The halo of the Milky Way

We show that the star counts in the spheroid of the Milky Way are not symmetric about the l = 0 degrees, l = 180 degrees plane. The minimum counts are found towards l = 155 degrees. The Galactic longitude of maximum star counts depends on the magnitude and color selection of the halo stars. We interpret this as evidence that the spheroid population is triaxial with a major axis oriented 65 degrees from the line of sight from the Sun to the Galactic center, and approximately perpendicular to the Galactic bar. Large local star concentrations from tidal debris and possible tidal debris are also observed. A full understanding of the Galactic spheroid population awaits position information and three dimensional space velocities for a representative set of stars in every substructure. Tangential velocities for many stars will be provided by current and planned astrometry missions, but no planned mission will measure stars faint enough to unravel the more distant parts of the spheroid, which contain the majority of the spatial substructure. This paper uses data from the Sloan Digital Sky Survey (SDSS) public data release DR3.