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

TL;DR

This study analyzes the chemical and dynamical properties of the Sagittarius stream's leading and trailing arms, revealing a unique low-eccentricity stellar population in the leading arm likely influenced by the Large Magellanic Cloud's gravitational effects.

Contribution

It identifies a distinct low-eccentricity stellar population in the Sagittarius stream's leading arm and attributes this to gravitational perturbations from the Large Magellanic Cloud.

Findings

The leading arm has a higher fraction of low-eccentricity stars than the trailing arm.

The Large Magellanic Cloud's gravitational pull influences the orbital properties of Sagittarius stream stars.

The chemical properties support a radial metallicity gradient in the progenitor galaxy.

Abstract

We present a chemical and dynamical analysis of the leading arm (LA) and trailing arm (TA) of the Sagittarius (Sgr) stream, as well as for the Sgr dwarf galaxy core (SC), using red giant branch, main sequence, and RR Lyrae stars from large spectroscopic survey data. The different chemical properties among the LA, TA, and SC generally agree with recent studies, and can be understood by radial metallicity gradient established in the progenitor of the Sgr dwarf, followed by preferential stellar stripping from the outer part of the Sgr progenitor. One striking finding is a relatively larger fraction of low-eccentricity stars (e < 0.4) in the LA than in the TA and SC. The TA and SC exhibit very similar distributions. Considering that a tidal tail stripped off from a dwarf galaxy maintains the orbital properties of its progenitor, we expect that the e-distribution of the LA should be similar to that of the TA and SC. Thus, the disparate behavior of the e-distribution of the LA is of particular interest. Following the analysis of Vasiliev et al., we attempt to explain the different e-distribution by introducing a time-dependent perturbation of the Milky Way by the Large Magellanic Cloud (LMC)'s gravitational pull, resulting in substantial evolution of the angular momentum of the LA stars to produce the low-e stars. In addition, we confirm from RR Lyrae stars with high eccentricity (e > 0.6) that the TA stars farther away from the SC are also affected by disturbances from the LMC.