  We have conducted a spectroscopic survey of the inner regions of the
Sagittarius (Sgr) dwarf galaxy using the AAOmega spectrograph on the
Anglo-Australian Telescope. We determine radial velocities for over 1800 Sgr
star members in 6 fields that cover an area 18.84 deg^2, with a typical
accuracy of ~2 km/s. Motivated by recent numerical models of the Sgr tidal
stream that predict a substantial amount of rotation in the dwarf remnant core,
we compare the kinematic data against N-body models that simulate the stream
progenitor as (i) a pressure-supported, mass-follows-light system, and (ii) a
late-type, rotating disc galaxy embedded in an extended dark matter halo. We
find that the models with little, or no intrinsic rotation clearly yield a
better match to the mean line-of-sight velocity in all surveyed fields, but
fail to reproduce the shape of the line-of-sight velocity distribution. This
result rules out models wherein the prominent bifurcation observed in the
leading tail of the Sgr stream was caused by a transfer from intrinsic angular
momentum from the progenitor satellite into the tidal stream. It also implies
that the trajectory of the young tidal tails has not been affected by internal
rotation in the progenitor system. Our finding indicates that new, more
elaborate dynamical models, in which the dark and luminous components are
treated independently, are necessary for simultaneously reproducing both the
internal kinematics of the Sgr dwarf and the available data for the associated
tidal stream.

Astrophysics of Galaxies