A numerical twist on the spin parameter, lambda (R)

A primary goal of integral field spectroscopic (IFS) surveys is to provide a statistical census of galaxies classified by their internal kinematics. As a result, the observational spin parameter, lambda(R), has become one of the most popular methods of quantifying the relative importance of velocity dispersion and rotation in supporting a galaxy's inner structure. The goal of this paper is to examine the relationship between the observationally deduced lambda(R) and one of the most commonly used theoretical spin parameters in the literature, the Bullock et al. lambda'. Using a set of N-body realizations of galaxies from which we construct mock IFS observations, we measure lambda(R) as an observer would, incorporating the effects of beam smearing and seeing conditions. Assuming parameters typical of current IFS surveys, we confirm that there are strong positive correlations between lambda(R) and measurement radius, and strong negative correlations between lambda(R) and size of the PSF, for late-type galaxies; these biases can be reduced using a recently proposed empirical correction. Once observational biases are corrected for, we find that lambda(R) provides a good approximation to similar to root 3/2 lambda' (R-eff), where lambda' is evaluated for the galactic stellar component within 1 R-eff.