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

Abstract

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; $[Fe/H]<-2$) content. We combine spectroscopic and astrometric information from SEGUE and $Gaia$ EDR3, respectively, with data products from a new large-scale run of $\text{StarHorse}$ 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 ($[Fe/H]>-1$) 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 ($[C/Fe]>+0.7$ and $[Fe/H]\le -1.5$) stars in Sgr stream, which is in tension with current observations of its remaining core where such objects are not found.

Accepted for ApJ. The arXiv was updated to match the accepted version

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