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Modeling complete distributions with incomplete observations: The velocity ellipsoid from Hipparcos data  David W. Hogg
; Michael R. Blanton
; Sam T. Roweis
; Kathryn V. Johnston
;  Date: 
3 May 2005  Journal:  Astrophys.J. 629 (2005) 268275  Subject:  astroph  Affiliation:  NYU), Michael R. Blanton (NYU), Sam T. Roweis (Toronto), Kathryn V. Johnston (Wesleyan  Abstract:  [abridged] A "missing data" algorithm is developed to model (ie, reconstruct) the threedimensional velocity distribution function of a sample of stars using data (velocity measurements) every one of which has one dimension unmeasured (the radial direction). It also accounts for covariant measurement uncertainties on the tangential velocity components. The algorithm is applied to tangential velocities measured in a kinematically unbiased sample of 11,865 stars taken from the Hipparcos catalog. The local stellar velocity distribution function of each of a set of 20 colorselected subsamples is modeled as a mixture of two threedimensional Gaussian ellipsoids of arbitrary relative responsibility. In the fitting, one Gaussian (the "halo") is fixed at the known mean velocity and velocity variance tensor of the Galaxy halo, and the other (the "disk") is allowed to take arbitrary mean and arbitrary variance tensor. The mean and variance tensor (commonly the "velocity ellipsoid") of the disk velocity distribution are both found to be strong functions of stellar color, with longlived populations showing larger velocity dispersion, slower mean rotation velocity, and smaller vertex deviation than shortlived populations. The local standard of rest (LSR) is inferred in the usual way and the Sun’s motion relative to the LSR is found to be (U,V,W)_{odot}=(10.1,4.0,6.7)+/(0.5,0.8,0.2) km/s. Artificial data sets are made and analyzed, with the same error properties as the Hipparcos data, to demonstrate that the analysis is unbiased. The results are shown to be insensitive to the assumption that the velocity distributions are Gaussian.  Source:  arXiv, astroph/0505057  Services:  Forum  Review  PDF  Favorites 


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