We present a new N -body model for the tidal disruption of the Sagittarius ( Sgr ) dwarf that is capable of simultaneously satisfying the majority of angular position , distance , and radial velocity constraints imposed by current wide-field surveys of its dynamically young ( \lesssim 3 Gyr ) tidal debris streams .
In particular , this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years .
While the model does not reproduce the apparent bifurcation observed in the leading debris stream , recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit .
The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential which can be described in terms of a triaxial dark matter halo whose minor/major axis ratio ( c / a ) _ { \Phi } = 0.72 and intermediate/major axis ratio ( b / a ) _ { \Phi } = 0.99 at radii 20 < r < 60 kpc .
The minor/intermediate/major axes of this halo lie along the directions ( l,b ) = ( 7 ^ { \circ } , 0 ^ { \circ } ) , ( 0 ^ { \circ } , 90 ^ { \circ } ) , and ( 97 ^ { \circ } , 0 ^ { \circ } ) respectively , corresponding to a nearly-oblate ellipsoid whose minor axis is contained within the Galactic disk plane .
This particular disk/halo orientation is difficult to reconcile within the general context of galactic dynamics ( and CDM models in particular ) , suggesting either that the orientation may have evolved significantly with time or that inclusion of other non-axisymmetric components ( such as the gravitational influence of the Magellanic Clouds ) in the model may obviate the need for triaxiality in the dark matter halo .
The apparent proper motion of Sgr in this model is estimated to be ( \mu _ { l } \textrm { cos } b, \mu _ { b } ) = ( -2.16 , 1.73 ) mas yr ^ { -1 } , corresponding to a Galactocentric space velocity ( U,V,W ) = ( 230 , -35 , 195 ) km s ^ { -1 } .
Based on the velocity dispersion in the stellar tidal streams , we estimate that Sgr has a current bound mass M _ { Sgr } = 2.5 ^ { +1.3 } _ { -1.0 } \times 10 ^ { 8 } M _ { \odot } .
We demonstrate that with simple assumptions about the star formation history of Sgr , tidal stripping models naturally give rise to gradients in the metallicity distribution function ( MDF ) along the stellar debris streams similar to those observed in recent studies .
These models predict a strong evolution in the MDF of the model Sgr dwarf with time , indicating that the chemical abundances of stars in Sgr at the present day may be significantly different than the abundances of those already contributed to the Galactic stellar halo .
We conclude by using the new N -body model to reevaluate previous claims of the association of miscellaneous halo substructure with the Sgr dwarf .