First Clear Signature of an Extended Dark Matter Halo in the Draco Dwarf Spheroidal

We present the first clear evidence for an extended dark matter halo in theDraco dwarf spheroidal galaxy based on a sample of new radial velocities for159 giant stars out to large projected radii. Using a two parameter family ofhalo models spanning a range of density profiles and velocity anisotropies, weare able to rule out (at about the 2.5 sigma confidence level) haloes in whichmass follows light. The data strongly favor models in which the dark matter issignificantly more extended than the visible dwarf. However, haloes withharmonic cores larger than the light distribution are also excluded. Whencombined with existing measurements of the proper motion of Draco, our datastrongly suggest that Draco has not been tidally truncated within ~1 kpc. Wealso show that the rising velocity dispersion at large radii represents aserious problem for modified gravity (MOND).Comment: to be published in ApJL; 5 pages, 4 figure