Past exam of the mathematics course of the University of Cambridge 2001 iii Paper 40 2 ii Solution Created 2026-10-03 Updated 2026-10-14
For the spherical Abel deprojection, put and . Then and . Consequently the integral appearing in the reconstruction of isDifferentiate before substituting the numerical value of :For the projected second stellar velocity moment, . The identical substitution in isotropic stellar pressure deprojection givesFor completeness, setting yields , where is the Euler beta function. The Gamma function recurrence gives and . This also checks the normalization of the power-law spherical projection kernel.
The radial Jeans equation is . Since , division by the tracer number density givesWriting , the total gravitational acceleration is thereforeTo find all gravitating mass density, apply the Poisson equation for Newtonian gravity, with the same sign convention:Equivalently, the enclosed mass is , and . The inferred total mass is and its relative potential is , a Plummer model. The tracer number density has a different radial exponent from this total mass density. One must not multiply by an arbitrary stellar mass and identify it with all matter: the inferred field can include unobserved stars, gas, and dark matter.