We present abundances of C, O, Ti, and Fe for eleven M-giant stars in the
trailing tidal arm of the Sagittarius dwarf (Sgr). The abundances were derived
by comparing synthetic spectra with high-resolution infrared spectra obtained
with the Phoenix spectrograph on the Gemini South telescope. The targeted stars
are drawn from two regions of the Sgr trailing arm separated by 66 degrees (5
stars) and 132 degrees (6 stars) from the main body of Sgr. The trailing arm
provides a more direct diagnostic of the chemical evolution of Sgr compared to
the extensively phase-mixed leading arm.
Within our restricted sample of ~2-3 Gyr old stars, we find that the stream
material exhibits a significant metallicity gradient of -(2.4\pm0.3)x10^{-3}
dex / degree (-(9.4\pm1.1)x10^{-4} dex / kpc) away from the main body of Sgr.
The tidal disruption of Sgr is a relatively recently event. We therefore
interpret the presence of a metallicity gradient in the debris as indicative of
a similar gradient in the progenitor. The fact that such a metallicity gradient
survived for almost a Hubble time indicates that the efficiency of radial
mixing was very low in the Sgr progenitor.
No significant gradient is seen to exist in the [alpha/Fe] abundance ratio
along the trailing arm. Our results may be accounted for by a radial decrease
in star formation efficiency and/or radial increase in the efficiency of
galactic wind-driven metal loss in the chemical evolution of the Sgr
progenitor. The [Ti/Fe] and [O/Fe] abundance ratios observed within the stream
are distinct from those of the Galactic halo. We conclude that the fraction of
the intermediate to metal-rich halo population contributed by the recent
dissolution (<3 Gyr) of Sgr-like dwarf galaxies can not be substantial.Comment: 22 pages, 7 figures, ApJ accepte
