I'm attempting to create a velocity profile for M31. I have a Sersic profile from a paper by Sofue et al (2009), but I'm unable to project it into 3D space in order to get the actual 3D density that I'd use to calculate the velocity at a given radius. Here's the formula from a 2008 paper by Noordermeer (http://arxiv.org/pdf/0801.0870v1.pdf). Where dI is the Sersic Profile, κ is the measured radius along the line-of-nodes, m is the 3D radius and p(m) is the density in 3D. But how is m in the formula above related to κ (kappa)? I get the general idea that we're projecting a 2D profile into 3D space, but without knowing either the radius or the line of sight dimension (ζ in the paper listed above), I don't see how the 3D radius, m, can be determined. What am I missing?