Phase-space Properties and Chemistry of the Sagittarius Stellar Stream Down to the Extremely Metal-poor ($[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]\lesssim -3$) Regime

In this work, we study the phase-space and chemical properties of Sagittarius (Sgr) stream, the tidal tails produced by the ongoing destruction of Sgr dwarf spheroidal (dSph) galaxy, focusing on its very metal-poor (VMP; $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]<-2$) content. We combine spectroscopic and astrometric information from SEGUE and $Gaia$ EDR3, respectively, with data products from a new large-scale run of $\mathtt{\text{StarHorse}}$ spectro-photometric code. Our selection criteria yields $\sim 1600$ stream members, including $>200$ VMP stars. We find the leading arm ($b>0^{\circ }$) of Sgr stream to be more metal-poor, by $\sim 0.2$ dex, than the trailing one ($b<0^{\circ }$). With a subsample of turnoff and subgiant stars, we estimate this substructure’s stellar population to be $\sim 1$ Gyr older than the thick disk’s. With the aid of an $N$-body model of the Sgr system, we verify that simulated particles stripped earlier ($>2$ Gyr ago) have present-day phase-space properties similar to lower-metallicity stream stars. Conversely, those stripped more recently ($<2$ Gyr) are preferentially more akin to metal-rich ($[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]>-1$) members of the stream. Such correlation between kinematics and chemistry can be explained by the existence of a dynamically hotter, less centrally-concentrated, and more metal-poor population in Sgr dSph prior to its disruption, implying that this galaxy was able to develop a metallicity gradient before its accretion. Finally, we discovered several carbon-enhanced metal-poor ($[\mathrm{C}\mathrm{/}\mathrm{F}\mathrm{e}]>+0.7$ and $[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]\le -1.5$) stars in Sgr stream, which is in tension with current observations of its remaining core where such objects are not found.