Metallicity and α-Element Abundance Gradients along the Sagittarius Stream as Seen by APOGEE

Abstract

Using 3D positions and kinematics of stars relative to the Sagittarius (Sgr) orbital plane and angular momentum, we identify 166 Sgr stream members observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) that also have Gaia DR2 astrometry. This sample of 63/103 stars in the Sgr trailing/leading arm is combined with an APOGEE sample of 710 members of the Sgr dwarf spheroidal core (385 of them newly presented here) to establish differences of 0.6 dex in median metallicity and 0.1 dex in [α/Fe] between our Sgr core and dynamically older stream samples. Mild chemical gradients are found internally along each arm, but these steepen when anchored by core stars. With a model of Sgr tidal disruption providing estimated dynamical ages (i.e., stripping times) for each stream star, we find a mean metallicity gradient of 0.12±0.03 dex Gyr −1 for stars stripped from Sgr over time. For the first time, an [α/Fe] gradient is also measured within the stream, at 0.02±0.01 dex Gyr −1 using magnesium abundances and at 0.04±0.01 dex Gyr −1 using silicon, which imply that the Sgr progenitor had significant radial abundance gradients. We discuss the magnitude of those inferred gradients and their implication for the nature of the Sgr progenitor within the context of the current family of Milky Way satellite galaxies, and we suggest that more sophisticated Sgr models are needed to properly interpret the growing chemodynamical detail we have on the Sgr system. Unified Astronomy Thesaurus concepts: Sagittarius dwarf spheroidal galaxy (1423); Milky Way stellar halo (1060); Chemical abundances (224); Galaxy evolution (594); Tidal tails (1701); Galaxy chemical evolution (580); Galaxy abundances (574); Stellar kinematics (1608); Stellar abundances (1577); Dwarf galaxies (416)

Elemental Abundances in M31: Gradients in the Giant Stellar Stream*

“…The presence of spatial [α/Fe] variations in Local Group dwarf galaxies, such as MW dwarf spheroidal satellite galaxies and the Magellanic Clouds (e.g., Kirby et al 2011;Nidever et al 2020), has generally not been quantified, thus largely precluding comparisons of observational expectations for [α/Fe] gradients to the GSS. The exceptions include chemical abundance studies of M31 satellite dwarf galaxies (Vargas et al 2014b), where no strong evidence for significant [α/Fe] gradients was found, and Sgr (Hayes et al 2020). Hayes et al measured an [α/Fe] gradient of (0.1 − 1.2) × 10 −3 dex deg −1 (or 0.06−0.15 dex in absolute difference) between the Sgr core and Sgr streams-in addition to weaker internal [α/Fe] gradients in the streamsthat they interpreted as reflecting Sgr's [Fe/H] gradient combined with the characteristic anticorrelation between [α/ Fe] and [Fe/H] in dwarf galaxies (e.g., Shetrone et al 2001;Venn et al 2004;Kirby et al 2011).…”

“…For the LMC, SMC, and M33, radial metallicity gradients of −(0.06-0.08) dex kpc −1 have been detected out to several disk scale lengths (LMC: Choudhury et al 2016;SMC: Dobbie et al 2014;Parisi et al 2016;Choudhury et al 2018;M33: Kim et al 2002;Tiede et al 2004;Barker et al 2007) that are similar to the gradient in the MW's disk (−0.06 dex kpc −1 ; e.g., Cheng et al 2012;Hayden et al 2014). In addition, metallicity differences of 0.4-0.6 have been observed between the Sgr core and Sgr streams (Chou et al 2007;Monaco et al 2007;Keller et al 2010;Hayes et al 2020), which translate to an intrinsic gradient of about −0.2 dex kpc −1 in the Sgr progenitor based on dynamical modeling (Law & Majewski 2010). 8 Although only weak internal gradients are measured along the Sgr streams ((−1.2−(−1.4)) × 10 −3 dex deg −1 , but consistent with zero within the uncertainties; Hayes et al 2020), combining these measurements with modeling provides evidence for a gradient with dynamical age (i.e., initial orbital radius; −0.12 ± 0.03 dex Gyr −1 ) in the Sgr progenitor.…”

“…In addition, metallicity differences of 0.4-0.6 have been observed between the Sgr core and Sgr streams (Chou et al 2007;Monaco et al 2007;Keller et al 2010;Hayes et al 2020), which translate to an intrinsic gradient of about −0.2 dex kpc −1 in the Sgr progenitor based on dynamical modeling (Law & Majewski 2010). 8 Although only weak internal gradients are measured along the Sgr streams ((−1.2−(−1.4)) × 10 −3 dex deg −1 , but consistent with zero within the uncertainties; Hayes et al 2020), combining these measurements with modeling provides evidence for a gradient with dynamical age (i.e., initial orbital radius; −0.12 ± 0.03 dex Gyr −1 ) in the Sgr progenitor. In comparison, smaller Local Group dwarf galaxies (M å  10 8.5 M e ) have diverse gradients, ranging from flat to as steep as −0.4 dex per half-light radius (e.g., Kirby et al 2011Kirby et al , 2017Leaman et al 2013;Vargas et al 2014b;Ho et al 2015;Kacharov et al 2017), with no clear relationship between the magnitude of a gradient and luminosity, host distance, or morphology (Ho et al 2015, cf.…”

“…Possible explanations for this steep gradient include a massive, disky Sgr progenitor similar to the LMC or, more likely, the lack of self-consistent modeling of continuing star formation in the Sgr core as the progenitor tidally disrupts over several gigayears(Hayes et al 2020). …”

The S2 Stream: the shreds of a primitive dwarf galaxy.*

“…Even though this is in fact the main tenet of the popular sub-field of Astrophysics known as Galactic Archaeology, not many such forensic studies are currently on record. Large numbers of detailed chemical abundances have so far only been collected and analysed for two stellar halo structures: the Sagittarius stream (see Monaco et al 2007;Chou et al 2007;de Boer et al 2015;Carlin et al 2018;Hayes et al 2020) and the Gaia Sausage (see Nissen & Schuster 2010;Helmi et al 2018;Mackereth et al 2019;Vincenzo et al 2019;Molaro et al 2020;Feuillet et al 2020). Additionally, a small number of stars in globular cluster streams have been under the magnifying glass of high-resolution spectroscopy (e.g.…”