Masses for the Local Group and the Milky Way |
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Authors: | Yang-Shyang Li Simon D. M. White |
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Affiliation: | Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, the Netherlands;Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany |
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Abstract: | We use the very large Millennium Simulation of the concordance Λ cold dark matter 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 per cent 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 present 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 × 1012 M⊙ or log M LG/M⊙= 12.72 , with an interquartile range of [12.58, 12.83] and a 5–95 per cent range of [12.26, 13.01]. Thus, a 95 per cent lower confidence limit on the true mass of the Local Group is 1.81 × 1012 M⊙ . 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 × 1012 M⊙ with a 95 per cent lower confidence limit of 0.80 × 1012 M⊙ . |
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Keywords: | Galaxy: formation galaxies: kinematics and dynamics Local Group dark matter |
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