Near-Gaussian distributions for modelling discrete stellar velocity data with heteroskedastic uncertainties

TL;DR

This paper introduces a new positive-definite distribution model for discrete stellar velocity data with heteroskedastic uncertainties, improving non-Gaussianity analysis and dark matter profile inference in galaxies.

Contribution

The authors develop a convolution-based method using uniform and Laplace kernels to model non-Gaussian velocity distributions without negative probabilities, applicable to discrete stellar data.

Findings

Applied to dwarf spheroidal galaxies, revealing positive kurtosis indicative of cored dark matter profiles.

Demonstrated the method's effectiveness in real and mock datasets for orbital anisotropy analysis.

Provided analytic Fourier transforms suitable for spectral fitting, avoiding unphysical negative wings.

Abstract

The velocity distributions of stellar tracers in general exhibit weak non-Gaussianity encoding information on the orbital composition of a galaxy and the underlying potential. The standard solution for measuring non-Gaussianity involves constructing a series expansion (e.g. the Gauss-Hermite series) which can produce regions of negative probability density. This is a significant issue for the modelling of discrete data with heteroskedastic uncertainties. Here, we introduce a method to construct positive-definite probability distributions by the convolution of a given kernel with a Gaussian distribution. Further convolutions by observational uncertainties are trivial. The statistics (moments and cumulants) of the resulting distributions are governed by the kernel distribution. Two kernels (uniform and Laplace) offer simple drop-in replacements for a Gauss-Hermite series for negative and positive excess kurtosis distributions with the option of skewness. We demonstrate the power of our method by an application to real and mock line-of-sight velocity datasets on dwarf spheroidal galaxies, where kurtosis is indicative of orbital anisotropy and hence a route to breaking the mass-anisotropy degeneracy for the identification of cusped versus cored dark matter profiles. Data on the Fornax dwarf spheroidal galaxy indicate positive excess kurtosis and hence favour a cored dark matter profile. Although designed for discrete data, the analytic Fourier transforms of the new models also make them appropriate for spectral fitting, which could improve the fits of high quality data by avoiding unphysical negative wings in the line-of-sight velocity distribution.