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

Abstract

In this work, we study the phase-space and chemical properties of the Sagittarius (Sgr) stream, the tidal tails produced by the ongoing destruction of the 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 the StarHorse spectrophotometric code. Our selection criteria yield ∼1600 stream members, including >200 VMP stars. We find the leading arm (b > 0°) of the Sgr stream to be more metal-poor, by ∼0.2 dex, than the trailing one (b < 0°). With a subsample of turnoff and subgiant stars, we estimate this substructure’s stellar population to be ∼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 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 identified several carbon-enhanced metal-poor ([C/Fe] > +0.7 and [Fe/H] ≤ −1.5) stars in the Sgr stream, which might be in tension with current observations of its remaining core where such objects are not found.

A Dynamically Distinct Stellar Population in the Leading Arm of the Sagittarius Stream

“…We derived mean values of [Mg/Fe] = +0.08 ± 0.07 (sys) ± 0.07 (ran), +0.08 ± 0.07 (sys) ± 0.06 (ran), and -0.05 ± 0.02 (ran) for the LA, TA, and SC, respectively. These chemical features of the Sgr members agree well with recent studies (e.g., Hayes et al 2020;Ramos et al 2022;Limberg et al 2023).…”

“…Note that as we have only the giant stars from APOGEE in the SC, we did not estimate the systematic error. Our derived mean values agree well with recent studies (e.g., Hayes et al 2020;Ramos et al 2022;Limberg et al 2023).…”

“…The observed metallicity difference between the two arms and the core is thought to be the result of the selective stripping of older, more metal-poor stars located in the outermost regions of the Sgr progenitor. It has also been reported that, within each arm, the metal-poor stars exhibit larger velocity dispersions than the relatively more metal-rich stars (Gibbons et al 2017;Johnson et al 2020;Limberg et al 2023).…”

All-sky Kinematics and Chemistry of Monoceros Stellar Overdensity

“…For our APOGEE sample, we used spectro-photoastrometric distances (Queiroz et al 2020(Queiroz et al , 2023, which take advantage of spectroscopic information during the isochrone-fitting procedure. These precise distance estimates provided by StarHorse enabled us to confidently identify members of the Monoceros overdensity for a chemo-kinematic analysis of its structure, as demonstrated in its successful application to studying other distant Galactic structures, such as stellar streams (Limberg et al 2023), overdensities (Perottoni et al 2022;Abuchaim et al 2023), and the bar/bulge (Queiroz et al 2021).…”

The APO-K2 Catalog. I. ∼7500 Red Giants with Fundamental Stellar Parameters from APOGEE DR17 Spectroscopy and K2-GAP Asteroseismology

“…As the Milky Way developed its structure, it accreted much smaller galaxies, and in some cases, the Milky Way is still actively accreting. For example, the tidal disruption of the Sagittarius dwarf galaxy is ongoing (Ibata et al 1994;Limberg et al 2023), the Magellanic Clouds appear to be on their first infall (Besla et al 2007), and there are dozens of globular cluster streams currently encircling our Galaxy (Bonaca et al 2020). These accreted bodies come with unique kinematic profiles, and they share integrals of motion (Helmi & de Zeeuw 2000;Font et al 2011;Simpson et al 2019) even several billion years later, allowing us, in some cases, to identify their stellar components.…”