Great Circle tidal streams: evidence for a nearly spherical massive dark halo around the Milky Way

Abstract

An all high-latitude sky survey for cool carbon giant stars in the Galactic halo has revealed 75 such stars, of which the majority are new detections. Of these, more than half are clustered on a Great Circle on the sky which intersects the center of Sagittarius dwarf galaxy (Sgr) and is parallel to its proper motion vector, while many of the remainder are outlying Magellanic Cloud C-stars. A pole-count analysis of the carbon star distribution clearly indicates that the Great Circle stream we have isolated is statistically significant, being a 5-6 sigma over-density. These two arguments strongly support our conclusion that a large fraction of the Halo carbon stars originated in Sgr. The stream orbits the Galaxy between the present location of Sgr, 16 kpc from the Galactic center, and the most distant stream carbon star, at ~60 kpc. It follows neither a polar nor a Galactic plane orbit, so that a large range in both Galactic R and z distances are probed. That the stream is observed as a Great Circle indicates that the Galaxy does not exert a significant torque upon the stream, so the Galactic potential must be nearly spherical in the regions probed by the stream. We present N-body experiments simulating this disruption process as a function of the distribution of mass in the Galactic halo. A likelihood analysis shows that, in the Galactocentric distance range 16 kpc < R < 60 kpc, the dark halo is most likely almost spherical. We rule out, at high confidence levels, the possibility that the Halo is significantly oblate, with isodensity contours of aspect q_m < 0.7. This result is quite unexpected and contests currently popular galaxy formation models. (Abridged)