Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2012-04-23
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
MNRAS, submitted
Scientific paper
We develop a method to extract the shape information of line profiles from discrete kinematic data. The Gauss-Hermite expansion, which is widely used to describe the line of sight velocity distributions extracted from absorption spectra of elliptical galaxies, is not readily applicable to samples of discrete stellar velocity measurements, accompanied by individual measurement errors and probabilities of membership. We introduce two parameter families of probability distributions describing symmetric and asymmetric distortions of the line profiles from Gaussianity. These are used as the basis of a maximum likelihood estimator to quantify the shape of the line profiles. Tests show that the method outperforms a Gauss-Hermite expansion for discrete data, with a lower limit for the relative gain of approx 2 for sample sizes N approx 800. To ensure that our methods can give reliable descriptions of the shape, we develop an efficient test to assess the statistical quality of the obtained fit. As an application, we turn our attention to the discrete velocity datasets of the dwarf spheroidals of the Milky Way. In Sculptor, the symmetric deviations are everywhere consistent with velocity distributions more peaked than Gaussian. This may suggest a radially biased orbital structure. In Fornax, there is an evolution in the symmetric deviations of the line profile from a peakier to more flat-topped distribution on moving outwards. This may be an indication of tangential anisotropy in the outer parts. Our methods are sensitive enough to detect evidence for velocity distributions more peaked than Gaussian in Carina and Sextans, which suggest radial anisotropy in the outer parts of these two galaxies. This is all consistent with a picture in which Fornax may have had a different evolutionary history to Sculptor, Carina and Sextans.
Amorisco N. C.
Evans Wyn N.
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