Groping for truth from the galaxy's outermost satellites
Three explanations of Kerr's bending of the Galaxy's outer disk are briefly considered.
The motions of distant globular 11 clusters and satellites of the Galaxy give a best estimate of its mass of 2.5 × 1011 M⊙ out to 120 kpc. This estimate depends on the isotropy of the motions of distant objects which will be untrue if the distant dwarf spheroidal galaxies are fragile to tidal disruption. The large velocity dispersions recently found in them imply that they are sufficiently rugged to resist the tides. However, critical discussion of the stellar velocity observations shows that the observers of Carina have underestimated their errors so that its true velocity dispersion is much smaller. This implies that Carina's mass-to-light ratio is stellar and that it is tidally fragile.
When measurements close to the observational limits are required, two or three independent astronomers observing the same stars with independent equipment and reduction procedures are needed. Three independent sets of measurements of the same stars allow external errors to be found for each set.
The high velocity at the tip of the Magellanic Stream is hard to reproduce in low mass Galaxy models unless non-gravitational forces act on the gas.
Direct evidence favouring a large mass for the Milky Way is not yet forthcoming but accurate velocities for Eridanus, Leo I and Palomar 14 might well produce it.
It is shown that a mass of 1012 M⊙ for the Galaxy is not in serious conflict with available data and would allow concordance with the Mass of the Local Group determined from the timing argument.
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