A Tail of Two Populations: Chemo-dynamics of the Sagittarius Stream and
  Implications for its Original Mass
  We use the SDSS/SEGUE spectroscopic sample of stars in the leading and
trailing streams of the Sagittarius (Sgr) to demonstrate the existence of two
sub-populations with distinct chemistry and kinematics. The metallicity
distribution function (MDF) of the trailing stream is decomposed into two
Gaussians describing a metal-rich sub-population with means and dispersions
(-0.74, 0.18) dex and a metal-poor with (-1.33, 0.27) dex. The metal-rich
sub-population has a velocity dispersion ~ 8 km/s, whilst the metal-poor is
nearly twice as hot ~13 km/s. For the leading stream, the MDF is again
well-described by a superposition of two Gaussians, though somewhat shifted as
compared to the trailing stream. The metal-rich has mean and dispersion (-1.00,
0.34) dex, the metal-poor (-1.39, 0.22) dex. The velocity dispersions are
inflated by projection effects to give 15 - 30 km/s for the metal-poor, and 6 -
20 km/s for the metal-rich, depending on longitude. We infer that, like many
dwarf spheroidals, the Sgr progenitor possessed a more extended, metal-poor
stellar component and less extended, metal-rich one. We study the implications
of this result for the progenitor mass by simulating the disruption of the Sgr,
represented as King light profiles in dark halos of masses between $10^{10}$
and $10^{11} M_\odot$, in a three-component Milky Way whose halo is a live
Truncated Flat potential in the first phase of accretion and a triaxial Law \&
Majewski model in the second phase. We show that that the dark halo of the Sgr
must have been $\gtrsim 6 \times 10^{10} M_\odot$ to reproduce the run of
velocity dispersion with longitude for the metal-rich and metal-poor
sub-populations in the tails.
