A wide range of self-gravitating systems may be idealized as configurations of point masses interacting through gravity. But in galaxies, the timescale over which the cumulative effects of interactions between individual stars cause significant deflections is so long that galaxies can be accurately approximated as continuous systems. This produces a simpler description, the collisionless Boltzmann equation.
Consider an encounter with impact parameter b and velocity v between two stars of mass m. Using the impulse approximation, the transverse velocity acquired is
2 G m
(1) v_t = ----- .
b v
This approximation is well-justified in systems with large N
because large-angle deflections are very rare; the impact parameter
leading to a large deflection is
2
(2) b_min = G m / v = ~ R / N ,
where the second equality follows from the virial theorem. In the
entire system, approximately one close encounter occurs per crossing
time, regardless of N.
A key assumption made in considering the effects of interactions between individual stars is that encounters are not correlated with one another; thus collective effects are neglected. This assumption works well in many cases. If each encounter is uncorrelated with the last, the cumulative effect of many encounters is a random walk in velocity; perturbations add in quadrature. During a single passage through the system, a typical star has
(3) dn = (N / pi R^2) 2 pi b dbencounters with impact parameters between b and b+db. Here the first factor is just the surface density of stars, and the second factor is the area of an annulus with radius b and width db. Adding velocity perturbations in quadrature, the deflection due to these dn encounters is
2 2 2
(4) d v = v_t dn = 8 N (G m / R v) db/b ,
and the total velocity perturbation acquired in one crossing time is
/ R
2 | 2 2
(5) Delta v = | d v = 8 N (G m / R v) ln(R/b_min) .
|
/ b_min
Here the logarithmic factor arises from the integration over impact
parameter from b_min to R; each decade between
b_min and R contributes equally to the total
deflection. Finally, estimating the encounter velocity v
from the virial velocity V = sqrt(G N m / R) gives
2 8 ln N 2
(6) Delta v = ------ V
N
for the total change in a typical star's velocity per crossing time
t_c.
The relaxation time is the time over which the cumulative effect of stellar encounters becomes comparable to a star's initial velocity. From Eq. 6 this is
V^2 N
(7) t_r = --------- t_c = ------ t_c .
Delta v^2 8 ln N
In stellar systems with reasonably large N this time is much
longer than the crossing time; the evolution of such systems proceeds
on two widely-separated timescales. Relaxation due to stellar
encounters plays an important role in the evolution of star clusters;
galaxies, however, are generally immune due to their vast numbers of
stars.
A typical galaxy has 10^11 stars but is only 100 crossing times old, so the cumulative effects of encounters between stars are pretty insignificant. This justifies the next step, which is to idealize a galaxy as a continuous mass distribution, effectively taking the limit t_r -> infinity.
In this limit, each star moves in the smooth gravitational field Phi(x,t) of the galaxy. Thus instead of thinking about motion in a phase space of 6N dimensions, we can think about motion in a phase space of just 6 dimensions. This is a vast simplification!
Rather than keeping track of individual stars, a galaxy may be described by the one-body distribution function; let
(8) f(r,v,t) dr dvbe the mass of stars in the phase-space volume dr dv at (r,v) and time t. This provides a complete description if stars are uncorrelated, as assumed above.
The motion of matter in phase space is governed by the phase-flow,
. .
(9) (r, v) = (v, - grad Phi) .
How does this affect the total amount of mass in the phase space
volume dr dv? The rate of change of the mass is just the
inflow minus the outflow; that is, the flow obeys a continuity
equation in 6 dimensions:
df d . d .
(10) -- + -- (f r) + -- (f v) = 0 ,
dt dr dv
where the derivatives with respect to t, r, and
v are understood to be partial derivatives. Using the
expression for the phase-flow yields the collisionless Boltzmann
equation:
df df df
(11) -- + v . -- - grad Phi . -- = 0 .
dt dr dv
The collisionless Boltzmann equation or CBE describes the evolution of
the distribution function f(r,v,t). It serves as the
fundamental equation of galactic dynamics.
The gravitational field Phi(x,t) is given by Poisson's equation,
/
|
(12) div grad Phi = 4 pi G | dv f(r,v,t) .
|
/
Eqs. 11 & 12 may be viewed as a pair of coupled PDEs which together
completely describe the evolution of a galaxy.
Let (r,v) = (r(t),v(t)) be the orbit of a star. What is the rate of change of f(r,v,t) along the star's orbit? The answer is
Df df . df . df
(13) -- = -- + r . -- + v . --
Dt dt dr dv
df df df
= -- + v . -- - grad Phi . -- = 0 ,
dt dr dv
where the first equality is just the definition of the convective
derivative in phase-space, the second equality follows on substituting
the phase-flow (Eq. 9), and the last equality follows from the CBE
(Eq. 11). Thus, phase-space density is conserved along every
orbit.
This fundamental and completely general result shows that the CBE has a much greater level of symmetry than the N-body equations of motion; whereas the latter conserves a fairly small set of parameters, the CBE conserves f(r,v,t) along an infinite number of stellar orbits. We can take advantage of this infinite array of conservation laws to obtain some important results even when we can't explicitly solve the CBE.