# Recent mass estimates for Milky and the Local Group

1. Oct 21, 2007

### marcus

http://arxiv.org/abs/0710.3740
Masses for the Local Group and the Milky Way
Yang-Shyang Li (1), Simon D. M. White (2) ((1) Kapteyn Astronomical Institute, (2) Max-Plank-Institut für Astrophysik)
10 pages, 5 figures, Submitted to MNRAS
(Submitted on 19 Oct 2007)

"We use the very large Millennium Simulation of the concordance LambdaCDM cosmogony to calibrate the bias and error distribution of Timing Argument estimators of the masses of the Local Group and of the Milky Way. From a large number of isolated spiral-spiral pairs similar to the Milky Way/Andromeda system, we find the interquartile range of the ratio of timing mass to true mass to be a factor of 1.8, while the 5% and 95% points of the distribution of this ratio are separated by a factor of 5.7. Here we define true mass as the sum of the 'virial' masses $$M_{200}$$ of the two dominant galaxies. For current best values of the distance and approach velocity of Andromeda this leads to a median likelihood estimate of the true mass of the Local Group of $$5.27\times 10^{12}M_\odot$$, or $$\log M_{LG}/M_\odot = 12.72$$, with an interquartile range of [12.58, 12.83] and a 5% to 95% range of [12.26, 13.01]. Thus a 95% lower confidence limit on the true mass of the Local Group is $$1.81\times 10^{12}M_\odot$$. A timing estimate of the Milky Way's mass based on the large recession velocity observed for the distant satellite Leo I works equally well, although with larger systematic uncertainties. It gives an estimated virial mass for the Milky Way of $$2.43 \times 10^{12}M_\odot$$ with a 95% lower confidence limit of $$0.80 \times 10^{12}M_\odot$$."