A new method for the identification of non-Gaussian line profiles in elliptical galaxies
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Abstract
It is usually assumed that the line profiles (i.e., the distributions of stars over line-of-sight velocities) of elliptical galaxies have Gaussian shapes, characterized by a line strength γ, mean radial velocity V, and velocity dispersion σ. We relax this unnecessarily restrictive assumption and propose a decomposition of the line profile into orthogonal functions: the Gauss-Hermite series. This series naturally leads to two extra parameters that measure deviations of the line profile from a Gaussian: a parameter h_3_ measuring asymmetric deviations and a parameter h_4_ measuring symmetric deviations. Model calculations for the outer parts of spherical galaxies yield line profiles that can deviate significantly from Gaussians. Even for models with only mild velocity dispersion anisotropy the rms deviations from a Gaussian can be of order 10%. Approximating these line profiles by Gaussians yields systematic errors in the estimates of the mean radial velocity and velocity dispersion of 10% or more. The new method is used to derive line profiles for the elliptical galaxies IC 1459, NGC 1374, and NGC 4278. All three galaxies have asymmetric line profiles on the major axis, similar to those found earlier in galaxies with kinematically distinct cores. In addition we find evidence for symmetric deviations from a Gaussian. By fitting Gaussians to these asymmetric line profiles the amplitude of the rotation curve can be overestimated by 30% or more. The results for h_3_ and h_4_ are not strongly dependent on the spectral resolution of the observations. The results confirm the notion that elliptical galaxies have complex structures, due to their complex formation history. It is expected that accurate measurements of line profiles will provide additional constraints on models of galaxy structure and formation.