Abstract
We present the first clear evidence for an extended dark matter halo in the Draco dwarf spheroidal galaxy based on a sample of new radial velocities for 159 giant stars out to large projected radii. Using a two-parameter family of halo models spanning a range of density profiles and velocity anisotropies, we are able to rule out (at about the 2.5 ? confidence level) halos in which mass follows light. The data strongly favor models in which the dark matter is significantly more extended than the visible dwarf. However, halos with harmonic cores larger than the light distribution are also excluded. When combined with existing measurements of the proper motion of Draco, our data strongly suggest that Draco has not been tidally truncated within ~1 kpc. We also show that the rising velocity dispersion at large radii represents a serious problem for modified gravity (MOND).
Highlights
The central velocity dispersions of many Local Group dwarf spheroidal galaxies are significantly larger than expected for self-gravitating systems (e.g., Mateo 1998)
The primary motivation of this Letter is to break this degeneracy for Draco by means of improved modeling and a larger data set with many more stars in the outer parts
A dwarf spheroidal (dSph) galaxy in orbit about the Milky Way is truncated by the Galactic tidal field during perigalacticon passages (Oh & Lin 1992)
Summary
The central velocity dispersions of many Local Group dwarf spheroidal (dSph) galaxies are significantly larger than expected for self-gravitating systems (e.g., Mateo 1998). Given the apparent absence of dark matter in globular clusters (e.g., Dirsch & Richtler 1995), dSph galaxies are the smallest dark matter–dominated stellar systems in the universe As such, they have emerged as crucial testing grounds for competing theories of dark matter. From our data set it is possible to measure the line-of-sight velocity dispersion profile for a strongly dark matter–dominated dSph galaxy for the first time. The velocity dispersion is clearly flat or gently rising with increasing radius This plot already rules out the best-fit mass-followslight King model (OD01), for which the dispersion falls to zero at rtidal p 49Ј.4 and should have fallen to ∼0.59 times the central dispersion (or ∼5 km sϪ1) by 22Ј
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