A Tail of Two Populations: Chemo-dynamics of the Sagittarius Stream and Implications for its Original Mass

Abstract

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.