "error function" obtained by integrating the normal distribution. ρ {\displaystyle \rho } is the density of the matter field.