A 2MASS All‐Sky View of the Sagittarius Dwarf Galaxy. V. Variation of the Metallicity Distribution Function along the Sagittarius Stream

We present reliable measurements of the metallicity distribution function(MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarfspheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy ofcandidate M giant members of the Sgr system. The Sgr MDF is found to evolvesignificantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgrleading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidalstripping) age. This is direct evidence that there can be significant chemicaldifferences between current dSph satellites and the bulk of the stars they havecontributed to the halo. Our results suggest that Sgr experienced a significantchange in binding energy over the past several Gyr, which has substantiallydecreased its tidal boundary across a radial range over which there must havebeen a significant metallicity gradient in the progenitor galaxy. By accountingfor MDF variation along the debris arms, we approximate the MDF Sgr would havehad several Gyr ago. We also analyze the MDF of a moving group of M giants wepreviously discovered towards the North Galactic Cap having opposite radialvelocities to the infalling Sgr leading arm stars there and propose that mostof these represent Sgr trailing arm stars overlapping the Sgr leading arm inthis part of the sky. If so, these trailing arm stars further demonstrate thestrong MDF evolution within the Sgr stream.Comment: The new version is a much-expanded version moved from ApJL to ApJ and will appear in Dec. 1 editio